Pre-meeting background material for the 2011 International Workshop
on the Science and Conservation of Asian Horseshoe Crabs
(13-16 June 2011, Hong Kong, China]
Carl N. Shuster Jr.
NOTE: Because I will not be able to attend the workshop, this paper is
intended for distribution to participants ahead of time so that if any questions arise
on it or other topics, they can be sent to me before 30 April 2011 so that I can
respond prior to the workshop. In addition, a video keynote address will be
presented at the workshop.
SUGGESTIONS: In preparation of background material for the workshop I:
 Used Biology and Conservation of Horseshoe Crabs (2009), The American
Horseshoe Crab (2003), and Biology of Horseshoe Crabs (1988) as guides;
 Assembled and read recent papers on populations (viz.: papers co-authored
by David R. Smith et al.; Winston Watson III et al., etc.);
 Assembled and read recent papers on habitat (viz.: Jackson, Nordstrom
2009; Jackson, Smith, Nordstrom 2008; Jackson, Nordstrom, Smith 2002;
etc.);
 Prepared the attached background information, based primarily on what
authors have sent to me (my thanks to them, as I do not frequently get to
libraries) and some internet searches. Within the past two decades there
probably have been more publications on Limulus polyphemus than in all
previous years on a wide range of topics including spawning, fecundity, egg
transport, population dynamics, distribution and migrations, fossil species,
and habitat changes, etc. So much, it seems to me, that much effort will be
needed to assimilate what has accumulated.
 Developed a table of questions and answers pertaining to Limulus, as a guide
to preparation of what would be more pertinent to the Asian species.
The following material may not be the information that you expected or
hoped for. If so, I am willing to attempt to provide it and to answer questions on
Limulus, prior to the workshop.
Please send questions to [email protected] before 30 April 2011.
1
Limulus polyphemus as a Model for the Conservation of
Horseshoe Crabs
Carl N. Shuster Jr.
Virginia Institute/School of Marine Science, The College of William & Mary
March 2011
PROLOGUE
I am honored and humbled by the invitation to keynote ways in which
Limulus might serve as a model for the conservation of Asian horseshoe crabs.
This is a challenge because we are in an upward trend of research on Limulus that
is related, in part, to conservation and management of the species and there are so
many others who are well-aware and involved in Limulus conservation issues. One
such expert, the late Dr. Robert B. Barlow, known for his many contributions to
understanding vision, circadian rhythms, and spawning in Limulus would probably
advise up to look after the “little ones” – meaning, of course, populations of lesser
numbers of horseshoe crabs. He would speak emotionally but accurately and
succinctly of his experience in having his research population wiped out by
commercial harvesting (Weiner and Barlow 1999); (Figure1). Due to his incisive
insights and his many contributions on Limulus, I respectfully request that this
workshop be dedicated to the memory of Dr. Robert B. Barlow.
2
INTRODUCTION
A call for international coordination in the pursuit of the conservation and
management of horseshoe crabs was a theme of the symposium held in 2007 at
Dowling College, U.S.A. (Tanacredi et al. 2009). It and the report of its panel on
horseshoe crab management (Berkson et al. 2009) were direct precursors to this
workshop. The panel, comprised of representatives from five countries, responded
to eight questions prepared by Berkson. They generally agreed that:
 Basic data necessary for assessment of the status of the horseshoe crab
populations had not been collected.
 Habitat degradation and loss was the primary threat to the populations.
 Primary focus must be on protecting spawning and nursery habitats.
 A citizenry educated on the importance of the crabs, especially the
policymakers, is essential.
 Information and lessons learned during research and management in other
countries should be made more directly available through regularly scheduled
meetings.
 Funding should be sought, especially at the ecosystem-level.
 In contrast to the general unanimity on the first seven questions, opinions
regarding the future of horseshoe crab conservation ranged from pessimism
(Taiwan and China), to cautious optimism (India), to guarded optimism because
there was still time to thwart some major threats (Mexico), to outright optimism
(USA) due to successful coordination of coastwise management by the Atlantic
States Marine Fisheries Commission.
Defining Conservation
Everyone “knows” what conservation is, but do we agree on a definition? As
a country boy, trained at a land grant college, it is perhaps natural that I emerged as
a believer in conservation as the wise use of a resource. That may not be the
concept applicable to the goals of this workshop. In any case, I believe we should
define the term. As part of the discussion I would point out that we may not be able
to save all populations of horseshoe crabs. If not, is the information obtained and
recorded on a population an adjunct to saving it or at least remembering it?
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Scope of the Following Material
Participants of this workshop most likely will be called upon to evaluate
available information on the Asian species, determine what else should be
obtained, and then decide, at the very least, what actions could be taken to
conserve and protect the Asian horseshoe crabs. This paper calls attention to what
has been learned about the science, conservation, and management of Limulus
populations that may provide a useful background for the development of a
conservation strategy for the Indo-Pacific horseshoe crabs. It concentrates on the
first two of the four workshop themes (Populations, Habitats,
Exploitation/Utilization, and Public Consensus).
Why Look To Limulus?
The suggestion that Limulus polyphemus, the American horseshoe crab, can
serve as a model for the conservation of the Indo-Pacific species is based on a large
amount of scientific information on the natural history and ecology of Limulus
polyphemus, as well as programs to manage its populations. This information and
experience provides a sufficient background to begin the discussion on
conservation, despite major differences in the natural and political patterns of the
distribution of Limulus and that of the Indo-Pacific species.
One glance at the distribution of the Asiatic species is sufficient to
appreciate the dedication of Dr. Sekiguchi and his colleagues in the difficult task of
mapping and studying such diversely situated populations (Figure 2). Even a
casual inspection of the figure suggests that the number of horseshoe crab
populations around Borneo is an invitation for an in depth study, particularly due
to the presence of all three Asiatic species. There is a long history of research on
the Asiatic species, especially Tachypleus tridentatus, exemplified by an excellent
treatise on the Biology of Horseshoe Crabs edited by Koichi Sekiguchi (1988).
Thus the challenge is to select and utilize what is pertinent from our collective
experiences. Much research on Limulus has occurred in recent years; even a
summary of that research would create a book. Because I have selected items
emphasizing populations and habitats that illustrate or contribute to the
conservation theme of this workshop, this selection does not evenly reflect all that
is available.
4
In contrast, all of the populations L. polyphemus, except the more ancient
ones on the Yucatan peninsula, are located within the boundaries of the United
States of America and relatively easy to get to. Because the sequence of spawning
along the coast is staggered, being slightly later and later as one travels northward,
it is possible to observe the spawning of a majority of the populations on the
Atlantic coast from Florida to Maine in one summer (Figure 3). In another
summer, the Gulf of Mexico population, from Florida to the Mississippi delta,
could be examined, including an expedition to Yucatan. These distributions have
the distinct advantage of not only that of relative accessibility but also the
opportunity to quickly observe many populations along an appreciable
environmental gradient, from about 21o to 44o N, during one or two spawning
seasons. In a world-wide search Mikkelsen (1988) has also contributed to
observations on distribution of the four species.
An important difference between managing/conserving the American and
the Asiatic species of horseshoe crabs is that fewer nations and jurisdictions are
involved in North America.
POPULATIONS
There are at least two guiding principles when considering the ecology of
horseshoe crabs -- they are environmental generalists and they exist in discrete
populations. Generally the adaptation of horseshoe crabs to their habitat or
environmental factors have been subjects of prime interest (see Towle and Henry
2003). Recently questions have been raised about the genetic viability of
populations have been raised (Faurby et al. 2010). Perhaps more closely pertinent
topics are considered here: population size; distribution limits including influence
of abundance; egg size and spawning events, etc.
