A M . ZOOLOGIST, 9:895-901 (1969).
The Gastropod, Siphonaria pectinata: a Factor in Destruction
of Beach Rock
ALAN K. CRAIG, SHELDON DOBKIN, ROBERT B. GRIMM, AND J. BLAINE DAVIDSON
Departments of Geography, Biological Sciences, and Ocean Engineering,
Florida Atlantic University, Boca Raton, Florida 33432
SYNOPSIS. The marine gastropod, Siphonaria pectinata, has an active part in the
complex formation of secondary solution features characteristic of outcroppings of
beach rock in south Florida. Analyses were made of the distribution, anatomy, and
feeding activity of this snail. The results show that these molluscs are concentrated in
areas of maximum algal growth. While grazing they rasp substrata presoftened by
algae so that new rock surfaces are continually exposed to biochemical erosion.
Exposures of calcareous beach rock are
found along the eastern coast of the
United States from a point near Palm
Beach, Florida, southward to Key West.
This low-lying shoreline is largely composed of sand and other poorly consolidated sediments. Tabular masses of beach
rock cropping out in the swash zone often
represent the only well indurated substratum in the beach environment. This paper
contains data we have collected relating
to organic destruction of this carbonate
material.
PREVIOUS WORK
Interest in the origin and distribution of
beach rock has been evident in the work
of many geologists and geographers concerned with coastal morphology. Important contributions include the descriptions
by Branner (1904), Field (1919), and
Daly (1924). After the pan-tropical distribution of this cemented beach sand was
firmly established by these early reports,
subsequent investigators concentrated on
solving the complex problem of genesis of
beach rock. Steers (1940), Ginsburg
(1953), and Russell (1962) published theoretical explanations of its origin, but only
McLean (1964) has specialized in research
on factors that contribute to the destruction
of beach rock.
This study was supported in part by the BrownHazen Fund of the Research Corporation, New
York, N. Y. Line drawings were prepared by A.
Watkins.
The rapidity with which lithification
can occur is well documented by Fairbridge (1948), Kuenen (1950), Doran
(1956), and Russell (1955, 1959). Actual cementation of beach sand into large tabular
masses having a slight seaward inclination
involves the deposition of acicular aragonitic crystals oriented perpendicularly to
the grain surface. This secondary crystallization extends into the normal void spaces
of beach sand so that lithification through
loss of porosity takes place. The end result
is a dense, well indurated, calcarenite outcrop which grades imperceptibly into loose
sand at depth and laterally. Lithification
is enhanced by continued exposure of an
outcrop to the atmosphere. This hardening
process involving evaporation has not
been satisfactorily explained but is common to a variety of both organic and inorganic carbonate substrates.
Destruction of beach rock by organic
agents is still poorly understood and has
traditionally been neglected as being relatively inconsequential when compared to
mechanical abrasion as the primary agent
of erosion. Our studies further confirm this
ranking but also show that certain organic
erosive effects have been overlooked.
ZOOGEOGRAPHY OF TEST SITES
Three widely separated calcarenite outcrops were selected in 1967 for study. Site
"A" is situated in a protected but currentswept location provided by an artificial
tidal inlet excavated in 1924 to connect the
895
896
ALAN K. CRAIG, SHELDON DOBKIN, ROBERT B. GRIMM, AND J. BLAINE DAVIDSON
FIG. 1. Beach rock at Site "B".
Intracoastal Waterway with the Atlantic
Ocean. This exposure is less than 1 meter
above mean sea level, seldom subjected to
spray or wave attack, and has strongly developed secondary solution features in the
recently hardened substrate.
Site "B" (Fig. 1) may be the shoreline
feature responsible for the early Spanish
place name of Boca Raton, or Rat's
Mouth, which in this case is a rather graphic description of the series of ragged
beach-rock outcrops found along this
coast. Site "B" is geologically atypical. It
includes an uninterrupted vertical sequence that begins in submerged beach
rock having well developed reef-like spur
and groove structures, and then continues
intertidally. Above mean sea level it forms
a conspicuous outcrop by grading into a
massive fossil dune deposit of highly indurated, cross-bedded, eolianite 4 m in thickness.
Site "C" is low, being fully exposed to
strong wave attack and abrasive effects.
