Gastropod reproductive behavior
Mating systemsGastropod reproductive behavior is as varied as the animals themselves (Baur, 1998). Hermaphroditism is universal among the pulmonate gastropods, nearly universal among opisthobranch gastropods, but rare in all other gastropod taxa (primarily marine snails). While the term simultaneous hermaphroditism refers to the condition in which an adult animal has both a functioning male system and a functioning female system, it does not necessarily imply that an individual engages both sexual functions in any single mating. Although inseminations are always internal, that is, by penial insertion, copulation patterns differ among taxonomic groups.
Bizarre mating practices
- Animals such as Aplysia californica, which have their male and female genitalia located on different parts of the body and normally mate unilaterally, can form daisy chains which may contain as many as six mating individuals (Fig. 3). The first individual in the chain acts only as a male, the last individual acts only as a female, and all animals in between act as female to one partner and male to a second partner.
- Certain terrestrial slugs, notably in the genus Limax, mate by hanging their penes from a vertical perch, then twisting their bodies and their penes around one another before sperm is exchanged at the tips of the penes (Fig. 4). Only when the penes are retracted is the partner’s sperm internalized. For reasons unknown, evolution in this group of animals has favored penes of ever increasing length. In one Italian species, the penis length during mating measures 92.5 cm.
- Dart shooting is a feature of courtship in about 6 of 75 families of terrestrial molluscs. The term "dart shooting" is something of a misnomer because the dart does not actually fly through the air, although it is forcefully expelled. The dart itself appears in a variety of sizes, shapes and numbers in different species. Most species have just a single dart which is shot once, but some species shoot multiple darts and still other species stab repeatedly with the same dart. Courting individuals belonging to the Japanese species Euhadra subnimbosa, for example, use the same dart to stab their partners on average 3,311 times (Koene and Chiba, 2006)! The function of dart shooting is explained below.
- The sacoglossan opisthobranchs deliver sperm not by inserting the penis into a female pore, but by hypodermic injection anywhere on the partner’s body. This is usually a reciprocal event within a mating pair.
- The banana slugs (genus Ariolimax) mate in the normal manner by internal insemination and sperm exchange, but sometimes one of the penes becomes stuck in the partner’s female tract and cannot be retracted. In such instances, one or both of the slugs will gnaw at the stuck penis (apophallation) until the slugs are free to go their separate ways.
- Numerous species of freshwater and terrestrial pulmonates contain hermaphroditic individuals that lack penes (aphally). Such animals reproduce either by outcrossing as females or by using their own sperm to fertilize their own eggs (selfing). Indeed, selfing also occurs in some species that are fully functional as simultaneous hermaphrodites, but that have trouble finding partners.
Potential mates must be located and identified using the chemical senses (olfaction and contact sensations) because gastropods have no sense of hearing and little or no vision. Sea slugs in the genus Aplysia release a bouquet of pheromones when laying eggs, and these substances attract conspecifics that may then mate with the egg-laying individual. Several of these pheromones have been identified and sequenced; they have been given the names “attractin”, “seducin”, "enticin", etc. (Painter et al., 2004). Terrestrial species may locate potential mates by sensing cues in mucus trails, then following the trials to find the source. Contact chemosensation, as well as tactile stimulation, is probably important for courtship interactions in all species. Generally, gastropods do not discriminate among potential partners, although body size seems to be assessed in some species where mates of large size are preferred.
Sperm competition is responsible for many sexually selected traits in gastropods. In some, but not all species, the received sperm can be stored for many months to years before it is used to fertilize eggs. Meanwhile, the receiving individual may mate with additional sperm donors. When the multiply mated individual eventually fertilizes its eggs, it will generally select sperm at random (as in a raffle) from the stored pool. Together, the features of promiscuity, sperm storage and internal fertilization combine to cause intense sperm competition in gastropods. Therefore, evolution has favored traits that enhance the survival and use of the sperm that are transferred during matings. Some species use a spermatophore to package the sperm and thereby protect them during transfer. Mating order may also be important when successive transfers are made to the same recipient from different sperm donors, a phenomenon known as sperm precedence. For example, in Cornu aspersus (= Helix aspersa = Cantareus aspersum), early donors fare better than later donors in terms of siring offspring. Other species, however, exhibit second donor sperm precedence or no apparent precedence. Another important consequence of sperm competition is the very large numbers of sperm contained in each ejaculate (about 5.5 x 106 in Cornu aspersus). Because of sperm competition, the costs of reproduction via the male function can approach, or even equal, that of the female function (Greeff and Michiels, 1999).