Environmental Generalists.
Horseshoe crabs are capable of adapting/adjusting to and surviving manifold
variations in their environment (Eldredge 1991, Loveland et al. 1997). They are
also rugged, not only in relation to the hydroclimate but also to physical impact
due to occasional high energy waves, dredges, and loggerhead turtles that
5
bash/dent or rip away part of their carapace; healing often follows even extensive
wounds.
It can be argued that, in addition to their physiological capacity to adjust to
estuarine conditions, their survival as a group is due to an architecturally solid,
three-part body design with two articulations – a piano-hinge between the first and
second parts and a universal joint between the telson and the mid-part. The lineally
arranged thirteen, multi-articulated appendages all operate under the carapace, as in
a caisson, protected from outside interferences and capable of carrying out, more
effectively, such functions as burrowing, spawning, feeding, and locomotion. That
they cannot back up appears to be the only major flaw in their mechanical abilities.
Discrete Populations.
Because Limulus polyphemus has morphometrically distinct populations,
Shuster (1955) advanced the possibility that the species existed as physiological
races. The morphometric discreteness was amply confirmed by Riska (1981). DNA
and related analyses clinched the concept and substantiated without a doubt that
Limulus existed in genetically discrete populations (Saunders et al. 1970, Selander
et al. 1970, Pierce et al. 2000, King et al. 2005).
Thus, although Limulidae have been considered to be “living fossils,” based
on their morphology, they have been actually keeping pace, physiologically, with
changes in their long-term environment. Ranges in the behavior and physiological
characteristics of horseshoe crab species are usually presented in summations
based upon many disparate populations. Because this results in a species range of
characteristics rather than that of one of its populations, I wonder: Do we really
know the extent of these ranges in each of even several populations? Not being
sure how many populations there might be, I undertook a quick tabulation of sites
where Limulus has been reported. As rough as that review was, it seemed as
though Limulus wanders almost everywhere (e.g., Figure 4).
I wonder if it would be worthwhile to survey the U.S. coastline (or any
coastline) in detail and determine how many horseshoe crabs populations exist and
approximately the total number of these crabs. Actually, because most states have
at least surveyed their horseshoe crab populations, with some states actively
conserving that resource, it should be relatively easy to prepare an atlas on the
6
abundance and distribution of Limulus polyphemus, including a review of their
habitats. Perhaps an atlas could be made of the Indio-Pacific species, starting with
the results from the first such survey by Sekiguchi and colleagues (see Sekiguchi
1988).
Does Population Size Matter?
Delaware Bay, touted as the largest population and epicenter of Limulus, is
probably not the best source of the answers we seek for the conservation of the
Asian species which may be in the populations of lesser numbers category. Dose
this require specific definitions? What do we mean by a “larger” or “lesser”
population? What are their characteristics? Is one factor the stress level of coping
with the hydroclimate (e.g., the salinity, temperature, oxygen mix) of their
environment? Is another the restrictions placed on a population due to the size of
the environment? Or by the distances that the members of a population travel to
compete, in essence, the life history of the population?
In considering these questions and others where, geographically, do we look
for populations that might provide the answers? For Limulus, the most northern
populations in New England waters are among the “lesser” populations and appear
to be pushing the envelope of existence, a least in a cold-water hydroclimate.
Those in Maine, especially at Taunton Bay (Moore and Perrin 2007), fit this
category. Pleasant Bay, Massachusetts is also semi- to largely restricted in its
perambulations (Carmichael et al. 2003, Smith et al. 2009). Other populations are
also pushing the envelope of existence as those in a hyper-saline, high-temperature,
micro-tidal lagoon at Cape Canaveral, Florida (Ehlinger & Tankersley 2009). We
could also consider the populations on the Yucatan Peninsula (Zaldivar-Rae et al.
2009) that was isolated from all other Limulus populations in the distant past; how
much is this reflected in their basic biology/ecology? Genetic (King et al. 2005)
makeup? Considering the distribution of the Asiatic species (Figure 2), are there
any long-term isolations? Are there any particularly interesting populations?
How Many Asian Populations of HSCs? Many of the mainland populations
in Asia are no longer as sizeable as in former years. While it is desirable to protect
them and to enhance their chances of survival, if for no other reason than to have a
national resource that the public can treasure, should all populations, even those in
7
most obscure regions be considered? If the objective is to protect the species then
most of the populations of all three species should be considered.
Should mating and spawning behaviors be considered? In this there are a
few marked contrasts between Limulus and the three Indo-Pacific species. Yet it
seems that the basic spawning unit in all species is single-pair mating – a female
will not spawn (at least in Limulus) if a male is not attached (in amplexus)
according to Brockmann (2003). The fact that the Asiatic species have two pairs
of claspers versus one pair in Limulus seems to be correlated with spawning
behavior. The males of Tachypleus tridentatus (Botton et al. 1996) and
presumably also the other Indo-Pacific species, secure a more secure lock on the
female opisthosomas and the attachment lasts much longer than in Limulus. The
weaker attachment in the American species may have led to its alternative behavior
of mainly seeking mates on beaches; this results in an abundance of males, with
most participating as unattached satellites during spawning.
Spawning Schedule Depends Upon Number of Eggs? A rough calculation
indicates that a female Limulus lays 20,000 eggs during a high tide -- about the
number of eggs in the major oviducts at the time of spawning (depending upon the
age, size, and health of the individual). This, 5 high tides, compares favorably with
the two to six spawnings recorded by female crabs tagged with a combined
acoustic and radio transmitter and a plastic ID button (Brousseau et al. 2004).
Between each spawning the tagged females tended to remain offshore within an
average of 299m near the same beach. They usually repeatedly returned to the
same beach within an average of 351m from the tagging site, thus creating a
seasonal beach-site-spawning-fidelity. Objectively, where else were they going to
go? To recap: if a female containing approximately 100,000 mature eggs makes 5
nests a tide with each nest containing about 4,000 eggs, she could completely
spawn within five tides. Then what? Presumably these spawned-out females leave
the area for deeper water in the bay or on the continental shelf (unless abundant
prey is available and the shallow near-shore area). A fair number possibly leave
Delaware Bay soon after spawning; at least some do, as observed by Oates (pers.
comm., 2005) via use of cameras mounted on a benthic sled. Earlier Shuster and
Botton (1985) and Botton and Ropes (1987) recognized the possibility of an
offshore migration. This may also be the case for Tachypleus tridentatus
(Sekiguchi 1988). Do the other Asian species have a similar spawning “schedule”?
8
Would such behavior, of spawned-out females retiring to deeper waters within a
bay or on the shelf have any bearing on conservation or management plans?
Botton and Loveland (1989) pointed out, due to the likelihood of stranding,
that spawning was a hazardous activity. Areas, where the crabs lack the ability to
orient themselves with respect to water when level/flat tidal flats are exposed
during low tides, are also hazardous (Botton & Loveland 1987). Considering that
the number of times a female horseshoe crab comes to a beach to spawn could
increase the chance of her stranding, would the number of trips be correlated with
the number of eggs and the time it takes to deposit them?
