Sea Slug - Reproduction

Reproduction

Sea slugs mating habits are quite unique in that they are hermaphrodites, equipped with both male and female reproductive organs. Although these subtidal mollusk live alone for most of the year, during the mating season in the summer the sea slugs congregate in breeding colonies. In these mating pools the sea slugs climb all over each other transferring sperm from one to another. This goes on for a few days every year.

Even more amazing than the congregation of sea slugs is how they manage to find each other. Considering that they are virtually blind, the mollusk rely on a pheromone cocktail to find each other and mate. The solution produced by the sea slugs contain substances which tell other sea slugs around them that they are in need of more sperm. After one sea slug creates and releases the compound chemical solution, it makes all the sea slugs irresistible to one another. So, the sea slugs find each other on the ocean floor neither by sight nor sound, but by chemical attraction to one another.

During the mating process, the sea slugs can mate in single pairs of two with one playing the role of male and the other of female, but they usually form chains of mating slugs. This is the only way that the sea slugs can act as both male and females. In these chains, the mollusk at the front is playing the role of female, the one at the end male, and all the others in-between play both roles. This is the only way in which the sea slugs can be receiving sperm as it passes its own onto a third party. This is due to the fact that the sea slugs have their reproductive organs on different sides. The male organ is found on the right side of the head while the female organ is found under the sea slug in its mantle cavity. This makes it so that in a pair of two mating sea slugs it is impossible for either of the slugs to act as both male and female at the same time.

Sea slug copulation can be either a reciprocal sperm exchange or a controlled reciprocal sperm exchange. Mating in many sea slugs appears to be based on controlled sperm trading between the individual sea slugs, but there are some species in which mating appears to occur freely due to the unconditional willingness of both sea slugs to give and receive sperm for reproduction. In unconditional mating the two sea slugs attract, meet and mate freely exchanging their sperm to one another.

In controlled reciprocal sperm exchange, however, there appears to be a lack of confidence between the sea slugs and mating takes longer and takes certain steps and precautions by each animal involved. It appears that during this mating process one of the slugs first releases a small package of its sperm to the other and waits for a response from the other sea slug. The other sea slug then reciprocates the action and sends its own package of sperm. This is done through hypodermic transfers onto the dorsal surfaces of each other. These transfers are always done in balanced pairs and are only interrupted by synchronized circling movements. If the transfers are reciprocated, then a final step is taken where sperm transfer is done by standard insemination through the female organ. If the initial transfer is not reciprocated then mating stops and the sea slugs move on. It has also been noted that reciprocal matings last longer and are more common than unilateral matings in which only one of the sea slugs receives sperm from the other.