JOURNAL OF CRUSTACEAN BIOLOGY, 22(4): 728–741, 2002
REPRODUCTIVE MORPHOLOGY AND BIOLOGY OF MALE AND
FEMALE MANTIS SHRIMP (STOMATOPODA: SQUILLIDAE)
Jennifer L. Wortham-Neal
Department of Biology, University of Louisiana at Lafayette, Lafayette, Louisiana 70504-2451, U.S.A.
Present address: Department of Biology, University of Evansville, Evansville, Indiana 47722, U.S.A.
([email protected])
ABSTRACT
Male and female reproductive anatomy of a ‘‘spearer’’ mantis shrimp, Squilla empusa, is described
using light and scanning electron microscopy as well as dissections. The genital region of females is
located medially on the sixth thoracic sternite. It consists of a pair of gonopores connected by a medial
genital slit, which leads to a cuticlar sperm storage organ that is shed with every molt. Sperm and
accessory material have been located in the seminal receptacle. The accessory material appears to serve as
a sperm plug. Females have three internally located cement glands that are visible through the exoskeleton
on the thoracic sternite surface. The cement-gland material forms a matrix that holds individual embryos
together in a uniform mass. Cement glands develop in synchrony with the ovaries, and development is
divided into three stages. Posterior to the gonopores is a medial pore from which material from the cement
gland is released. Reproductively active females have ovaries that are oriented anteriorly to posteriorly and
are visible dorsally and ventrally through the exoskeleton. Males have paired penes that arise from the last
pair of walking legs on the eighth thoracic sternite. The distal end of each penis has two openings: 1) one
from the vas deferens that transfers sperm and 2) one from the accessory gland duct that contains sperm
plug material for the female seminal receptacle. Male penes are not symmetrical; the left penis is
significantly longer when compared to the right penis. Under laboratory conditions, most females that did
not have immediate access to males before oviposition produced unfertilized eggs. Two females produced
fertilized eggs; one lacked contact with a male for four weeks, and one had constant male contact. The
seminal receptacle may normally serve as short-term sperm storage, even though long-term storage was
documented. Molting is not related to oviposition. Females produce consecutive broods of eggs an average
of 40.6 days apart. Based on the location of the oviducts with respect to the female seminal receptacle,
fertilization occurs immediately after the eggs are extruded from the oviducts.
Genitalia vary and are diverse, possibly because of sexual selection (Eberhard, 1985).
Although the primary function of mating is
sperm transfer, postmating sexual selection, in
terms of sperm competition and cryptic female
choice, has become a topic of great interest to
evolutionary biologists (Parker, 1970; Thornhill, 1983). In order to understand postmating
sexual selection and how it operates in specific
taxa, an understanding of the structure and
function of different reproductive organs is imperative.
Studies on reproductive morphology in a
phylogenic context can answer two questions:
1) how have reproductive morphologies, mating behaviors, and fertilization coevolved, and
2) is reproductive anatomy conserved in phylogeny? Morphology of the reproductive tract influences 1) which male fertilizes the eggs, 2)
which male’s sperm are stored, and 3) which
type of mating system is involved. When in-
semination and fertilization occur at relatively
different times, natural selection appears to
have favored methods that increase sperm survival inside the female reproductive tract.
Many studies have analyzed the impact of morphology on the evolution of mating systems
(crustaceans: see review articles in Bauer and
Martin, 1991; spiders: Kaster and Jakob, 1997;
copepods: Barthelemy et al., 1998; Vigoni
et al., 1999; nudibranch: Hasse and Karlsson,
2000). Genital anatomy and fertilization mechanics have also been used for phylogenetic
studies (isopods: Wilson, 1991; penaeoid
shrimp: Bauer, 1991; copepods: Blades-Eckelbarger, 1991; Barthelemy et al., 1998, Barthelemy, 1999; Corni et al., 2000; spiders: Kaster
and Jakob, 1997; Drosophila sp.: Pitnick et al.,
1999).
Mantis shrimp are benthic, marine, predatory
crustaceans that live in defendable burrows.
Also called stomatopods, mantis shrimp can be
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further divided into two groups based on the
morphology and function of their raptorial appendage (Caldwell and Dingle, 1976; Caldwell,
1991). ‘‘Smashers’’ live in preexisting cavities
that are limited in abundance and are made of
hard substrate. They kill and feed on hardshelled prey and have complex communication
and agonistic behaviors. ‘‘Spearers’’ live in
self-excavated burrows that are not limited in
abundance and are made of sand or mud.
‘‘Spearers’’ kill and feed on soft-bodied prey,
are considered to be less aggressive than
‘‘smashers’’ (Caldwell and Dingle, 1975), and
have not been studied as much because of their
deeper, murkier habitats and less complex behaviors (see review in Wortham-Neal, 2002).