The function of dart shooting
The so-called “love” dart is an especially interesting example of a trait evolved in the context of sperm competition. Speculation as to its function has a long history, dating back at least as far as the 17th century. Mostly, it was seen as a device to stimulate a potential mate during courtship, but recent studies in Cornu aspersus (= Helix aspersa = Cantareus aspersum) suggest a very different function. In this species, the single dart is solid and has a sharp tip; it is made of pure calcium carbonate crystals. The dart is rapidly expelled from the genital pore in the later stages of courtship and it often, but not always, penetrates into the flesh of the potential mate (Fig. 5). A role for the dart in either mate selection or arousal is excluded by the fact that the outcome of a dart shot has absolutely no influence on the subsequent mating behavior of either member of the mating pair. Rather, dart shots that hit the partner result in more of the shooter’s sperm becoming stored by the receiving snail compared to cases where the dart misses the intended target (Rogers and Chase, 2001). As a consequence, snails that hit their partners with a dart sire more offspring than competing snails whose darts miss the same partner. The mucus that clings to the surface of the dart is crucial to its effectiveness. The mucus contains an unidentified molecule, probably a peptide, that is injected into the partner’s blood when the dart ruptures the skin. This molecule is thought to act upon the receiver's female tract to enhance the survival of the shooter's sperm (Chase and Blanchard, 2006).
Neural controls for mating
Identified cell clusters
The expression of male sexual behavior is mainly controlled by neurons in the central nervous system. A population of neurons with homologous representations in several gastropod species has been identified in the right cerebral ganglion (Koene et al., 2000). In Aplysia califonica, the neurons are located in the H-cluster, the only cluster among a total of 13 that is not present bilaterally in the cerebral ganglia. In Lymnaea stagnalis the neurons are in the right anterior lobe, and in Cornu aspersus they are found in the right mesocerebral lobe. In the latter two examples, Lymnaea and Cornu/Helix/Cantaeus, the anatomical structures are bilaterally represented, but considerably larger in the right cerebral ganglion than in the left cerebral ganglion. The significance of the right-side bias is that it coincides with the right-side placement of the genital organs. Anatomical studies have demonstrated that neurons in the named clusters send axonal projections to the penis, dart sac and related male structures, while electrophysiological studies have linked activity in these neurons to motor behaviors related to copulation and dart shooting (Chase, 1986). For example, recordings from intact animals using fine, implanted wires revealed dramatic increases in spiking activity during eversion of the penis. Additional neurons implicated in the control of male mating behavior are located in the right pedal ganglion.
An intriguing aspect of the neurons implicated in male sexual behavior is that many of them show immunoreactivity for the neuropeptide APGWamide. Furthermore, injections of APGWamide into intact animals, or application of APGWamide in vitro causes penial eversion. The peptides conopressin, FMRFamide and GFADamide are also implicated in the expression of male sexual behaviors.
Together, these findings indicate that many of the neurons responsible for male mating behavior in gastropods are phylogenetically conserved (Fig. 6).
Regulation of mating frequency and sexual roles
The expression of mating behavior, like that of other motivated behaviors, is regulated in order to maximize the benefits while minimizing the costs. One example of this is that animals that have recently mated as males are unlikely to quickly re-mate as males. Consequently, for experimental studies, animals are generally kept in social isolation to increase the mating probabilities. In Lymnaea stagnalis, the readiness to mate as a male (proclivity) is regulated by the volume of the prostate gland (De Boer et al., 1997). Animals that have an insufficient supply of seminal fluid do not mate. As the supply of seminal fluid increases, so too does the rate of discharge in the nerve that innervates the prostate. Lesions of this nerve reduce male sexual drive. Presumably, input from this nerve changes the excitability of neurons that trigger courtship behavior. In other snails, for example Cornu aspersus, that mate irregularly but that do not use seminal fluid (because the sperm are contained in a spermatophore), the mating intervals are obviously regulated in some other, unknown, manner. Also, a complicating factor with Cornu aspersus, and with all other gastropods that mate simultaneously and reciprocally, is that proclivity is potentially influenced by both male and female drives. In these cases, it is likely that mating frequency is regulated by a combination of male and female drives. An open question is how sex roles are established in individuals that can change their roles from one mating event to another (Anthes et al., 2006).