Since the site is completely covered by
spring tides, a distinct spray-zone habitat
does not occur. A large scale (1:120) contour map (Fig. 2) of 3rd order accuracy
was prepared as a base for detailed zoogeographic studies of gastropods and associated neritic fauna. Preparation of this
map, which extends more than 2 m below
sea level, was facilitated by a combination
of conventional surveying techniques supplemented by diving equipment. Spot elevations with respect to an arbitrary sealevel datum (based on the upper limit of
the barnacle, Tetraclita squamosa) were
established by transit and a 5-m rod from a
single, centrally located, temporary bench
mark. After completing the subaerial survey, we carried the rod progressively farther out to sea. Positioning for readings in
the strong currents of the surge zone
was accomplished by a heavily weighted,
SCUBA-equipped, rod man. He was aided
by an assistant in snorkel equipment who
steadied the rod and transmitted signals
from the instrument man at the completion of each reading. Approximately
100 spot elevations were recorded in 3.5
hours.
There are prominent spur and groove
structures present in Site "C" leading to
the base of the submerged scarp where
through wave turbulence abrasive debris
has ground out several conspicuous hollows (Fig. 2). Similar relict features were
found on the uppermost surface of Site
"B" where they constitute evidence of
eustatic change in Pleistocene sea levels.
Gross mechanical erosion of this degree
is conspicuous. Less obvious are the slow
attritional effects of organisms ranging in
size from bacteria to the chiton, Acnnthopleura (Fig. 3). The latter locally attains a maximum length of 8 cm. While
chitons are demonstrable rock grazers, they
are few compared to the patelliform pulmonate gastropod, Siphonaria pectinatn
Linne (Fig. 4). This snail is numerous
throughout the intertidal portion of the
test sites, where it shares the habitats with
the common gastropods, Littorina zic-zac
and Echininus nodulosus. Less numerous
molluscs include Nerita peloronta, Nerita
versicolor, and the pelecypod, Liobcrus
castaneus. A heavy encrustation of the barnacle, Tetraclita squamosa, occurs where
the substratum has not been subjected to
seasonal or transitory sand burial.
DESTRUCTION OF BEACH ROCK
897
JAP ROCK SITE
FIG. 2. Distribution of Siphonaria pectiiiala at Site "C"
RADULAR MORPHOLOGY OF
Siphonaria
pec-
linata
Of the various gastropods present at the
test sites, the false limpet, Siphonaria pectinata, was found in greatest abundance.
Consequently, we emphasized in our
studies those aspects of its distribution,
feeding activities, and ecology relevant to
destruction of beach rock. Members of the
genus Siphonaria are patelliform basommatophorans, considered by Hyman (1967)
to be the most primitive of pulmonate
gastropods. As a group they appear to
have abandoned a terrestrial habitat, and
have invaded the intertidal zone throughout tropical and subtropical waters over
much of the world. Their patelliform
shape can be considered an adaptation to
the habitat.
Two species of Siplionaria are found in
South Florida; 5. pectinata occurs from
northeastern Florida southward to the Upper Keys, while S. alternata is reported in
the Lower Keys. According to Voss
(1959), S. pectinata
has an amphiAmerican as well as amphi-Atlantic dis-
FIG. 3. Acanlhopleura grazing on red algae.
898
ALAN K. CRAIG, SHELDON DOBKIN, ROBERT B. GRIMM, AND J. BLAINE DAVIDSON
>•
FIG. 4. Siphonaria pectinata clustered on beach
rock.
tribution. The ability of this snail to attach itself to the bottoms of wooden ships
would explain its wide distribution, but a
rigorous taxonomic study may disclose
varietal differences in the widely separated
populations.
Voss (1959) found Siphonaria pectinata
living in greatest concentration on rock
surfaces covered only by a film of microscopic algae. They are believed to graze on
macroscopic algae as well. Our observations confirm these findings. Our investigation included study of the feeding apparatus in order to determine if the snail can
destroy substrata by mechanical means.
Kohler (1893) described the anatomy of
Siphonaria pectinata but stated that his
illustrations of the radula were "not instructive," an opinion with which the
present authors concur. We removed radulae from several specimens, examined them
under a stereomicroscope, and drew them
with the aid of a camera lucida (Fig. 5). A
radula was also illustrated in position in
the buccal cavity (Fig. 6).
