Demystifying the Shrimp Penis: How Big is a Shrimp’s Male Organ?

Light and scanning electron microscopy, as well as dissections, are used to describe the male and female reproductive anatomy of the “spearer” mantis shrimp Squilla empusa. The genital region of females is located medially on the sixth thoracic sternite. It has two gonopores connected by a slit in the middle of the genital area. This slit leads to a cuticular sperm storage organ that falls off with each 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. Females that are reproducing have ovaries that are oriented from front to back and can be seen through the exoskeleton on both the dorsal and ventral sides. Males have paired penes that arise from the last pair of walking legs on the eighth thoracic sternite. At the very end of each penis, there are two openings: one from the vas deferens, which moves sperm, and one from the accessory gland duct, which holds 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 laid fertilized eggs; one didn’t touch a male for four weeks, while the other was always touching a male. 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. Because of where the oviducts are in relation to the female seminal receptacle, fertilization happens as soon as the eggs are pushed out of the oviducts.

Genitalia vary and are diverse, possibly because of sexual selection (Eberhard, 1985). Astrosperm transfer is the main purpose of mating, but evolutionary biologists are now very interested in sexual selection after mating, including sperm competition and female choice that is hard to understand (Parker, 1970; Thornhill, 1983). To fully grasp postmating sexual selection and how it works in certain taxa, it is necessary to know how different reproductive organs are built and how they work.

Mantis shrimp are benthic, marine, predatory crustaceans that live in defendable burrows. Mantis shrimp, which are also known as stomatopods, can be split into two groups based on the shape 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 hard-shelled 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” hunt and eat soft-bodied animals. They are thought to be less aggressive than “smashers” (Caldwell and Dingle, 1975), and their behaviors aren’t as complex, so they haven’t been studied as much (see Wortham-Neal, 2002).

Mantis shrimp have separate sexes and oviparous females that lay yolky eggs. The females care for the eggs by brooding them. They clean the embryos that are brooded and move water around them with their maxillipeds. The males have paired testes and the females have paired ovaries with ventral cement glands. The egg mass stays together during brooding because of the cement gland material. Internal fertilization has been proposed and reported in the literature but without any direct evidence (Caldwell, 1991). A sperm storage structure and sperm storage have also been talked about (Gerstaecker, 1889; Tirmizi and Kazmi, 1984; Caldwell, 1991), but not much is known about its function, location, or shape.

This is a “spearer” mantis shrimp, Squilla empusa Say. It lives in the mud and sandy bottoms of the Atlantic Ocean from Maine, U.S. S. A. , to South America and in most areas of the Gulf of Mexico (Manning, 1969). Squilla 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). Spawning happens in the Gulf of Mexico for eight months, from January to August. From September to December, not many pregnant fish are caught (Rockett et al. , 1984; Wortham-Neal, personal observation).

The goal of this study was to look at how a mantis shrimp reproduces and its genital systems by comparing them to a “spearer” species. The male and female genital areas of Squilla empusa are described and analyzed in great detail. The reproductive biology of this species is also talked about in detail. Since molting is linked to mating and spawning in many crustaceans (Gleeson, 1991; see Bauer, 1996), observations about reproduction and molting are given.

It was caught at night with a shrimp trawl on the R/V Tommy Munroe (Gulf Coast Research Laboratory) in the Gulf of Mexico off the coasts of Louisiana, Mississippi, and Alabama in June, November, and December 1997, and in January, June, and October 1998. 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 keep the mantis shrimp from fighting and eating each other, they were kept in separate containers that were submerged in a round, recirculating, underground-filtration aquaculture tank that held 5,700 l. People were fed peeled penaeoid shrimp. The water temperature was between 21°C and 23°C, and the light-to-dark cycle was 14L:10D. The salinity was between 30 ppt and 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 amount of time the eggs were brooded and where they were in each container were both written down. e. on the floor of the burrow, being cared for by the female, or on the floor of the container The goal of these observations was to find out how long the females in the field were brooding and how many of them had stored sperm.

To find out how long sperm can be stored and whether females need to be able to access males right away before oviposition, several reproductively active females that were about to spawn were put in a test arena with a male that was the same size as them (i.e. e. , one female and one male in each test arena). The females were chosen because they had cement glands that were fully developed and ovaries that fused in the telson, making a “triangle” on the ventral surface (Deecaraman and Subramoniam, 1980a, 1983). Two fake burrows were put in the test area so that animals wouldn’t fight over who would own the one 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 lab, a test arena with several males and several females was set up. The animals were of different sizes and stages of reproduction.

The mantis shrimp were first fixed in Formalin, then washed with water, and finally moved to 70% alcohol to be stored permanently. Males and females were dissected. Potassium hydroxide was used to dissolve tissue for exoskeleton observations. For scanning electron microscopy (SEM), specimens were dried out in a series of alcohols that ranged from 0% to 100% before being immersed in hexamethyldisilazane, air-drying, and sticking to aluminum stubs with double-stick tape. We put colloidal graphite on the edges of the stubs to make them more conductive, and then we sputtered 20 nm gold on them for two minutes. A JEOL 6300-F field emission scanning electron microscope was used to look at the samples at 15 kv and 20 kv voltages.