Mantis shrimp have separate sexes and oviparous females that lay yolky eggs. Females
provide maternal care by brooding the egg
mass, using the maxillipeds to clean the
brooded embryos and to circulate water
among them. Males have paired testes, and females have paired ovaries with ventral cement
glands; the cement-gland material holds the
egg mass together during brooding. Internal
fertilization has been proposed and reported in
the literature but without any direct evidence
(Caldwell, 1991). Also, sperm storage and a
sperm-storage structure have been discussed
(Gerstaecker, 1889; Tirmizi and Kazmi, 1984;
Caldwell, 1991) but with very little detailed
observations on the function, location, and
morphology.
Stomatopods have a diverse, complex range
of social behaviors and mating systems (see
review in Caldwell, 1991). Although much research exists on the social and reproductive
behaviors of members of some families, little
is known about their reproductive biology,
and detailed functional and descriptive analyses are rare. Descriptions and illustrations of
stomatopod genital systems are limited, incomplete, or lacking detail (Gerstaecker, 1889;
Giesbrecht, 1910; Balss, 1938; Deecaraman
and Subramoniam, 1980a, 1980b, 1983;
McLaughlin, 1980; Tirmizi and Kazmi, 1984).
In spite of the importance of reproductive
biology, no study has focused on descriptions
of the male and female reproductive anatomy
of mantis shrimp.
Squilla empusa Say, a ‘‘spearer’’ mantis
shrimp, is distributed in the mud and sandy
bottoms of the Atlantic Ocean from Maine,
U.S.A., to South America and in most areas of
the Gulf of Mexico (Manning, 1969). Squilla
729
empusa excavates burrows constructed of sand
and mud and usually occurs in high-salinity
waters (Franks et al., 1972). The species can
reach a total length of 165.0 mm in males and
185.0 mm in females (Manning, 1969). In the
Gulf of Mexico, spawning occurs in an
8-month period from January to August; few
gravid individuals are collected from September to December (Rockett et al., 1984; Wortham-Neal, personal observation).
The purpose of this study was to examine
the reproductive morphology and genital systems of a mantis shrimp, using a ‘‘spearer’’
species as a model. Detailed descriptions and
functional analyses of the male and female genital areas as well as details of the reproductive
biology of Squilla empusa are presented and
discussed. Because spawning and mating are
associated with molting in many crustaceans
(Gleeson, 1991; see review in Bauer, 1996),
observations related to reproduction and molting are presented.
MATERIALS AND METHODS
In the Gulf of Mexico off the coasts of Louisiana,
Mississippi, and Alabama, Squilla empusa were collected at
night in June, November, and December 1997 and January,
June, and October 1998 using a shrimp trawl on the R/V
Tommy Munroe (Gulf Coast Research Laboratory). These
organisms live in high densities, and over 100 individuals
could be collected during a 30-min trawling period. Mantis
shrimp were transported to The University of Louisiana at
Lafayette in collecting bags with oxygenated sea water. To
prevent fighting and cannibalism, mantis shrimp were
housed individually in containers 32 3 16 3 9 cm that
were submerged in a round, recirculating, undergroundfiltration aquaculture tank (5,700 l). Individuals were fed
peeled penaeoid shrimp, and water temperature ranged from
218 to 238C with a photoperiod of 14L : 10D, and salinity
ranged from 30 ppt to 33 ppt.
In the laboratory, field-collected males and females were
immediately separated and placed into individual containers.
Each individual container had one artificial burrow made of
PVC pipe. Most individuals immediately entered the pipe
and treated it like a natural burrow (i.e., resided in it,
returned food to it, and cleaned it of excess food). The dates
that individuals molted were recorded to determine whether
molting, spawning, and fertilization were associated.
Females were monitored daily for spawning and the
brooding of embryos. The duration of brooding was
documented as well as the position of the egg mass in the
individual container (i.e., being brooded by the female, on
the floor of the burrow, or on the floor of the container). The
purpose of these observations was to determine brooding
duration and the number of females with stored sperm from
field populations.
To determine the duration of sperm storage and whether
females need immediate access to males before oviposition,
several reproductively active, prespawning females were
placed in a test arena with a size-matched male (i.e., one
female and one male in each test arena). Females were
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selected based on the presence of late-stage cement-gland
development and ovaries that fuse in the telson, forming a
‘‘triangle’’ on the ventral surface (Deecaraman and Subramoniam, 1980a, 1983). Two artificial burrows were placed
in the test area to prevent agonistic interactions for
ownership of a single burrow. Individuals were fed peeled
penaeoid shrimp every other day. Individuals were allowed
to interact until the female spawned. Females were
monitored every day, and the presence and location of the
egg mass were recorded. In a separate laboratory setup,
several females were placed in a test arena with several
males; individuals were of different sizes and reproductive
stages.