Despite their Wide Ranges, Horseshoe Crabs Have Limits. Examination of
the ranges of the extant species reveals that there are few large-scale limitations to
their distribution (Sekiguchi & Shuster 2009, Shuster & Sekiguchi 2009): they are
dependent upon low-energy beaches of estuaries; cannot jump an ocean abyss (are
limited by the extent of continental shelves); essentially limited to tidal regimes
that are diurnal (regular and irregular), and have not been able to attain breeding
populations in northern climes beyond 45oN. Locally horseshoe crabs usually exist
in discrete populations:
1. Depending upon where they are situated within the range of the species, it
seems obvious that most populations would be incapable of immediately surviving
in a much more extreme environment. This has been illustrated by a couple of
simple temperature trials (Mayer 1914, Fraenkel 1960). They were principally
concerned with lethal high temperatures. Cold temperatures are not necessarily
lethal as the crabs may survive being frozen in ice at Delaware Bay (according to
Thurlow Christian Nelson, Rutgers University, pers. comm. 1947). Limulus begins
spawning at a minimum temperature of 12oC in Maine waters, predominately
occurring at 14oC (Schaller et al. 2002). Due to the length of time the water is so
cold in Maine, this could result in insufficient time to grow (i.e., put on enough
weight to trigger the molting process). Born (1977: p.30) believes otherwise. He
suggested “… that temperature will not prove to be a primary limiting factor
influencing the distribution and breeding success of Limulus at the northern end of
its range.” Whatever that threshold may be, Limulus does not maintain populations
further north.
9
2. Populations at the extreme limits of their geographic range are likely to be
under more stress than those within their mid-range.
3. Populations isolated from others are more likely to be genetically more
removed.
4. From a physiological viewpoint those populations most under stress or
more isolated are most likely to reveal trends or stasis, respectively, in evolution.
5. As a species, horseshoe crabs exhibit wide tolerances to environmental
parameters, but members of discrete populations have a lesser range of tolerance.
6. That horseshoe crabs move in the direction of benthic currents is common
knowledge (Anderson and Shuster 2003). Michael F. Oates (2007 unpublished) has
observed that migrating horseshoe crabs have an unusual sense and use of tidal
currents. Observed, with a benthic sled fitted with optical sensors, crabs migrating
from Delaware Bay burrow in during a slack tide and then continue on their way
when the current flows again in the direction they were travelling.
7. Horseshoe crabs are wanderers as evidenced by several tagging and
electronic-sensing programs (Shuster 1950, Baptist et al 1957, Swan 2005, Smith
et al. 2005, Watson et al. 2009) and, after the large ice shield that covered North
America some 11,000 to 13,000 years ago began to melt (Chrzastowski 1986), by
their expansion from southern climes to Maine within a few thousand years. King
et al. (2005) believe that female horseshoe crabs return to their natal bays to spawn
but the males, being more inclined to wander, are the sex responsible for gene
distribution. Yet, a 17-year tagging study reported by Swan (2005) clearly
demonstrated that females are also wanderers, some moving 100 km or more from
their tagging sites. Nowhere is this better illustrated than the range of crabs
entering and leaving Delaware Bay, from Toms Cove to Atlantic City (Figure 5).
Delaware Bay Population is Increasing Again
Horseshoe crabs were still abundant in Delaware Bay during the 1990s
(Figure 6), when environmentalists predicted their decline to extinction along with
the migratory shorebirds. Some reports are no better today. A recent interpretation
of genetic data claims that horseshoe crabs are declining (Faurby et al. 2010). Nor
have spawning indices (Smith et al. 2005) indicated a marked increase in
10
spawning. In contrast there is positive information. Whatever the population size
might have been during the past couple of decades, adults now number an
estimated 20 million (90% confidence interval: 13-28 million) of which 6.25
million were females (90% CI = 4.0-8.8 million); (Smith et al. 2005). That is a
substantial number. In 60 years of observing the abundance and distribution of the
crabs in the area, I have never seen so many as in the past four years. In recent
years on several beaches they have often paved a beach two-tiers deep (Figures 7
and 8). And, upon these tiers, a few females with attached males wander about,
even reaching the uppermost edge of the spawners before ultimately retreating
back to the bay. Seemingly these females having given up ever finding a spot in
which to spawn on that tide.
Abundance and Distribution
Over the years it has been increasingly evident that the distribution of
Limulus is, at least in part, due to its abundance. Nowhere is this as evident as the
far-ranging crabs of the Delaware Bay area (Shuster 1985, Swan 2005). By
comparison, crabs in populations of lesser numbers do not stray very far. At
Taunton Bay, Maine (Moore & Perrin 2007) or in four coastal embayments on
Cape Cod, Massachusetts (James-Pirri et al. 2005), distances travelled from
tagging sites were related to the size of the embayment and its accessibility (the
size of its entrance/exit area). The ranges of movement were generally within 2-3
km of the tagging site within the restricted entrance embayments and up to 7 km in
the larger ones. These and similar studies provide a key element to our
consideration of the conservation of the crabs – that Limulus populations differ
among embayments within a region, at least due to environmental conditions and
extent of enclosure. Behavioral patterns may also be part of this, as, for example,
cannibalism and different migration patterns of the adults – as when the females
leave Delaware Bay prior to the males. For the time being, the observation that
abundance drives distribution can be considered a working hypothesis.
Another Consideration
Somewhere in the development of conservation plans certain dimensional
variables should be considered. For example, what number of individuals
11
comprises a viable population and how large (extensive) must the habitat complex
of shoreline, shallow and deep water be? Should this be determined on a case-bycase basis?
Genetic Definition of Populations and Conservation.
The first reports on the discreteness of populations were based on
morphology and morphometric analyses (Shuster 1955, Riska 1981). Studies on
the genetic component of the evolution of Limulus have been shaping our views on
that evolution beginning with Saunders et al. (1986); Figure 9. King et al. (2005)
have suggested that there are regional management units of Limulus based upon an
apparent substantial gene flow between each population and with its nearest
neighbors, mediated perhaps by a male-biased dispersal (i.e., the females do not
wander as much). Five regional management units have been defined: Florida-Gulf
of Mexico; Florida-Atlantic; Southeast U.S. (South Carolina and Georgia); MidAtlantic; and, the Gulf of Maine.
Genetic Diversity and Survival: A Speculation. According to Degener (2010), Tim
King, based on a genetic study (Faurby et al., 2010), explained that while the
overall population of Limulus has declined since the end of the last ice age, his
bigger concern was the “effective population” decrease. The definition of an
“effective population” was the number of horseshoe crabs necessary to create the
genetic diversity to successfully adapt to changing conditions. Co-author Matthias
Obst (Press release 4 Oct. 2010, University of Gothenburg) essentially repeated
that conclusion: “We noted a clear drop in the number of horseshoe crabs at the
end of the Ice Age, a period characterized by significant global warming” and
added “Our results also show that future climate change may further reduce the
already vastly diminished population. Normally, horseshoe crabs would have no
problem coping with climate change, but the ongoing destruction of their habitats
makes them much more sensitive.” If Delaware Bay is an example of what may
occur elsewhere, one spawning site may disappear (Figure 10) while another is
developing. In a similar setting but in the extensive tidal stream/marsh complex
behind Fortesque, New Jersey, there are several new spawning sites that may be
expanding (observed by Shuster when accompanying Mr. Michael Oates who was
navigating through the numerous tidal stream waterways to observe the habitat
situation in 2005).
12
Limulus Increased After Last Ice Age
Several ice ages ago, horseshoe crabs may have freely wandered between
the Gulf of Mexico and the Atlantic Ocean (Figure 11). This may have been the
cause of the genetic makeup of Limulus populations now on either side of the
Florida peninsula (Figure 9).
