Mating of gastropods

The mating of gastropods is a vast and varied topic, because the taxonomic class Gastropoda is very large and diverse, a group comprising sea snails and sea slugs, freshwater snails and land snails and slugs. Gastropods are second only to the class Insecta in terms of total number of species. Some gastropods have separate sexes, others are hermaphroditic. Some hermaphroditic groups have simultaneous hermaphroditism, whereas some sequential hermaphroditism. In addition, numerous very different mating strategies are used within different taxa. This article currently focuses primarily on the mating habits of air-breathing terrestrial slugs. Land slugs can be thought of as land snails that over evolutionary time have either lost the shell completely, have a very reduced external shell, or have retained only internal remnants of a shell. Land slugs are a highly polyphyletic group, which means that many land slug families are not at all closely related to one another. The majority of land slugs are simultaneous hermaphrodites, meaning they possess both male and female reproductive organs that are functional at the same time. Some species regularly self-fertilise. Uniparental reproduction may also occur by apomixis, an asexual process. For the most part, however, land slugs do mate: they find partners, and engage in elaborate courtship rituals before actual sperm transfer takes place. It is common for slugs to mate in a simultaneous reciprocal manner, as occurs in the monophyletic groups Limacoidea and Philomycidae. Limacoidea comprises the family Limacidae (the keelback slugs) and the largest genus of slugs, Deroceras, which contains over 100 known species. Sperm transfer may be external (as in Deroceras) or internal (as in Ariolimax).

Sexual strategy When it comes to the mating behaviour of simultaneous hermaphrodites such as pulmonate land snails and pulmonate land slugs, as well as opisthobranch sea snails and opisthobranch sea slugs, there is the question of which sexual role or roles an individual will adopt in a mating encounter. If the individuals involved both have a preference for the same sexual role, then their mating interests are inherently incompatible. Further conflicts arise because a hermaphroditic individual may not necessarily use received sperm; it may use another partner's, or self-fertilise. According to risk-averse models, when this is the case, it would be more prudent for an individual to invest in eggs. However, alternative views state that the male role is preferred, as males benefit more from multiple matings than females do. Neither strategy is evolutionarily stable, as both sex roles are required for successful reproduction. Because sperm digestion in land slugs reduces the likelihood of a given set of sperm fertilising the partner's eggs, increased sperm investment is selected for. This can lead to a co-evolutionary cycle in which the amount of sperm digested and male allocation both increase until eventually, male and female gametes are equally invested in. The larger and more costly ejaculates become, the more reciprocal sperm transfer is favoured, as each slug receives compensation for its investment. Reciprocity, either simultaneous or serial, appears to alleviate the sexual role conflict, and is a feature of most slug matings.

Finding a mate

As land gastropods such as land snails and land slugs have no sense of hearing and very limited vision, they rely heavily on chemoception, primarily olfaction, and tactile perception. Mate location is also greatly mediated by airborne chemicals. Slugs often raise their head at the detection of pheromones in the air, and then turn to orient themselves and crawl towards the perceived source. Many slugs also exhibit some degree of mucus-trail-following in order to locate a mate. Trail following may be an alternative or complementary strategy to airborne cues, and can constitute a major part of precourtship behaviour. Using mucus trails to find a mate involves the discrimination between conspecific and heterospecific trails. It is therefore advantageous to incorporate species specific cues into trails to facilitate mate finding. In some species, such as banana slugs (Ariolimax), pheromones are secreted together with the mucus to attract mates. It may even be possible for a follower to perform a quality assessment of a potential mate based on its mucus trail. The mucus may reveal information on body size or parasite infection, giving an insight into fecundity. A larger size suggests a slug is highly fertile, whereas parasitism could mean it suffers from decreased egg production or even sterility. Trail following is a complex behavioural pattern which usually involves the active participation of both slugs. The follower keeps very close to the leader once they are in proximity. In species with pronounced trail following, such as Deroceras panormitanum, the leader flattens its tail laterally and waves it from side to side in front of the follower. It is not yet known whether tail waving is a visual cue or whether it wafts chemical attractants in the direction of the follower. Sometimes, the tail is waved between the follower's tentacles and occasionally makes contact, contributing to recognition between the slugs. The leading slug seemingly pauses to wait for the follower, tail waving, if the follower falls too far behind.

Courtship

Once a mate is found, land slugs undergo a prolonged courtship phase prior to copulation. Courtship can and usually does last for several hours. The two individuals get into position along the periphery of an imaginary circle with their heads towards the other's tail, and circle one another. The slugs crawl in a clockwise direction so that their right sides, containing the genital pores, continually face each other. This circling may serve to assess whether the other is ready to mate; the speed of circling generally decreases as courtship progresses. The slugs then lie close together, remaining in the antiparallel configuration. Each protrudes from its genital pore a sarcobelum, a highly manoeuvrable penile structure, with which it strokes its mate.

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