Mantis shrimp were initially fixed in Formalin, washed
with water, and then transferred to 70% alcohol for
permanent storage. Males and females were dissected.
Potassium hydroxide was used to dissolve tissue for
exoskeleton observations. For scanning electron microscopy
(SEM), specimens were dehydrated in a graded alcohol
series to 100% prior to immersion in hexamethyldisilazane,
air-drying, and mounting on aluminum stubs using doublestick tape. The stubs were coated along the edges with
colloidal graphite to enhance conductivity and sputtercoated with 20 nm gold for 2 min. Specimens were
examined using a JEOL 6300-F field emission scanning
electron microscope at voltages of 15 kv and 20 kv.
The genital regions and internal reproductive systems
were paraffin-carved using Paraplast-PlusÒ and a standard
rotary microtome. These specimens were critical-point dried
using CO2 and sputter-coated with 20 nm of gold. More
detail of paraffin carving procedures can be found in
Felgenhauer (1987).
A binomial test was used to determine whether the
number of all males and females collected from the field
differed in reproductive months and nonreproductive
months. I predicted that I would collect fewer females in
reproductive months (one-tailed test, females should be in a
burrow brooding embryos) but predicted to find equal
numbers of males and females (two-tailed test, both sexes
should be foraging at night) in nonreproductive months.
In order to determine whether there were differences in
size between the two male penes, thirty males were
randomly selected from a field sample, carapace length
was measured (mm), and both penes were removed at the
base near the eighth thoracic segment and walking leg. Each
penis was measured for the total length to the nearest 0.01
mm; because the penes are consistently naturally curved, the
measured length was a straight line from the proximal end to
the distal tip. Because an articulation region is visible, the
length of the penis from the proximal tip to the articulation
region was also measured and termed ‘‘half penis.’’ All
measurements were made using digital calipers under a
dissecting microscope.
For statistical analyses, some measurements were not
normally distributed, as determined using SAS (1999), and a
square root transformation did not conform the data to a
normal distribution. Thus, nonparametric statistics were
used (Siegel and Castellan, 1988) to determine differences
between the lengths of the penes and the sizes of the males.
A Wilcoxon signed-ranks test was used to determine
whether 1) the left and right penes of an individual were
equal in length, 2) the left and right half penes were equal in
length, and 3) the proportion of the left half penis to the total
length of the left penis was equal to the proportion of the
right half penis to the total length of the right penis. A
Spearman’s correlation analysis was used to determine
whether body size and penes sizes were related.
RESULTS
Female Reproductive Anatomy
Most observations and details of female reproductive morphology were made on the
sixth, seventh, and eighth thoracic sternites
(Fig. 1A, B). Females have paired ovaries that
lie between the dorsal heart and the ventral digestive glands and gut (Fig. 1C). Ripe ovaries
are orange or pink, are oriented anteriorly to
posteriorly, and are especially visible ventrally
(Fig. 1B). Ovarian development can be divided
into three stages that can be assessed visually.
Stage 1 represents no ovarian development.
Stage 2 represents a pink ovary in the thoracic
and abdominal body cavity, and Stage 3 represents fully developed ovaries that have fused in
the telson (Fig. 1B, E). Gravid females can be
differentiated from nongravid females by observing the ventral side of the telson. Nongravid females have a monochromatic telson (Fig.
1D), whereas gravid females have normal telson coloration along with a ventral median
pink triangular structure, which represents
fused ovaries (Fig. 1E).
Female genital structures are located on the
ventral, medial surface of the sixth thoracic
segment. This genital region is associated with
storage of seminal products, fertilization, oviposition, and release of cement-gland material.
The anterior region of the sixth thoracic sternite
has two gonopores that are lateral and a single
medial pore called the cement-gland pore (Fig.
2A, B). This pore, which was previously
termed the genital vulval opening (Deecaraman
and Subramoniam, 1983), can be closed (Fig.
2C) or open (Fig. 2D). The reproductive stage
and female size does not correlate with the
morphology of the cement-gland pore.
The genital area is organized by complex
anatomical characters and forms a relationship
between egg-laying structures and the transfer
and storage of seminal products. Males insert
their penes into the genital slit region, which
leads to a cuticular sperm storage organ (seminal receptacle) that is dorsal to the genital slit
and gonopores (Fig. 3A). The cement-gland
pore is not connected to the gonopores or to
the genital slit (Fig. 3C).