Today, Limulus ranges northward to Maine, with the largest population in
the Delaware Bay area. Because this bay did not exist during the last ice age or
until a few thousand years later, where were the horseshoe crabs? It seems logical
to assume that they moved northward from a southern site, perhaps as far south as
Georgia or Florida. Kreamer and Michels (2009) estimated that the spawning of
Limulus at Delaware Bay gradually increased over the last 3,000 0r 4,000 years
during the time that the bay was evolving (Figure 12). Delaware Bay now supports
a population of some 20 million adults (Smith et al. 2005). The size of this
population challenges the conclusion (Obst: University of Gothenburg press
release 4 Oct. 2010) that there was “…a clear drop in the number of horseshoe
crabs at the end of the Ice Age.”
Is Geologic Time A Factor? About 140 million years may be the geologic
timeline within which horseshoe crabs have diverged into the four extant species
(Figure 14). Limulus diverged from the Indo-Pacific species some 150 to 135
million years ago (mya), followed by a split in the genus Tachypleus between 110
and 20 mya which led to T. gigas and the branch that produced T. tridentatus and
Carcinoscorpius rotundicauda some 43 to 8 mya (Sugita 1988, Shishikura &
Nakamura 1988). If major ice ages occurred about every 100,000 years during the
past one million years (Schirber 2005), global warming probably peaked during the
intervals. The result of the interplay between genetic mutations and environmental
changes during these episodes of global temperature changes produced the four
species that have survived to today. That is the essential point -- four species of
horseshoe crabs or their antecedents have survived several global climate changes.
Wrap Up re Populations
Due to my early studies in New England waters, I believe that the studies
there may be being more pertinent to conservation issues in Asian than information
on the Delaware Bay situation. Unfortunately there are few papers scheduled on
13
the lesser populations in this workshop. However, those authors should be able to
support the case for understanding what goes on in the lesser populations. Advance
supporting information is readily available in Biology and Conservations of
Horseshoe crabs (2009), Chabot and Watson (2010) and in the situation on the
Yucatan Peninsula (Zaldivar-Rae et al. 2009) and microtidal lagoons (Ehlinger &
Tankersley 2009).
HABITATS
While habitats are not discrete, in the same sense that populations are, a
good case can be made that many if not most habitats are unique. Indeed, many of
the behaviors exhibited by Limulus can be attributed to the adjustment of the crabs
to certain local environmental parameters such as tidal amplitude, benthic currents,
prey, and quantity and quality of the habitat.
Broadly considered, geological processes are what determine the extent of
habitats favorable to horseshoe crabs. Although these processes are not the topic of
this discussion, a series of studies at Delaware Bay should be collated as a handy
reference (see papers coauthored by Jackson & Nordstrom 2003, 2009). Assessing
and mapping critical horseshoe crab spawning habitats have also been undertaken
(Lathrop et al. 2006).
Suffice it to state here that the familiar sequence of erosion – transport deposition and the encroachment of land into aquatic areas or inundation of land by
water undoubtedly would create new habitats as older ones vanish. The general
reaction is to deplore the loss of habitat and to try to stop it rather than anticipate
creation of new habitats by natural processes or abetted by human endeavor (Titus
2003).
Sekiguchi and Shuster (2009) and Shuster and Sekiguchi (2009) reviewed
the information on restrictions to global distribution and local environmental
conditions, respectively. The conditions restricting global distribution largely
define where horseshoe crabs have and can exist. Adaptation to local conditions
illustrates the ecological generalist capability of horseshoe crabs. These
environmental and biological conditions are among several factors that invoke a
14
number of questions including anthropological stresses and environmental
changes. Speculation on the newest stress is that it will occur due to global
warming (Faurby et al. 2010).
On a lesser scale, locally, horseshoe crab eggs are transported by waves and
swash, as demonstrated at Delaware Bay (Nordstrom et al. 2006). When wave
energy is low the probability increases that eggs will remain on the beach
compared with dispersal and burial, with fewer eggs on the surface, during high
waves when more eggs are transported in the swash.
Global Constraints on Distribution
There are four major constraints on the distribution of the extant species of
horseshoe crabs. Presumably these restrictions were also effective during the times
of the fossil species:
1. Horseshoe crabs are unable to cross an oceanic deep. The continental shelf is
the pathway for their migrations between bays and land masses.
2. Those shores that provide successful incubation for horseshoe crab eggs are
generally bathed by regular or irregular diurnal tides.
3. Horseshoe crabs, at least the three Asian species, tolerate tropical conditions
and are found north and south of the equator (Figure 2). However, all the
populations are restricted at northern latitudes (Figures 2 and 3).
4. The extent and condition of contiguous estuarine habitats are essential.
Beaches provide incubators for the eggs, shallow coastal waters are nursery
areas of juveniles), and the deeper waters, often including the continental
shelf where large juveniles and adults also feed and hibernate.
Local Environments
Spawning areas are certainly more susceptible, if not the most susceptible, to
harm. But not all beaches, mangroves, or intertidal sand flats are equal in value as
incubators. Therefore, it would seem that evaluation of sites as incubators should
be a prime concern rather counting the number of horseshoe crabs that spawn on
them. Just because Limulus spawns in greater numbers on certain beaches does not
necessarily identify those beaches as superior incubators. Should we aim to
15
evaluate of a beach in terms of a ratio such as: the number of females versus the
number of eggs that develop to maturity?
Each of us has probably witnessed natural and man-caused loss of habitat – a
loss that has been assumed to be harmful to horseshoe crabs, as in Figure 15. But it
may be that local, short-term changes, such as erosion of some beaches, might not
have long-term impacts because new beaches could be forming elsewhere.
Anyway, horseshoe crabs are hardy creatures – they are survivors. Over the
millions of years that the four extant species have lived, there must have been
circumstances where the hardiness and other characteristics of the crabs were
important factors in their reaction and survival. Somehow they kept pace in
relation to changes in their environment.
Will Horseshoe Crab Habitats Be Destroyed During Climate Warming? Yes
but probably not to any great extent, as it is most likely that geologic processes at
the edges of the pertinent continents will be similar to those that have occurred
before and new suitable habitats will be amenable as habitats for Limulidae.
Examples of once fauna-rich aquatic environments that included horseshoe crabs,
now at the center of North America (Mason Creek, Shabica and Hays 1997) and
Bavaria, Germany (the Solnhofen limestones, Barthel et al. 1990), illustrate the
disappearance of species and habitats. But similar habitats develop elsewhere and
some species did survive major extinctions, e.g., Paleolimulus avitus.
Limulus habitats have undergone observable changes during the past 50
years. During that time, sea level has been rising almost imperceptibly and most of
the natural changes to the spawning beaches, abetted by sea level rise, have been
due primaerily to strorm erosion. Over the next several thousand years, sea level
rise will be inundating coastal landscapes of different types, ranging from coastal
plains to piedmont (Figure 13). This recalls a saying by the “old timers” of
Delaware: “There is only one county at high tide;” the other two, more southerly
are as flat as a pancake. There may be times when large areas are no longer
habitable by horseshoe crab. However, there is no reason to assume that geologic
processes in the future will markedly differ from current ones, so new habitats
suitable for horseshoe crab spawning probably will be created. Under these
conditions, a warming climate does not appear to be a serious factor in the survival
of horseshoe crabs. The Asian species already exist in tropical climes. Warmer
16
waters to the north would seem to offer an extenstion of the present range of
Limulus into Canadian waters where outliers have been occasionally reported.