The genital regions and internal reproductive systems were paraffin-carved using Paraplast-Plus® and a standard rotary microtome. These specimens were critical-point dried using |${‘rm CO}_{2}$| and sputter-coated with 20 nm of gold. More detail of paraffin carving procedures can be found in Felgenhauer (1987).

We used a binomial test to see if the number of males and females found in the field changed during reproductive months and nonreproductive months. I thought I would find the same number of males and females in nonreproductive months (two-tailed test, both sexes should be foraging at night) but fewer females during reproductive months (one-tailed test, females should be in a burrow brooding embryos).

Thirty males were chosen at random from a field sample. The carapace length was measured in millimeters, and both penes were cut off at the base near the eighth thoracic segment and walking leg to see if there were differences in size. Each penis was measured for the total length to the nearest 0. Since the penes are naturally curved, the length that was measured was a straight line from the tip closest to the body to the tip farthest away. Since there is a clear area of articulation, the penis’s length from the proximal tip to that area of articulation was also measured and named “half penis.” ” All measurements were made using digital calipers under a dissecting microscope.

SAS (1999) showed that some measurements were not normally distributed for statistical analyses, and a square root transformation did not change the data to fit a normal distribution. So, nonparametric statistics (Siegel and Castellan, 1988) were used to find out if there were differences between the males’ sizes and the lengths of the penes. A signed-ranks test called Wilcoxon was used to see if 1) a person’s left and right penes were the same length, 2) a person’s left and right half penes were the same length, and 3) the left half penis’s proportion to its total length was the same as the right half penis’ proportion to its total length. A Spearman’s correlation analysis was used to determine whether body size and penes sizes were related.

We saw and learned a lot about the reproductive anatomy of females 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. In Stage 2, there is a pink ovary in the thoracic and abdominal body cavity. In Stage 3, the ovaries are fully developed and 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), but pregnant females have normal telson coloration and a pink triangular structure on the ventral median, which means their ovaries are fused (Fig. 1E).

Squilla empusa, female. These pictures show a pregnant woman with stage 3 ovarian development on the dorsal side and a pregnant woman with stage 3 ovarian and cement gland development on the ventral side. They also show a SEM image of a female abdominal segment cut in half across, with a scale bar of |$= 600”mu {‘rm m};'{‘rm D}$|⁠, a female that is not reproducing, showing the ventral telson; and h stands for heart, dg for digestive gland, g for gut, gr for genital area, ma for maxillipeds, and ov for ovary.

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. In the front part of the sixth thoracic sternite, there are two gonopores that are on the sides and one medial pore that is 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.

SEM of the external sixth thoracic sternite of female Squilla empusa. Top of photographs is anterior and bottom of photographs is posterior. A: The genital area has submedian gonopores and a posterior cement-gland pore; B: The connection between the genital slit and the gonopores; C and D: The cement-gland openings of two different women. cgp = cement-gland pore; gp = gonopore; gs = genital slit. The scale bars show that |$’rm A} = 522”mu {‘rm m}$|⁠, |$’rm B} = 100”mu {‘rm m}$|⁠, |$’rm C} = 27”mu ‘rm m}$|⁠, and |$’rm D} = 40”mu ‘rm m}$|⁠.

The genital area is organized by complex anatomical features and is connected to structures that lay eggs and those that move and store seminal products. Males put their penises into the area of the genital slit, which leads to a cuticular organ for storing sperm called the seminal receptacle, which is behind the gonopores and the genital slit (Fig. 3A). The cement-gland pore is not connected to the gonopores or to the genital slit (Fig. 3C).

SEM of the inside of the ventral part of the sixth thoracic segment, showing the seminal receptacle (dorsal view, specimens cleared with potassium hydroxide) Top of photographs is anterior and bottom of photographs is posterior. A. The oviducts connect to the seminal receptacle; B. The oviduct, which has been dissolved in potassium hydroxide, opens to the seminal receptacle and the outside world; C. The seminal receptacle and the separate posterior cement-gland pore; D. The remnant of the oviduct cgp stands for cement-gland pore, e for external environment, gs for genital slit, o for oviduct, oc for oviducal channels, and sr for seminal receptacle. Scale bars |$= 100 ‘ ‘mu {‘rm m}$|⁠.

Observations on oviducts were made using SEM and from dissections of females. There is a separate tube called the oviducal channel that leads from the gonopores to the seminal receptacle and from the oviducts to the outside world (Fig. 3A, B). The oviducts are not ventral, straight extensions from the ovary. This may be because of where the cement glands are located, how they grow, and how big they are. Instead, the oviducts are lateral extensions of the ovary near the sixth thoracic segment. The oviducts go around the sides of the body cavity from the ventral side and then run along 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 cuticle covers the seminal receptacle and is shed every time the plant molts. This can be seen by looking at the females’ exuvium. 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).