Observations on oviducts were made using
SEM and from dissections of females. The oviducts open to the exterior at the gonopores and
into the seminal receptacle by a separate duct
called the oviducal channel (Fig. 3A, B). Possibly because of the location, development, and
WORTHAM-NEAL: REPRODUCTIVE MORPHOLOGY OF STOMATOPODS
731
Fig. 1. Squilla empusa, female. A, Reproductively gravid female with stage 3 ovarian development, dorsal view; B,
Ventral view of gravid female with stage 3 ovarian and cement gland development; C, SEM, paraffin carving, transverse
section of a female abdominal segment, scale bar ¼ 600 lm; D, Nonreproductively active female, ventral telson; E,
Reproductively active female, ventral telson with fused ovary forming a ‘‘triangle.’’ h ¼ heart; dg ¼ digestive gland; g ¼ gut;
gr ¼ genital region; ma ¼ maxillipeds; ov ¼ ovary.
bulkiness of the cement glands, the oviducts
are not ventral, perpendicular extensions from
the ovary. Instead, the oviducts are lateral extensions of the ovary near the sixth thoracic
segment. The oviducts wrap ventrally around
the lateral sides of the body cavity and then run
medially on the dorsal side of the sixth thoracic
sternite, ventral to the cement glands. Each oviduct terminates at a lateral gonopore, which
connects the oviduct with the external environment (Fig. 3D).
Males transfer sperm and accessory material
to the female. Females store these seminal products inside their seminal receptacle (Fig. 4A).
The seminal receptacle is surrounded by cuticle
and is shed every molt cycle, as seen by observing the exuvium of females. Two distinct materials have been observed inside the seminal
receptacle (Fig. 4B). Sperm are located in the
dorsal region of the seminal receptacle (Fig.
4B, C), and the accessory material occupies the
ventral region (Fig. 4B, D). Empty seminal receptacles have also been observed (Fig. 4E),
with remnants of sperm still present (Fig. 4F).
There are two semicircular muscular regions at
the dorsal end of the seminal receptacle (Fig.
4F, G), which may function to force seminal
products out of the seminal receptacle. Material
in the seminal receptacle and the oviducts are
connected (Fig. 4H).
Females have three internal cement glands,
which are visible through the exoskeleton on
the sixth, seventh, and eighth thoracic sternites
(Fig. 1B). These glands develop in synchrony
with the ovaries and form dense white patches
in reproductively active and mature females.
The cement-gland material is extruded through
the cement-gland pore (Fig 4A). As observed
through dissections of females, the glands develop medially to laterally (Fig. 5A) and are
connected together by a medial duct oriented
perpendicular to the glands. In a female that is
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Fig. 2. SEM of the external sixth thoracic sternite of female Squilla empusa. Top of photographs is anterior and bottom of
photographs is posterior. A, Genital region with submedian gonopores and a posterior cement-gland pore; B, Connection
between genital slit and gonopores; C, D, Cement-gland openings of two different females. cgp ¼ cement-gland pore; gp ¼
gonopore; gs ¼ genital slit. Scale bars, A ¼ 522 lm; B ¼ 100 lm; C ¼ 27 lm; D ¼ 40 lm.
Fig. 3. SEM of the interior of the ventral part of the sixth thoracic segment, showing the seminal receptacle (dorsal view,
potassium hydroxide-cleared specimens). Top of photographs is anterior and bottom of photographs is posterior. A,
Connection of oviducts to the seminal receptacle; B, Oviduct (dissolved in potassium hydroxide) opens to the seminal
receptacle and the exterior; C, Seminal receptacle and separate posterior cement-gland pore; D, Remnant of oviduct. cgp ¼
cement-gland pore; e ¼ external environment; gs ¼ genital slit; o ¼ oviduct; oc ¼ oviducal channels; sr ¼ seminal
receptacle. Scale bars ¼ 100 lm.
WORTHAM-NEAL: REPRODUCTIVE MORPHOLOGY OF STOMATOPODS
733
Fig. 4. SEM of female seminal receptacle. Top of photographs is ventral, bottom is dorsal, left is posterior, and right is anterior. A,
Longitudinal section through medial sixth and seventh sternites; B, Material in seminal receptacle; C, Sperm in seminal receptacle; D,
Accessory-gland material (sperm plug); E–F, Empty seminal receptacle with some sperm remaining; G, ‘‘Muscular’’ fibers at the
dorsal base of the seminal receptacle; H, Frontal section of genital region connecting the gonopores with the seminal receptacle. c ¼
cuticle; cg ¼ cement gland; cgp ¼ cement gland pore; m ¼ ‘‘muscular’’ fibers; o ¼ oviduct; s ¼ sperm; sp ¼ sperm plug; sr ¼ seminal
receptacle. Scale bars, A ¼ 267 lm; B ¼ 120 lm; C ¼ 17 lm; D ¼ 20 lm; E ¼ 279 lm; F ¼ 100 lm; G ¼ 20 lm; H ¼ 240 lm.