There is also the possibility that the relative confinement of horseshoe crabs
within a limited access/egress habitat might make them more prone to genetic
stasis or to an abrupt change in a local condition such as a food source (e.g., when
a major food source – blue mussels – is threatened by increasing commercial
dredge harvests). The fate of such populations, it seems to me, is less fortunate
than for those with a wide-open access to a larger habitat via a larger body of
water. But since all of this may play out over a geologic time scale, does it really
matter? Or should we concentrate on populations in the more favorable habitats?
Impact of Global Change: Speculation
Several news releases that followed the publication of Faurby et al. (2010)
have speculated that global warming is one of the causes of declines in populations
of Limulus polyphemus. By implication global warming should have impacted each
of the extant species during their length geologic history. Sea-level rise is certainly
associated with global warming and, if Delaware Bay is an example, beach quality
and quality have been demonstrably diminished, especially by erosion due to
storms (Figure 15). However, there are ample irregularities in the elevation of land
such that encroachment of rising seas would also be irregular. Further, the
tolerance and adaptability of horseshoe crabs to a wide range of temperature
(Mayer 1914, Fraenkel 1960, Reynolds and Casterlin 1979) suggest that they
should be sufficient to survive any water temperature fluctuations due to global
warming. It would seem that the speculation about losses of beaches and
temperature fluctuations in the hydroclimate is overdone when examined within
the context of geomorphic changes in the coastline and the hydroclimate during the
millions of years that the four extant species has existed, This probably was a
certainty for Limulus after the last great ice ace (13,000 – 11,000 years ago), at
least along the Atlantic coast of North America. The distribution of the Asian crabs
throughout the equatorial area suggests that warm temperatures are not all that
limiting. Apparently no studies have addressed the question of long-term exposure
to higher temperatures, especially in relation to ovulation and fecundity.
17
Also, prior interpretations, as expressed by Avise (2002), held that the extant
horseshoe crabs “…are exceptional in levels and patterns of genetic variability,
both within and across species. On other words, most of their genes give no
indication of “aberrant” evolutionary generic rates or processes compared with
other kinds of animals.” Avise also observed that the body plan that evolved so
long ago apparently worked so well that it was retained “Yet, internally, their
molecular genetic clocks have kept on ticking.” He also cautions “…don’t
underestimate these resilient beasts...” they “…have witnessed mountain ranges
rise from the sea and continents drift across the full face of the planet.”
Visualizing Environmental Variables
Independent variables such as water temperature and salinity can be
combined in a climatograph as an aid in the visualizing spatial and temporal
differences of these variables in the same or several other habitats. The graphic
method adopted from terrestrial studies by Hedgpeth (1957) has been used here to
compare the water environments (salinity and temperature) of Great Bay, New
Hampshire, and Delaware Bay (Figure 16). Variations between these two
environmental factors can be due to differences in the evaporation and
precipitation as well as to the relative amounts of freshwater and sea water in the
bays and to their temperatures. During the spring rains, flows from the rivers and
streams are generally high and the air temperature has a greater effect upon the
river water than the colder seawater; thus river flow may affect both temperature
and salinity. The opposite effect may occur during the autumn rains when the river
water. Influenced by air temperatures, has cooled more rapidly than the larger body
of coastal water.
EXPLOITATION versus UTILIZATION
The term “exploitation,” as contrasted to the overall objectives of
“conservation,” seems a too radical characterization of the uses of horseshoe crab
resources. A more representative term may be “utilization,” in the sense that
conservation is the wise use of resources.
Multiple uses of Limulus polyphemus are well-known (Berkson and Shuster
1999; Walls et al. 2002), including the extensive use of the species in scientific
18
research. Because each use has a different impact upon the crabs and their
ecological associates, it may be necessary to consider different conservation
measures in relation to different uses. The extremes are obvious – complete
protection for ecological reasons (e.g., to protect the crabs and dependent species
such as loggerhead turtles and migratory shorebirds) versus unlimited use. In
between uses involve those such as the number of crabs used and the relatively low
mortality of the crabs bled for LAL production versus their ultimate death as bait.
Managing for Conservation. The conservation objective, whether to preserve
or protect a local crab population, protect an endangered species, or manage the
species to benefit other species, etc., may require different methods but, in most
cases, overall habitat conditions, protection of spawning crabs, and impacts
(including harvesting) on a population will probably be the focal situations in need
of the greater consideration.
Controlled handling procedures of horseshoe crabs in the production of
Limulus amebocyte lysate (LAL) – harvest method, transport, bleeding, and return
to the sea – are necessary to reduce external stressors that produce mortalities (e.g.,
lengthy air exposure, elevated temperatures, etc.); Hurton and Berkson (2006).
After the introduction of bait bags using parts of horseshoe crabs by a Frank
“Thumper” Eicherly IV. a waterman whose home pot is Bowers, Delaware, Robert
Fisher, researcher in the Virginia Institute of Marine Science - VA Sea Grant
Program, determined that one-half a crab, male or female, versus the use of a
whole crab, showed a slight decrease in total catch of whelk but the decrease was
not statistically different (Fisher 2000). However, further decrease in bait portions,
to one-third or one-fourth a crab, sharply reduced the catch.
By far the greatest stimulus for the management and conservation of Limulus
arose in recent decades due to a perceived impact on migratory shorebirds by the
harvest of horseshoe crabs mainly to bait whelk traps in the mid-Atlantic bight
centered on Delaware Bay. All Atlantic states of the United States, Florida to
Maine, through the Atlantic States Marine Fisheries Commission (ASMFC 2010),
have participated in the management of the total horseshoe crab resource and of
local discrete populations since 1999; the Gulf Coast Marine Fisheries
Commission has been less involved to the present time. The ASMFC management
19
has involved spatial, temporal, and biologically-based restrictions on harvests and
the establishment of horseshoe crab reserves.
Management of horseshoe crabs in the Delaware Bay area, where the
emphasis is upon the availability of the crab eggs for migratory shore birds that
stop in the area to refuel on flights from the south on the way to northern Canada to
breed, directly involves the fisheries in the states of Delaware, New Jersey,
Maryland, and Virginia. Thus, in this case, the most sensible and direct way to
manage egg production is to protect the adult females. This can be accomplished
by banning harvest of all crabs or banning harvest of the females and limiting the
taking of adult males. In recent years New Jersey has banned all harvest while
Delaware has limited the harvest to 100,000 males per year and only after the
spawning season. Banning harvesting during the spawning season has two benefits:
lessening impacts on the breeding crabs and on the sensitive migratory shore birds.
The harvest of Limulus males is viable due to the special spawning behavior in
which the males tend to remain in the near-shore region and repeatedly come
ashore to mate throughout the season, often creating ratios of 3 to 5 males per
female on the beaches. This harvest of a portion of the skewed sex ratio favoring
Limulus males on the beaches would not be possible in the Indo-Pacific species
where single-pair mating (one female with her amplexed male) is the dominate
mating behavior. However, the essential management strategy, for either type of
mating behavior, is to protect the animals during their spawning years, particularly
the females (Figure 17), and their spawning sites.
Sometime during the development of conservation plans, consideration
should be given to the consideration of certain dimensional variables. For example:
what number of individuals comprise a viable population, and how large
(extensive) must the habitat complex of shoreline, shallow and deep water be? A
recent series of studies (e.g., Smith et al. 2009 and others with Smith) should be
collated as a compendium on methods of studying populations and the kinds of
results that can be obtained.
