Mating of gastropods
The majority of land slugs are simultaneous hermaphrodites, meaning they possess both male and female reproductive organs that are functional at the same time.
[1] 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.
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.
[7] 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.
[9] 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.
[10] Trail following may be an alternative or complementary strategy to airborne cues, and can constitute a major part of precourtship behaviour.
A larger size suggests a slug is highly fertile,[11] whereas parasitism could mean it suffers from decreased egg production or even sterility.
[15] These secretions have several possible functions, including physiological stimulation and synchronisation,[3] partner labelling[16] or manipulation.
It is also during courtship that the ejaculate is prepared; sperm flows from storage into the penis and, in some species, is assembled into packages such as spermatophores.
Mating refers to the phase of sperm transfer and lasts from the beginning of penis eversion to the moment the genitalia lose contact with the other slug.
This manner of mating requires that the pair of genitalia be exactly opposed prior to copulation, a task made difficult in slugs as they have no sense of hearing and very limited vision.
The everted penis is several fold larger than its usual size, and appears as a bulbous, transparent structure.
At least one species, the grey field slug (Deroceras reticulatum), shows a primitive degree of sperm packaging.
In most cases, both partners donate and receive simultaneously via intertwined penises; unilateral sperm transfer is only an occasional occurrence.
When an individual of a species with external sperm transfer lacks a penis, it can still reproduce uniparenterally, such as by self-fertilisation, provided it possesses the other components of a normal reproductive system.
When an appending penial gland, an accessory structure part of the male reproductive system, is present, it is also briefly everted during mating.
These secretions, like those of the sarcobelum during courtship, have several possible functions including acting as an allohormone which manipulates the partner to increase chances of paternity.
[18] This may be achieved through inhibiting additional mating, changing sperm transport or storage, or increasing egg laying in the partner.
A pair of slugs climb a tree or wall to an overhang and suspend together head down from a 10–25 cm long mucus strand before everting their penises and exchanging sperm in mid air.
In other cases, sperm transfer is internal and takes place by inserting the everted penis into the partner slug.
[28] Only a very small percentage escape to higher regions of the female reproductive system for storage (in the spermatheca) and later fertilisation.
Apophallation may be made necessary in order for separation to occur by the partner tightly gripping the penis via a special muscle of the female reproductive tract.
The amputee is at least prevented from mating in the future as a male, which could divert resources from producing eggs with the amputator's sperm.
Noel et al.[29] concluded that both outbred and inbred populations of Physa acuta can efficiently purge deleterious mutations.