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ready to spawn, the cement glands form three
parallel stripes on the ventral side. Cement
gland development is divided into three stages
that can be visually assessed: stage 1—no
gland development and no ventral ‘‘stripes’’
(Fig. 5B); stage 2—gland development into
three parallel lines (one visible on the each of
the sixth, seventh, and eighth thoracic segments, Fig. 5C); and stage 3—gland development into three dense, thick lines that are
connected medially (Fig. 1B, 5D). In stage 3,
the cement-gland material fills the ventral region of the thoracic cavity.
Male Reproductive Anatomy
The male penes (Fig. 6A) are located at the
base of the last pair of walking legs on the
eighth thoracic segment. The penes have an articulation region located at about one-half of
the total length of the penes (Fig. 6A, B). The
distal end of the penes has two openings. The
circular orifice at the distal tip is the opening of
the accessory gland (accessory gland orifice),
whereas the oval opening is the end of the vas
deferens (genital orifice) (Deecaraman and
Subramoniam, 1980b) (Fig. 6C). Thus, material
from the accessory gland is transferred to the
female through the accessory gland orifice, and
sperm is transferred to the female through the
genital orifice. The penes have two separate
ducts, one leading to each orifice (Fig. 6D).
The material in the accessory gland duct (Fig.
6D) is similar to the material found in the female seminal receptacle and may contribute to
a sperm plug (Fig. 4D).
In males, the paired testes begin at the third
abdominal segment and extend to the telson
where they fuse. The testes are located ventral
to the dorsal heart but dorsal to the digestive
gland and gut (Fig. 6E), and individual sperm
are visible in the testes (Fig. 6F, G). The vas
deferens begins anterior of the fourth abdominal segment and leads to the eighth segment
where it enters the penis (Fig. 7A). Males have
paired accessory glands that extend to the 8th
thoracic segment (Deecaraman and Subramoniam, 1980b). Males package sperm and accessory-gland material into a sperm cord (Fig.
7B), which is then transferred to the female
and stored in the seminal receptacle.
Molting and Reproductive Biology
More females were collected than males in
the reproductive months (nmale ¼ 618, nfemale ¼
679, z ¼ 1.67, P ¼ 0.0475) and in the
Fig. 5. Female cement gland development. Development begins
medially and extends laterally in the body cavity. A, SEM of transverse
section through posterior sixth thoracic sternite, scale bar ¼ 1,000 lm;
B, Stage 1; C, Stage 2; D, Stage 3. cg ¼ cement gland; cgp ¼ cementgland pore; 6 ¼ sixth thoracic sternite; 7 ¼ seventh thoracic sternite;
8 ¼ eighth thoracic sternite.
WORTHAM-NEAL: REPRODUCTIVE MORPHOLOGY OF STOMATOPODS
735
Fig. 6. SEM of male reproductive organs. A, penis; B, Articulation region of penis; C, Distal end of male penis; D,
Transverse section through distal end of penis; E, Transverse of abdominal segment; F, Testes; G, Sperm in testes. a ¼
articulation region; agd ¼ accessory gland duct; ago¼ accessory gland orifice; d ¼ distal end; dg ¼ digestive gland; g ¼ gut;
go ¼ genital orifice; h ¼ heart; t ¼ testes; ts ¼ testicular sac; vd ¼ vas deferens. Scale bars, A ¼ 857 lm; B ¼ 150 lm; C ¼
86 lm; D ¼ 55 lm; E ¼ 364 lm; F ¼ 100 lm; G ¼ 24 lm.
nonreproductive months (nmale ¼ 139, nfemale ¼
175, z ¼ 1.98, P ¼ 0.0478). The mean
period between female molting and spawning
was an average of 38.1 days before spawning
(SD ¼ 11.8 d, range 23–58 d, n ¼ 8) and 35.7
days after spawning (SD ¼ 24.8 d, range
10–101 d, n ¼ 23). Molting could not be
predicted based on reproductive condition.
Females molted randomly; some females had
late-staged ovaries and cement glands (stage
2 and stage 3), whereas others had no visible
cement-gland (stage 1) or ovarian development.
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Brooding of Unfertilized
and Fertilized Eggs
Seven females produced consecutive broods
in the laboratory an average of 40.6 days apart
(SD ¼ 21.0, range 15–64 d). The smallest and
largest female that produced broods had carapace lengths of 16.5 mm and 29.5 mm, respectively. No individual below the carapace length
of 15.0 mm was observed to have any cementgland or ovarian development. The duration
that females brooded unfertilized eggs differed
significantly between females that had continuous contact with a male prior to oviposition
(n ¼ 10, mean ¼ 5.50, SD ¼ 1.18, range ¼ 3)
and isolated females (no male contact for over
one week; n ¼ 36, mean ¼ 3.69, SD ¼ 2.15,
range ¼ 7) (v2 ¼ 5.70, P ¼ 0.017). Thus, females brooded unfertilized eggs longer if they
had recent male contact. In observations when
one reproductively active female was placed
in a test arena with a size-matched male, six
females brooded unfertilized eggs, males
cannibalized three females, and one female
never spawned. All females ceased feeding
while brooding.