20
PUBLIC CONSENSUS
If the term “consensus” is being used to imply the opinion of the majority,
this probably can be reached on broad issues but narrow issues may have to be
resolved locally. Public consensus should be based, among other factors, upon
awareness of the value (i.e., the range of uses of horseshoe crabs) and the health of
the horseshoe crabs (Mattei 2008).
Scientists have not reached a consensus on whether the relative numbers of
young adults versus middle aged and old specimens can be used as an index on the
“health” of a population. The biggest obstacle to identifying the relative age is due
to the level of impact of the abrasive environment of the crabs. Because their
uneven distribution ensures differences in environments, their carapaces “age” and
collect epibionts differently. However, there is no mistaking a first-year adult. It
has a shiny carapace that is abundantly “feathered” on the adjacent margins of the
prosoma and opisthosoma, is virtually non-scarred (bear no black scratches or wear
areas), has no or few epibionts, and is more aggressive than older crabs (feisty,
“fighting” when handled); Figure 18.
Several books on Limulus written to appeal to a youthful audience and some
of these are quite suitable to introduce adult readers to horseshoe crabs. These
appear to have had wide dissemination. Similar books are probably also available
on the Asian species; if not, this could be one way of reaching a youthful audience.
Horseshoe crabs have long been revered in some Asian countries as national
icons. In comparison, only recently, 2002, has a state in the U.S.A., designated
Limulus polyphemus as the State marine animal.
CONCERNING CONCEPTS
I have used concepts occasionally to emphasize a point. Concepts and
definitions are the cornerstones of the framework on which I place information on
horseshoe crabs. Of these the “environmental generalist” concept is prime, the one
on which all information must assessed. The concept of “discrete populations” has
been repeatedly been supported by morphological and genetic data and is wellaccepted. Whereas, “spawning beach fidelity” is ephemeral rarely lasting more
21
than a season. If we accept seasonal “fidelity” as absolute, then it could be argued
that Delaware Bay supports several populations, which it probably does anyway,
during the spawning season. Another point of guidance – horseshoe crabs are not
fish. They are one of the oldest surviving lines of arthropods and should not be
hemmed in, exclusively, even by what we think of other evolutionary lines except,
perhaps, the trilobites. That horseshoe crabs are “wanderers” was discussed earlier.
And the crabs are significantly guided by benthic currents (Barthel 1974).
EPILOGUE
An Appendix is offered, not as a summary of this paper, but as one way in
which information might be organized to aid consideration of the conservation of
the Indo-Pacific species of horseshoe crabs. The anticipation is that this appendix
and the preceding the text will be circulated to all registered participants well in
advance of the workshop. Participants can determine if these two, text and
appendix, are pertinent to local and national problems concerning horseshoe crabs
and scope out their answers and questions so that, at the workshop, they can hit the
ground running -- utilizing what is pertinent about Limulus in the conservation of
the Indo-Pacific species. I except all participants will have a working knowledge of
the information in Tanacredi et al. (2009); from this book I would concentrate on
the recent studies by Smith et al. (2009), Jackson and Nordstrom (2009), and
Watson et al. (2009), the references that they cite, and my citations of “lesser”
populations in my paper.
A Last Minute Addition. Just as I was finishing up this paper I chanced upon a
Current Zoology website:
http://www.actazool.org/issuedetail.asp?volume=56&number=5&issue_id+503
Twelve papers in this volume, co-edited by Christopher C. Chabot and Winston H.
Watson III, addressed the theme of “Horseshoe crab behavior: Patterns and
processes.” The papers dealt mainly with horseshoe crabs in relatively small
estuaries where their migrations are somewhat limited. I was also favorably
impressed by the paper by a new-comer, Wan-Jean Lee, who used a camera “on a
22
clothes line” to study and photograph, unmolestingly, intensive scavenging on an
intertidal flat of Great Bay, New Hampshire, by adult crabs.
This special issue was dedicated in a personal, heartfelt tribute to the late Dr.
Robert B. Barlow who was an inspiration to many. Much of his enthusiasm
showed in the intensity in which he probed for an understanding of the animal and
from his deep interest in and concern for Limulus.
What To Do Now?
In getting ready for the workshop, the preceding discussion and questions -besides those that may be or are more pertinent to the Asian species – should be
answered or identified. This bit of homework was the reason I requested that my
written article be circulated prior to the workshop. Meanwhile, Michael “Mike” F.
Oates, a documentarian, is making a video with me that will be presented at the
workshop.
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Towle D.W. and Henry R.P. (2003) Coping with environmental changes: physiological
challenges. In: Shuster C.N. Jr., Barlow R.B. and Brockmann H.J. (eds.) The American
Horseshoe crab. Cambridge, Harvard University Press: 224-244.
U.S. Fish & Wildlife Service. (2010) Draft Environmental Assessment for Dune Work at
Prime Hook National Wildlife Refuge: 42pp.
Walls E.A., Berkson J., Smith S.A. (2002) The horseshoe crab, Limulus polyphemus:
200 million years of existence, 100 years of study. Reviews on Fisheries Science10(1): 39-73.
Watson W.H. III, Schaller S.Y., Chabot C.C. (2009) The relationship between small- and
large-scale movements of horseshoe crabs in the Great Bay estuary and Limulus behavior in the
laboratory. In: Tanacredi J.T., Botton M.L. Smith D.R. (eds). Biology and Conservation of
Horseshoe Crabs. New York, NY; Springer: 131-147.
Widener J.C., Barlow R.B. (1999) Decline of a horseshoe crab population on Cape Cod.
Biol. Bull. 197: 300-301.
Zaldivar-Rae J., Sapien-Silva R.E., Rosales-Raya M., and Brockmann H.J. (2009)
American horseshoe crabs, Limulus polyphemus in Mexico: open possibilities. In. Tanacredi, J.T.
(eds.), Biology and Conservation of Horseshoe Crabs: Springer Science: 97-113.
29
Pre-meeting background material for the 2011 International Workshop
on the Science and Conservation of Asian Horseshoe Crabs
(13-16 June 2011, Hong Kong, China]
Limulus polyphemus as a Model for the Conservation of
Horseshoe Crabs
Carl N. Shuster Jr.1
Virginia Institute of Marine Science, The College of William and Mary
March2011
FIGURES
1
Home Address: 3733 North 25th Street, Arlington, Virginia, USA 22207-5011
[email protected]
30
2
Fig. 1. Maximum number of Limulus counted in three 10-m2 quadrats during the 1984, 1990,
and 1900 spawning seasons in Mashnee Dike (from Widener & Barlow 1999). The height of
each bar gives the maximum number of males and females in a single half-hourly survey each
day. The open and closed circles indicate the times of the new and full moons. Surveys were not
done on 28 and 30 May 1990 because of bad weather. Data were interpolated for these two days
to calculate spawning activity. Periods of observations were 14 May to 8 July 1984; 19 May to
22 June 1990, and 24 May to 16 June 1999.
31
Fig. 2. The continental shelf (cs) is the
avenue of migration/contact between the
three Asian species of horseshoe crabs
(based on Sekiguchi 1988). One species or
more was found near most of the research
sites (small arrows); yellow circles =
Tachypleus gigas; green circles =
Carcinoscorpius rotundicauda; and, red
circles = Tachypleus tridentatus. JA =
Japan; CH = China; IN = India, and A =
Australia. The south-north axis of the
distribution of Tt is similar to Limulus
polyphemus, the American horseshoe crab.