In Siphonaria pectinata the radula is a
toothed, ribbon-like device lying over the
dorsomedian surface of the odontophore.
In general, these relationships are similar
to those described by Fretter and Graham
(1962) for prosobranch molluscs.
The radula of Siphonaria pectinata is
trowel-like in form and bears a scoopshaped depression toward its posterior end
(Figs. 5, 6). Size of the radula and quantity of teeth are recognized as functions of
age. In the adult specimen illustrated,
there were approximately 100 rows of
teeth, the number of teeth per row varying
between ca. 50 and 100. Many of the teeth
were unicuspicl while others were bicuspid (Fig. 7). All were arranged in transverse rows consisting of a median tooth
flanked by lateral teeth that decreased in
size. It is not possible to distinguish between lateral and marginal teeth in S. pectinata. As Hyman (1967) has indicated,
the arrangement and number of teeth on
pulmonate radulae, unlike those of the
prosobranch molluscs, are of no taxonomic
value except that details may occasionally
facilitate identification of species.
Our studies indicate that Siphonaria
pectinata possesses a radula capable of
contributing to mechanical erosion of carbonate substratum, particularly where presoftening by ancillary organisms and chemical agents takes place.
FIG. 5. Detailed drawing of excised radula of Siphonaria pectinata. x25
DESTRUCTION OF BEACH ROCK
FIG. 6. Radula of Siphonaria peclinala in buccal
cavity. x25
ROLE OF ALGAE
899
ten observed grazing among or near these
red algae, it is probably significant that
Siphonaria seemed to avoid this plant.
In addition to these macroscopic algae,
there was an important community of microscopic algae. They produced the faint
blue-green or gray-green film present over
much of the rock within the splash zone.
Above this zone on the Butt's Cave outcrop, the gray-green coloration graded imperceptibly from dark gray to a black algal
coating that is characteristic of beach rock
and eolianite in tropical environments.
Since Siphonaria pectinata feeds on surfaces heavily populated by the blue-greens,
we directed our attention to this microhabitat as a model for study of biologic destruction of the substratum by these snails.
Small samples of beach rock were collected
and broken surfaces were examined. The
blue-green coloration penetrated 2 mm
into the rock matrix. This surficial layer
contrasts sharply with the pale creamcolored interior of the unweathered beach
rock. Fragmented samples of the algal layer were examined microscopically and
several blue-green algae were observed,
among which the most prominent was a
tufted form with short, abruptly tapering,
curved filaments having basal and inter-
A separate study of the algal community
at Butt's Cave (Site "B") was made along
a transect through the greatest variety of
habitats. In this relatively exposed environment, erosion by sand abrasion is predominant, so that most of the outcrop appeared devoid of macroscopic algae. However, in areas protected from scour, numerous colonies were located which have been
identified as genera of Cladophora (2 or
more), Enteromorpha, Monostroma, Chaetomorpha, Giffordia, Sphacelaria, and Polysiphonia. Two species of blue-green algae
(Lyngbya}) were observed among the
filamentous green algae and several species
of diatoms were occasionally present, particularly as epiphytes on Giffordia. A
short, cartilaginous red alga was relatively
common at this site, ranging from a position
low in the intertidal zone where it occurred
on exposed gently sloping surfaces, to the
undersides of overhanging rock, or on
steeply sloping faces in the splash zone.
While the chiton, Acanthopleura, was of- n e . 7. Teeth of Siphonaria peclinala. x200
900
ALAN K. CRAIC, SHELDON DOHKIN, ROBKRT B. GRIMM., AND J. BLAINE DAVIDSON
calary heterocysts. This was determined to
be the genus Calothrix, but the species was
not identified.
This alga has special significance to our
study in view of the several reports cited in
Desikachary (1959) in which similar algae have been described as capable of
perforating calcareous substrates. Taylor
(1960) and Desikachary (1959) were used
in identifying genera present at our test
sites. In addition to Calothrix, the bluegreen genera, Microcoleus and Oscillaloria
were found within the rock. After treating
the rock fragments with acid, additional
filamentous algae were observed but it was
not possible to identify them.