When females brooded unfertilized eggs
(n ¼ 46), females would discard the egg mass
outside the burrow onto the test arena floor or
onto the container floor. The egg mass would
be in several pieces, and females often fed on
the discarded egg mass. When the discarded
egg masses were studied under a compound
microscope, ciliates were visible on the individual eggs, and no development was ever seen.
Females (n ¼ 2) were observed brooding fertilized embryos. One female had continuous access to several males of several different sizes;
the embryos hatched into larvae after 27 days.
The second female had no male contact for 24
days, and the embryos hatched into larvae after
23 days. Both females stopped brooding the
embryos one day before hatching and discarded
the intact egg masses onto the floor of the
aquarium or individual container. No embryos
were present in their maxillipeds, and the
females were not observed feeding on the
embryos.
Male Penes Differences
The total lengths of the left and right penes
are significantly correlated to the individual’s
body size (Fig. 8A, 8B). The size of the left penis is significantly larger than the size of the
right (n ¼ 30, Tþ ¼ 99, z ¼ 2.75, P ¼ 0.003);
Fig. 7. Male, Squilla empusa. A, SEM of vas deferens and
sperm, scale bar ¼ 20 lm; B, Eighth thoracic sternite and
walking legs with penes and ejaculated sperm cord. lp ¼ left
penis; rp ¼ right penis; sc ¼ sperm cord; vd ¼ vas deferens;
w ¼ walking leg.
however, the half-penis regions of the left and
right penis do not differ (n ¼ 30, Tþ ¼ 253.5,
z ¼ 0.43, P ¼ 0.33).
DISCUSSION
Even though stomatopods are economically,
ecologically, and systematically important, few
studies have focused on their reproductive morphology, and little has been inferred regarding
the impact of reproductive morphology on the
evolution of mating behaviors. Besides the
study by Gerstaeker (1889), other descriptions
of mantis shrimp reproductive anatomy have
been short, with little text that corresponds to
figures (Deecaraman and Subramoniam, 1980a;
McLaughlin, 1980; Tirmizi and Kazmi, 1984),
or focused on the male reproductive system
(Deecaraman and Subramoniam, 1980b, 1983).
Thus, a comprehensive description of male and
female reproductive morphology was not available until the present study.
WORTHAM-NEAL: REPRODUCTIVE MORPHOLOGY OF STOMATOPODS
737
Fig. 8. Spearman’s correlation analysis and mean lengths of the total body size and right and left penes. Values in [ ] are
means and standard deviations while values in ( ) are the Spearman’s correlation coefficient (r). All correlations were
statistically significant at a P-value of ,0.0001. Body size is an individual’s carapace length; all measurements (n ¼ 30) are
in mm. A, Right penis [8.22 6 1.41] and carapace length [17.95 6 2.47](0.95); B, Left penis [8.45 6 1.42] and carapace
length–(0.93).
Sperm Storage
The terminology of the crustacean spermstorage organs varies in the literature and is
usually related to specific groups (copepods:
genital atrium, Barthelemy et al., 1998; shrimp:
thelcum, seminal receptacle, or spermathecae,
Bauer, 1991, 1994; crabs: thelyca, spermathecae, or seminal receptacle, Diesel, 1991; Subramoniam, 1991; mantis shrimp: female vaginal
pouch: Subramoniam, 1993; seminal receptacle: Tirmizi and Kazmi, 1984). ‘‘Seminal receptacle’’ has generally been referred to as an
internal structure or an invagination of the
exoskeleton in which sperm material is stored
after copulation (Bauer, 1994; Barthelemy
et al., 1998). In brachyuran crabs, the term
‘‘seminal receptacle’’ is used to describe a
pouch-like sperm-storage organ that is connected to the ovaries by the oviducts (Diesel,
1991). In female Squilla empusa, a pouch-like,
cuticular sperm-storage organ is connected to
the exterior as well as the oviducts; hence, the
term ‘‘seminal receptacle’’ is used.
If the seminal fluids stored in the seminal receptacle are not connected to the oviducts, fertilization is external, whereas fertilization is
internal when the sperm-storage organ is connected to the oviducts (Subramoniam, 1993).
In Squilla empusa, the oviducts end in the
same region as the seminal receptacle and are
connected to the seminal receptacle by the oviducal channel. Fertilization is likely to be internal and to occur inside the cuticular genital
region; the eggs and sperm meet and then are
released to the exterior.