Forms of deposition
Most species are oviparous, meaning that fertilized eggs are encapsulated but otherwise externalized with little or no embryonic development. Land snails deposit their eggs in a nest that has been dug out of moist, loose soil by peristaltic movements of the animal’s foot (Fig. 7). Aquatic gastropods generally deposit their eggs in gelatinous masses that are attached to a hard surface. Opisthobranchs lay huge numbers of fertilized eggs. For example, a single specimen of Aplysia californica was observed depositing one mass that contained 140,000 eggs (Kandel, 1979). Moreover, during the spawning season, an individual Aplysia californica will typically lay eggs at intervals of 1 or 2 days. By contrast, land snails lay eggs only two or three times per season, with egg numbers per clutch ranging from a few dozen to more than 100. Lymnaea stagnalis lays egg masses containing between 50 and 150 eggs once a week during the breeding season.
Egg laying behavior in Aplysia
The behaviors that are associated with egg laying appear in a stereotyped sequence (Ferguson et al., 1989). First, the animal slows its locomotion and begins to move its head and neck in certain characteristic ways. These movements become increasingly frequent so that by the time the animal is ready to oviposit, it is immobile but far from quiescent. Large, side-to-side movements of the head appear initially, probably to aid in finding a suitable substrate. Once a site has been selected, the substrate is prepared by small, up-and-down undulations of the head. Weaving movements of the head appear after the eggs have begun to emerge; these are side-to-side movements that serve to distribute the egg string. The eggs come out from the genital pore, which is located near the base of the right tentacle. As soon as the egg cordon leaves the genital pore it enters an external groove that lies within the skin on the right side of the neck; this directs the cordon towards the mouth. As the head moves from side to side, the egg string is brought into contact with the mouth where it receives a sticky secretion that facilitates its attachment to the substrate. Since the posterior portion of the animal’s foot remains attached to the substrate, the eggs tend to pile up, sometimes in a knotted mass. Finally, dorsal-ventral movements of the head are used to press the eggs onto the substrate. The total duration of the foregoing events is approximately three hours.
Egg laying behavior in Lymnaea
Three phases of egg laying behavior can be distinguished (Hermann et al., 1994). The first phase is a resting phase in which the animal stops locomoting, slightly contracts its foot and draws the shell partly over the tentacles. Two behaviors, turning and rasping, dominate during the next phase, which lasts for 1 hour or more. Turning behavior prior to egg laying involves only the shell and mantle; the foot remains in place. A single turn moves the shell 60o - 90o, always in a counter clockwise direction relative to the resting position. The new position is maintained for several minutes before the shell is returned to the resting position. Two to four such turns typically occur before oviposition. Their function is probably to help move the egg mass through the reproductive tract and into the genital pore. The rasping is done to clean the substrate surface so that the egg mass will adhere. Although the rasping movements prior to egg laying appear similar to those shown during feeding, the duration of the protraction phase is significantly longer during egg laying compared to during feeding. In the final phase of egg laying, the animal moves slowly forward while extruding the egg mass and pushing it against the substrate.
Neural and Hormonal Control Mechanismsbag cells. When stimulated to produce prolonged discharges of action potentials, the bag cells release at least five peptide hormones, one of which, the “egg-laying hormone” (ELH), is responsible for ovulation (Conn and Kaczmarek, 1989). The caudodorsal cells (CDCs) of Lymnaea stagnalis are similar to the bag cells of Aplysia californica. They lie in bilateral clusters of the cerebral ganglion, and they release two structurally related ovulation hormones during prolonged bursts of action potentials (Vreugdenhil et al., 1988). One of these, CDCH-1, contains an amino acid sequence that is 39% identical to the amino acid sequence of Aplysia ELH.
The mechanisms responsible for the body movements and lack of locomotion during egg laying are controversial (see Chase, 2002). According to one view, the cocktail of hormones released by the neuroendocrine cells is fully responsible. These messengers are thought to act directly on the neurons, and perhaps the muscles, that cause the particular motor acts that are described in earlier sections of this article. Evidence for this point of view derives from the observation that injections of bag cell homogenates or CDC homogenates produce a sequence of egg-laying behaviors that closely resembles that which occurs spontaneously. Also, when tested in vitro, some bag cell peptides and some CDC peptides modulate the electrical activities of some identified neurons. Other experiments, however, indicate that internal sensory feedback plays an essential role in mediating egg-laying behaviors. Preventing the movement of eggs down the genital tract, for example, is sufficient to block the expression of certain post-ovulatory head movements in animals that are injected with bag cell homogenates. It is therefore likely that hormonal and neuronal signals combine to produce the integrated response that is egg-laying behavior (Fig. 8).
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