ACIDITY OF TIDAL POOLS
Erosion of beach rock typically gives rise
to numerous tidal pools that appear to be
the result of solution activity. Since seawater does not ordinarily dissolve this carbonate material, we conducted studies to
determine the changes in pH attributable
to the presence of organic agents.
A Beckman meter (model 180) was used
to obtain a series of pH measurements in
small isolated pools of about 600-800 ml
capacity. Sites were chosen where conditions indicated that feeding and metabolic
activity of the snails had been in progress
for at least several hours. The pH of seawater in pools having substantial algal
growths but not occupied by gastropods
varied between 6.85 and 6.82. Where Siphonnria were present, readings were consistently around 6.45. In one small pool of
about 400 ml capacity where 4 Siphonaria
were active, the pH was observed to fall
from 6.82 to 6.22 during a period of 100
min. In this same pool we observed the rise
of minute gas bubbles from the rock surface to the top of the sea water where they
accumulated at the still surface as a conspicuous frothy layer. This gas was not
identified and was not commonly observed
in these tidal pools.
We were unable to describe what changes
in pH, if any, were generated by the
snails, although empirical evidence sug-
TAHLE ]. Percentage of insoluble residue present
in representative samples of sand and beach rock
at Boca Raton, Florida, after prolonged treatment
with excess 5% SCI.
% of Insoluble
Eesidue
Habitat
Dune sanrl
Bench sand (swash zone)
Beach sand (winter berm)
Beach rock (unweathered)
Beach rock (algal layer)
27.8
30.1
31.8
33.5
34.2
gests they are capable of lowering the pH
at least during long periods of inactivity.
Distinctive "halos" of bleached substratum, referred to by Voss (1959) as "home
scars," surrounded by a narrow rim of dark
green stain, are often uncovered when
adult specimens are removed from their
resting places.
PETROGRAPHY
Samples of substratum were collected
from a number of locations at each site for
studies of solubility and gross petrography.
Unconsolidated sands from the present
beach and inland stabilized dunes were also
sampled for comparison with beach rock.
Table 1 shows the percentage of total sample weight remaining after prolonged treatment with dilute HC1 acid.
These data indicate an unexpectedly
high percentage of insoluble components
mixed with clastic carbonate material in
sand and beach rock. The significantly
lower figure of 27.82% for the older
weathered dune sand as compared to
30.12% for contemporary beach sand suggests there may be differences in sources of
these materials. If both sands were derived from the same locality, we would
expect a greater amount of insoluble residue to have accumulated in the older
leached dune sands.
Of greater significance is the relatively
high percentage of insoluble residue in the
surficial algal layer. Microscopic examinations conducted in the field indicate that a
very thin (0.5 mm) layer of substratum is
often reduced to a soil-like consistency in
those areas of the splash zone where blue-
901
DESTRUCTION OF BEACH ROCK
green algae are particularly abundant. In
this micro-habitat individual grains of
quartz are abundant in the pasty matrix o£
partially decomposed cementing material.
Identical particles of comminuted silica
are also numerous in the fecal pellets of
Siphonaria pectinata, proving that ingestion of the substratum occurs. We interpret
this slight but distinct difference between
insoluble residue found in unweathered
beach rock and the algal layer of beach
rock to represent organically induced solution of carbonate constituents in the substrate.
CONCLUSIONS
By coordinating a series of multidisciplinary field studies focused on the problem of erosion of beach rock, we demonstrated the role of Siphonaria pectinata as
a contributory agent in the creation of solution features that are characteristic secondary surface features of beach-rock outcrops. The radula of these animals appears
to be an efficient mechanism for removing
rock, particularly where there has been
preconditioning of the substratum by the
metabolic activities of rock-penetrating,
blue-green algae. These boring algae may
be essential for softening the aragonitic cementing material of the rock. Grazing by
gastropods rasps off the thin layer of substratum penetrated by the algae. Our
studies demonstrated a thin surficial layer
of rock of soil-like consistency where algal
growth is at a maximum. Solubility analysis of the layer discloses an abnormally
high percentage of insoluble residue.
These residues result from dissolution of
interstitial cement by organically derived
acid during periods of low tide. Our
studies suggest progressive development of
acidic conditions in still tidal pools oE limited volume where algae are consumed by
actively feeding gastropods.
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