The morphology of the reproductive tract
and sperm-storage organs is important in determining sperm precedence patterns and which
male’s sperm fertilizes the majority of the eggs
(Pitnick et al., 1999). Diesel (1991) and Kaster
and Jakob (1997) determined that sperm storage morphology is correlated to sperm priority
and precedence. In Squilla empusa, the seminal receptacle has two openings, the genital slit
and the laterally dorsal openings near the oviducts (oviducal channels). Sperm precedence
patterns in mantis shrimp remain unstudied.
In females with seminal receptacles, sperm
can be stored for long-term or short-term use,
but sperm must survive long enough within the
female reproductive tract to allow egg fertilization. Females of the ‘‘spearers’’ Pseudosquilla
ciliata (see Hatziolos and Caldwell, 1983),
Oratosquilla oratoria (see Hamano, 1988),
and Squilla empusa (present study), and the
‘‘smashers’’ Odontodactylus scyllarus and
Neogonodactylus (¼Gonodactylus) bredini (see
Caldwell, 1991) can store sperm for several
weeks. In my study, most females produced
unfertilized eggs, suggesting that their seminal
738
JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 4, 2002
receptacles did not contain stored sperm. One
hypothesis that explains this high frequency of
females producing unfertilized eggs is that
some females molt synchronously (Reaka,
1976), and those in my study had recently
molted in the field. Thus, their seminal receptacles and stored seminal products were shed
with their exoskeleton, and the females had not
yet mated. Another hypothesis is that females
only mate just before egg-laying and that the
females caught out of their burrows were not
breeding.
In many taxa, the seminal secretions solidify
inside the sperm-storage organ and form a
‘‘sperm plug’’ (Subramoniam, 1993). A possible function of a sperm plug is to block the female’s reproductive tract from further transfer
of male secretions. There have been several
proposed functions of the male accessory gland
material: 1) to break down the sperm cord after
it has been transferred to the female, 2) to provide females with nutritional benefits by the
material being incorporated into the ovaries, 3)
to stimulate or initiate vitellogenesis and ovulation, 4) to inhibit female reproductive receptivity, and 5) to act as ‘‘prosperm’’ substance that
enhances the longevity or survival of a male’s
own sperm (Deecaraman and Subramoniam,
1980b, 1983; Subramoniam, 1993; Arnqvist
and Nilsson, 2000). In mantis shrimp, the duration and function of the ‘‘sperm plug’’ material
is still unclear. I propose that the accessorygland material serves as a sperm plug, which
prevents sperm loss from the female’s seminal
receptacle.
Reproductive Biology
More females than males were collected all
year. My hypothesis that fewer females than
males would be collected during the reproductive months (because the females should be in
burrows brooding eggs) was not supported.
There are many possible explanations for this
unpredicted result: 1) the trawl net could have
trapped animals living at the burrow entrance,
thus still collecting females, or 2) the sex ratio
is yearly-biased toward females so that no statistical difference was evident. Because many
females collected from the field have cement
gland development, the hypothesis that females
breed in synchrony but were not reproductively
active during collection times seems improbable.
Female Reproductive System
Deecaraman and Subramoniam (1983) reported a difference in the female genital region
(cement-gland pore) depending on whether or
not the female had mated. Mated females had
an everted and lucid opening. However, the
diagrams by Deecaraman and Subramoniam
(1983) were unclear, and no other information
in the text was provided. In my study, no relationship was found between reproductive state
or individual size and the morphology of the
cement-gland pore.
Deecaraman and Subramoniam (1980a)
investigated the histology of cement-gland
material in a spearer, Miyakea (¼Squilla)
holoschista, and they divided development into
four stages based on histology. Their study did
not describe or document the external appearance of the cement glands; their stages were
based on the internal histology of preserved
specimens. Because an objective of my study
was to determine the reproductive condition of
live animals, I did not use histology/preserved
animals as a basis for staging cement gland development. Deecaraman and Subramoniam
(1980a) proposed stages that are not practical
for staging the reproductive condition of live
females, and thus no correlation could be made
with my study. The proposed cement gland
stages in my study allow the researcher to stage
live females.
Tirmizi and Kazmi (1984) reported that the
cement glands open on the 6–8 thoracic sternites by many fine ducts. In my study, the only
opening to the external environment documented for cement gland material is through the
cement-gland pore. No ducts were observed.
Using SEM, I did observe numerous fine depressions/pores in the cuticle of the thoracic
sternites, and their function is unknown.