Could this be a basis for more ecological,
similarities between Tt and Lp than those
between Lp and Tg or Cr?
Fig. 3. Distribution of Limulus
polyphemus; coastal areas in Yucatan (YN)
and the United States, between the two sets
of arrows, support Lp populations. Sixteen
of these were examined by Shuster (1955,
1979): two from Florida in the Gulf of
Mexico and the rest from South Carolina
to Maine on the Atlantic coast, USA.
32
Fig. 4. Wherever the distribution of horseshoe crabs is examined in detail, the crabs seem to
be everywhere. The red dots indicate recapture of sonar-tagged crabs on the shores of
Connecticut, Staten Island, Long Island, and Narragansett Bay, Rhode Island (from Mattei
2008).
33
Fig. 5. Estimated distribution of three populations of horseshoe crabs in the Middle Atlantic
Bight: Upper Chesapeake Bay; Lower Chesapeake Bay and the continental shelf off the bay
entrance; and Delaware Bay and the adjoining shelf area (from Shuster 1985). The fewer
numbers throughout Chesapeake Bay is largely due to the lack of extensive sandy beaches (tidal
marshes form much of the undeveloped shoreline). The upper Chesapeake Bay has supported a
relatively small population, at least since 1953 (Shuster 1955, 1979). Horseshoe crab spawning
sites exist within most embayments along the Atlantic shore of the DelMarVa Peninsula; the
largest is at Toms Cove, Chincoteague, Virginia.
34
Fig. 6. Slaughter Beach, Delaware in 1999 (photograph by Shuster). The crabs are bunched up
near the foot of the beach during an ebbing tide. The stranded crabs mark the upper limits of
earlier spawning. This was during the time when the media were beginning to report that the
crabs were headed for extinction along with migratory shorebirds that fed on Limulus eggs,
especially the Red Knot. Management of Limulus harvests at the state level began in the 1990s in
the Mid-Atlantic States (Shuster 2003).
35
Fig. 7. A moderately heavy Delaware Bay spawning at Mispillion Harbor at 1548 hrs on 19
May 2006 (courtesy of Greg Breese, Delaware Bay Estuary Project, U.S. Fish & Wildlife
Service). Nest depressions in the upper right corner of the scene and the positions of several of
the spawners, especially in the lower right corner, suggest this was during an ebbing of the tide.
This illustration was used in several news items during October 2010 that warned of a negative
impact on horseshoe crabs by global warming. Such a large number of spawners, as seen in this
sample, does not fit that speculation. Especially since such numbers were also seen on other
beaches. According to observers of spawning in the early 1990s, the author included, the number
of spawners then and in this scene were comparable -- it was during the in-between-years that
the horseshoe crab population was possibly only 40% to 60% as great. Whether conservation
measures such as male-only harvests, an embargo on the taking of adult female crabs, and the
establishment of a large sanctuary off Delaware Bay (Figure XX) were mainly responsible for
the resurgence of the population can be argued, but it certainly is increasing!
36
Fig. 8. Limulus spawning on Pickering Beach, Delaware on Delaware Bay, on the new moon
in May 2010. This was a heavy spawning with the band of spawners extending at least four
meters from the uppermost spawners on the beach to the disappearance of the band into the bay;
these numbers also extended at least 100 m along this beach. There is only one old male in this
view (the one with the blackened carapace on the extreme right) and a mid-aged male (half-off
the bottom of the picture). The rest of the crabs, mostly males, as the females are beneath them,
are first-year spawners (the ones with the lighter-colored, shiny shells) and second- and thirdyear spawners (slightly older ones with black scratch marks and discolored rims); courtesy of
Glenn Gauvry, Ecological Research.
37
Fig. 9. The 15 populations examined by Saunders et al. (1986): Florida – Panama City,
Panacea, Tampa Bay, Ft. Myers, Islamorada Key, Stuart, Cape Canaveral; Georgia – three sires
including Sapelo Island; North Carolina -- Beaufort; New York – Long Island; Massachusetts –
two sites including Pleasant Bay; and, New Hampshire – Dover Point. There were two major
mtDNA clonal assemblages: horseshoe crabs in Florida waters versus those from Georgia to
New Hampshire.
38
Fig. 10. This panorama was taken in the summer of 1979 near the old exit of the Broadkill
River into Delaware Bay (formerly at the south end of Broadkill Beach, near the left foreground
in this view but completely closed by 1979). Small numbers of Limulus were still spawning in
this area in the 1970’s. The river entrance closured finally after several decades of openings and
temporary closures associated with storms.
39
Fig. 11. One million to 700,000 years ago (mya) a balmy tropical environment, the
Aftonia Interglacial Stage, existed in a southern Florida (Petuch 1992). A large inland
marine sea, the Okeechobean Sea, was fringed by coral reefs. Mollusks, chiefly
gastropods, were abundant; oysters and scallops were also abundant. Two major channels
(straits) connected the sea with oceanic water to the west and east; strong currents passed
through these channels during tidal changes. No fossil horseshoe crabs have been found
there but they almost certainly inhabited this sea.
40
Fig. 12. Three stages in the formation of present-day Delaware Bay (from Chrzastowski
1986): left to right – 15 to 13 thousand years ago when the entrance to the ultimate Delaware
River was at the edge of the continental shelf; 11 thousand years ago when the ancestral bay was
flanked by capes; 8 thousand years ago when mean sea level (MSL) was still -20 m below that of
today. The present land area is shown by diagonal lines; black areas along the margins of the
land indicate previous tide marshes. These modifications of the coast illustrate that the shoreline
is ever-changing.
41
Fig 13. Changes in the mid-Atlantic coastline between ice ages, showing comparisons of the
present coastline to estimated changes from the entrance to Chesapeake Bay to Long Island
Sound, 12,000 years before the present to 75,000 years in the future (courtesy of John C. Kraft,
University of Delaware, 1999).
42
Fig. 14. Two estimates of the geological ages of the extant species of Limulidae.
Phylogenetic relationships based on:
(Top) the results of micro-complement fixation measurements; it is assumed that American and
Asian species diverged 135 million years ago (Shishikura et al. 1982, from Sugita 1988).
(Bottom) the “Mutation distance” among peptide “C” of the four species (from Shishikura and
Nakamura 1988).
[NOTE: Mesolimulus walchi lived during the Upper Jurassic some 150 million years ago
(Barthel et al. 1990; Eldredge 1991); specimens, mainly juveniles are preserved in Solnhofen
limestone, Bavaria, Germany. Also see the distribution pattern of extinct and extant species
during the Mesozoic and Cenozoic by Sekiguchi (1988, pages 410-414).
43
Fig. 15. A “blowout” of a sandy beach exposed an underlying peat bank north of Fowlers
Beach in an uninhabited area, Delaware. This type of erosion has occurred several times, turning
freshwater impoundments behind the beach that had been created as waterfowl refuges, into
saline environments. It is also evidence of the invasion of the shoreline by marshland that was
subsequently covered with sediments that were being transported seaward (photo by John
Chirea, pilot, of Prime Hook, Delaware in 2007; courtesy of Glenn Gauvry, ERDG). A 2010
U.S. Fish & Wildlife Draft Environmental Assessment describes a program to restore the dune
system to minimize impacts of coastal flooding.
44
Fig. 16. A salinity/temperature
hydroclimagraph for Great Bay, New
Hampshire (based Watson et al. 2009)
compared with one on the Bar Grounds (oyster
beds) in a mid-bay section of Delaware Bay
(Shuster 1960).