SEM of female seminal receptacle. Top of photographs is ventral, bottom is dorsal, left is posterior, and right is anterior. This picture shows a longitudinal section through the middle sixth and seventh sternites; it shows material in the seminal receptacle, sperm in the seminal receptacle, accessory gland material (sperm plug), an empty seminal receptacle with some sperm still inside, and “muscular” fibers at the base of the seminal receptacle. It also shows a frontal section of the genital area connecting the gonopores to the seminal receptacle. There are “muscular” fibers, an oviduct, sperm, a sperm plug, and a seminal receptacle. The letters c, g, m, o, and s stand for parts of the cell. The scale bars show that |$’rm A} = 267”mu {‘rm m}$|⁠; |$’rm B} = 120”mu {‘rm m}$|⁠; |$’rm C} = 17”mu {‘rm m}$|⁠; |$’rm D} = 20”mu {‘rm m}$|⁠; |$’rm E} = 279”mu {‘rm m}$|⁠; |$’rm F} = 100”mu {‘rm m}$|⁠; |$’rm G} = 20”mu {‘rm m}$|⁠; |$’rm H} = 240”mu {‘rm m}$|⁠

On the sixth, seventh, and eighth thoracic sternites of females, you can see through the exoskeleton to find three cement glands inside. 1B). These glands grow at the same time as the ovaries and form dense white patches on mature females who are able to reproduce. 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. When a female is ready to spawn, the cement glands on the underside of her body meet in three rows. There are three stages of cement gland development that can be seen: stage 1 has no gland development and no ventral “stripes” (Fig. 5B); stage 2: the glands divide into three parallel lines, one on 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.

Female cement gland development. Development begins medially and extends laterally in the body cavity. Fig. A: SEM of a cross-section through the posterior sixth thoracic sternite; Fig. B: Stage 1; Fig. C: Stage 2; Fig. D: Stage 3 cg stands for “cement gland” and “cgp” for “cement-gland pore.” 6 refers to the sixth thoracic sternite, 7 to the seventh, and 8 to the eighth.

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. There are two openings on the end of the vas deferens: the oval one is the end of the vas deferens and the circular one is the opening of the accessory gland (Deecaraman and Subramoniam, 1980b) (Fig. 6C). Materials from the accessory gland are sent to the female through the opening in the accessory gland, and sperm are sent to the female through the opening in the genital area. The penes have two separate ducts, one leading to each orifice (Fig. 6D). The material in the accessory gland duct (Fig. 6D) is like the stuff in a female’s seminal receptacle and might help make a sperm plug (Fig. 4D).

SEM of male reproductive organs. A, the penis; B, the area where the penis articulates; C, the distal end of a male penis; D, a cross-section through the distal end of the penis; E, a cross-section of the abdominal segment; F, the testes; and G, sperm in the testes. a: location of the joint; agd: accessory gland duct; ago: accessory gland orifice; d: end; dg: digestive gland; g: gut; go: genital orifice; h: heart; t: testes; ts: testicular sac; vd: vas deferens The scale bars show that |$’rm A} = 857”mu {‘rm m}$|⁠; |$’rm B} = 150”mu {‘rm m}$|⁠; |$’rm C} = 86”mu {‘rm m}$|⁠; |$’rm D} = 55”mu {‘rm m}$|⁠; |$’rm E} = 364”mu {‘rm m}$|⁠; |$’rm F} = 100”mu {‘rm m}$|⁠; |$’rm G} = 24”mu {‘rm m}$|⁠

In males, the paired testes start at the third abdominal segment and go all the way to the telson, where they join together. 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). As shown in Figure, the vas deferens starts in front of the fourth abdominal segment and goes to the eighth segment, where it enters the penis. 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.

Male, Squilla empusa. SEM of the vas deferens and sperm; B, the eighth thoracic sternite and walking legs with penes and an ejaculated sperm cord. The scale bar is 20 ”mu {‘rm m}$|⁠. Left penis (lp) and Right Penis (rp) are shown. Sperm cord (sc) and vas deferens (vd) are used for reference.

Molting and Reproductive Biology

There were more females than males caught during the breeding season (n_{{rm male}} = 618, n_{{rm female}} = 679, z = -1). 67, P = 0. 0475)$| and during the months when they don’t have babies, |$(n_{{rm male}} = 139, n_{{rm female}} = 175, z = -1) 98, P = 0. 0478)$|⁠. The mean period between female molting and spawning was an average of 38. 1 days before spawning (⁠|${rm SD} = 11. 8 {rm d}$|⁠, range 23–58 d, |$n = 8$|⁠) and 35. 7 days after spawning (⁠|${rm SD} = 24. 8 {rm d}$|⁠, range 10–101 d, |$n = 23$|⁠). Molting could not be predicted based on reproductive condition. Random molting happened in the females. Some had late-staged ovaries and cement glands (stages 2 and 3), while others didn’t have any cement glands (stage 1) or ovarian development that could be seen.

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