The proposed functions of the cement glands
in mantis shrimp and other crustaceans are to
1) enable the sperm and egg to interact, 2) bind
the embryos together in the form of a mass
(Tirmizi and Kamzi, 1984), 3) aid in attaching
the embryos to the brooding appendages, 4)
provide nutrition to the embryos, and 5) prevent microbial infection (Deecaraman and Subramoniam, 1980a). In mantis shrimp, because
the cement-gland material is extruded through
the posterior cement-gland pore (not through
the gonopores), the cement-gland material does
not come in contact with the sperm and thus
cannot aid in the union of the egg and sperm.
WORTHAM-NEAL: REPRODUCTIVE MORPHOLOGY OF STOMATOPODS
In stomatopods, it is probable that the cementgland material helps bind the embryos together
to form a mass as well as aid the female in
brooding the eggs in her maxillipeds.
Male Reproductive System
In crustaceans, male reproductive organs
vary in name. The name of the external male
copulatory organ also varies in crustaceans. In
isopods, the terms ‘‘penile papillae’’ and
‘‘penes’’ are used interchangeably to describe
the projections that bear the external openings
of the vas deferens (Wilson, 1991). In Brachyura, the male ‘‘gonopods’’ are modified first
and second pleopods, whereas the vas deferens
contact the external environment at the last
thoracic segment at the ‘‘penes’’ or ‘‘penis’’
(Subramoniam, 1993). Male mantis shrimp
have a modified first pleopod and its function
is generally unknown (Manning, 1969); however, Tirmizi and Kamzi (1984) proposed that
the first pleopod (called ‘‘petasma’’ in their
study) may function to help guide the male
penes into the female seminal receptacle. Deecaraman and Subramoniam (1980b) termed the
male external reproductive organ of another
spearer mantis shrimp ‘‘intromittant organs.’’
Because the vas deferens of male mantis
shrimp contact the external environment at the
eighth thoracic segment and walking legs, the
term ‘‘penes’’ was used.
Deecaraman and Subramoniam (1980b)
drew the distal tip of the male penes in another
squillid (Miyakea (¼Squilla) holoschista), and
my study supports their observations. The
penes have been reported to become erect during copulation (Deecaraman and Subramoniam,
1983), and the region distal to the articulation
is likely to become rigid. In Oratosquilla, Tirmizi and Kazmi (1984) reported that penes appear to be divided into two halves that are
characterized by different degrees of chitinization. The penes of two other squillids (Squilla
chydaea and Gibbesia (¼Squilla) neglecta)
were identical to Squilla empusa in the morphology of the distal end (Wortham-Neal, unpublished observations). Because the left penis
was larger than the right penis but the halfpenes regions were statistically equal, the difference in length between the left and right
penes is in the region between the distal tip and
the articulation point. The benefit of having
asymmetrical penes remains untested.
Deecaraman and Subramoniam (1980b) reported that the male vas deferens could be di-
739
vided into three regions based on their function
and morphology: proximal, middle, and distal.
The proximal region is believed to be secretory and high in protein material, whereas the
middle region serves as a storage area for
spermatozoa. The distal region is also secretory
and aids in conducting the sperm cord during
copulation. The distal region is located in the
male penis. The middle region is reported in
figure 7A.
Although Kemp (1911) reported that male
stomatopods transfer spermatophores to the
female, other researchers have reported that,
instead, the sperm of mantis shrimp are embedded in mucopolysacchride cement that
forms a sperm cord, which is transferred to the
female (Komai, 1920; Deecaraman and Subramoniam, 1980b). The present study confirms
that a sperm cord is released from the penes of
males.
CONCLUSION
Variation in reproductive morphology may
influence selection pressures on mating behaviors in mantis shrimp. Male Squilla empusa
may not benefit by guarding females because
of the long and apparently unpredictable intermolt period and production of consecutive
broods of eggs. The best strategy for optimal
reproductive success from the male’s perspective may be pure-search (Wickler and Seibt,
1981). Males and nonbrooding females leave
their burrows at night (individuals are collected
only at night, personal observation; McCluskey, 1977). This pure-search strategy predicts
that males should mate with all behaviorally
receptive females. The male may then transfer sperm-plug material along with sperm to
females. This strategy of males should decrease
the cost of postcopulatory mate-guarding of
females. Males may then leave this female to
search for another receptive female.
ACKNOWLEDGEMENTS
I thank V. Townsend, Jr., for his assistance with
specimen sectioning and micrographs. I thank R. Bauer,
R. Jaeger, and three anonymous reviewers for critically
reading and making suggestions that improved the manuscript. This work was funded by The University of
Louisiana at Lafayette Department of Biology and the
Graduate Student Organization, a Lerner-Gray Grant from
the American Museum of Natural History, and a grant from
Louisiana Universities Marine Consortium (LUMCON)
Foundation, Inc.
740
JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 4, 2002
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RECEIVED: 16 May 2001.
ACCEPTED: 5 April 2002.