This graph shows the marked contrast
between the affect of freshwater inflow in the
estuary (Great Bay) and on the coast in April
and May despite the dominance of seawater the
rest of the year.
Delaware Bay: A = a graph of average
monthly conditions for a 5-year period
(1955-1959); B and C are contrasting years -1955 (a dry summer). -- 1956 (when rainfall
from hurricane “Diane” caused a rapid
increase of 3.5 times the river flow within 2
days).
45
Fig. 17. A horseshoe crab reserve was established off the entrance to Delaware Bay by
NOAA, National Marine Fisheries Service in 2001 to protect the heart of the Delaware Bay
spawning population, the female Limulus component; the large juveniles that molt in late fall and
could potentially spawning the following year and the young adults (2-4-year-olds).
46
Fig. 18. A young adult female Limulus presumed to be in her second year of adulthood. The
carapace of this female is slightly dulled, in comparison to the lustrous appearance of the
carapace of a newly molted animal, but the carapace still clearly shows the fine mosaic detail of
the ridges (clear) and valleys (black lines). The angle at which light shone on the opisthosoma
highlighted the swales between the fused segments of the Carboniferous ancestor. Also there is a
small mating scar on the opisthosoma.
47
APPENDIX -- A tabular summary of potential conservation-pertinent information about horseshoe crabs,
using Limulus polyphemus as an example.
POPULATIONS
Questions
1. Do mated pairs or single
adults of the Asian species
always return to their natal
beaches to spawn in
subsequent years?
Any differences in
harvesting?
2. Are the wanderings of the
sexes significantly
different?
Answers & Questions
A one year ephemeral
fidelity of Lp is not
commonly repeated, year
to year. Usually local
circumstances mediate this
- as along-shore currents,
& abundance of spawners.
Lp sexes appear to move
independently; females go
seaward before the males.
Problems re Asian spp.?
Single pair mating could
restrict management; e.g.,
if only males are collected
(as with Lp), mated pairs
would be unconnected.
Do Asian spp. Have similar Conduct research if it is
behavioral patterns? Does
deemed essential to
it really matter?
under-stand crab
behavior.
Will the multi-nation
Conduct research to rank
distribution require special the level of criticality for
cooperation and
each population?
coordination?
3. How many populations of Not known for Lp, but
a species are necessary for
after the last ice age
its survival?
probably at least double
the number of populations
occurred.
4. How long does it take a
Lp females probably could Could stranding be less;
female to complete an
at an average of four high e.g., Tt seems to mainly
annual spawning? Is risk of tides. For Lp risk of
spawn underwater?
stranding increased?
stranding is 10%.
48
Asian Actions?
Inhibit the taking of
mated pairs; harvest only
non-amplexed females;
designate some spawning
beaches sanctuaries.
Essential to
understanding population
dynamics? If so, conduct
research.
5. Is fecundity a factor? All
extant spp. egg numbers &
sizes differ.
6. What number of crabs is
needed to maintain a viable
population?
7. What is an “effective
population?” What can be
done to help an effective
population evolve?
8. Is it necessary to consider
the concept of effective
populations as important in
their conservation?
9. Even if an effective
population could be a
conservation measure,
would a healthy population
be a satisfactory
intermediate goal?
10. If the genetic change
posited by Faurby et al.
(2010) occurred within
some 12,000 years after the
last ice age, what occurred
during several previous ice
ages?
Lp has the smallest–sized
eggs and produces many
more that the Asian spp.
Recent genetic studies say
populations may be static
or in a bottleneck status
regardless of numbers.
Considering the survival
of the 4 extant species for
millions of years?
Fewer eggs, larger-sized
eggs a survival advantage
for the early instars?
Most populations have not
been studied.
Considering the survival
of the 4 extant species for
millions of years?
Possibly too esoteric to
reasonably consider.
Consider anyway?
It would certainly appear
to be. The major problem
is lack of a reliable ID for
“healthy.”
Examine whether this
could be used on some
populations to advance
their conservation.
Develop clear guidelines
if this concept has merit
and is applicable.
It probably is not just the
last ice age but other local
and global factors.
It posits and interesting if
speculative problem.
A “back burner” issue?
49
?
Conduct research if
deemed important.
Determine genetic status
of, at least, populations in
consideration for
conservation.
?
11. Is a population the most
manageable unit of a
species for conservation?
12. To what extent is it
desirable or even possible to
identify all populations?
13. Should all populations
be conserved, a majority, or
selected ones?
14. Because the separate
evolution of the extant
species began some 135
million years ago, is it
possible that a period of
time less than 12,000 years
would produce an effective
population?
Yes.
Probably the most practical Use.
approach.
Certainly those
populations most at risk
can be identified.
Depending upon the
circumstances, one of the
options should be viable.
The time line for species
appears to be millions of
years; see Figure 14 in
text.
The time line for an
“effective” population
would be less?
Identify, also, the reasons
Proceed as necessary.
for the risks and what can
be done to reduce them.
How discrete are
Consider?
populations of the Asian
spp?
What occurs within
Consider?
millions of years is more of
academic interest than of
practical use. What
actuates an “effective”
population? Would this
concept be helpful in
conservation
considerations?
50
HABITAT
Questions
Answers & Questions
Problems re Asian spp.?
1. Are global constraints of This appears to be an
Sea level rise, in geologic
any importance?
academic question, not one time, would probably
of every-day importance.
change access routes
between populations.
2. Do speculations on the
impact of global warming
have any place in
considerations about
conservation?
3. Should habitat
restoration/preservation be
a part of horseshoe crab
conservation?
Certainly sea level rise will
occur. In long-term
considerations it may not
matter. Locally and in the
“now” situations it would.
It has been successful in
the USA in conjunction
with beach fill to protect
buildings and dunes.
Patching up local shortterm impacts is done
mostly to protect human
activities and projects.
Asian Actions?
Determine if sea level rise
will markedly affect
horseshoe crab
conservation planning. If
so, how and how soon?
Consider combining
horseshoe crab and human
projects in conservation
planning.
This is probably a case-by- Consider.
case kind of decision.
UTILIZATION
Questions
1. Should conservation
measures be developed for
each use of HSCs?
Answers
Probably not. A lot would
depend upon the significance of the use and how
much it would advance the
conservation effort.
Problems re Asian spp.?
HSCs used in USA (for
fertilizers, bait, LAL,
watching birds feed on
HSC eggs, feed live stock
& poultry, etc.); human
food in some countries.
51
Asian Actions?
As required.
PUBLIC CONCENSUS
Questions
1. Will public concerns
and awareness be
surveyed to arrive at a
public consensus?
2. If so, what topics will
be surveyed?
3. Will it include what
steps to take to achieve
conservation of HSCs?
Answers & Questions
Is public awareness a
problem? What would be
involved? Spot-checking or
a census?
Uses of HSCs? Any other
values?
And steps to include
politicians and government
fisheries officials?
4. Will special workshops
or classes be held to
educate teachers and
pupils?
This has been successful in
several states in the USA.
Problems re Asian spp.?
Would any survey be of
any value? Or would it be
better to immediately
proceed with education?
Results needed at local,
state, or national level?
Will meetings occur among
local officials (government,
fisheries, and education)
with visiting scientists?
In 1996 I spoke about Lp to
a public school science
class in Japan; education
seems to be a universal
goal.
52
Asian Actions?
Take appropriate action.
Publish results in some
form?
Involve key people at all
levels and in all kinds of
activities.
As necessary.