Opisthobranchs are simultaneous hermaphrodites, meaning that they are both male and female at the same time, so mating is usually reciprocal, and both individuals exchange genetic material and lay eggs.
Polycera quadrilineata (Atlantic) in copula
As a living strategy, hermaphroditism has the advantage that any mature individual of a species is a potential partner of any other individual of the same species, something that increases the chances of fertilization, as each individual may (and does) lay eggs. Self-fertilization is possible in many species, but unusual because of the impoverishment of the genetic material involved.
The reproductive organs are located at the right side of the body so coupled individuals must face opposite directions with their right sides together.
Opisthobranchs usually lay their eggs on the animal they prey on, or over any prominent object nearby. As divers know, often happens that the only indicator that opisthobranchs are present in a given place is the presence of their eggmasses, so only after a thorough search in the surroundings the adult animals are found. The size, shape and color of the opisthobranchs’ spawn has a great variation from one species to another, so, it is an aid in the identification and classification of species.
Doris verrucosa with spawn
Opisthobranchs development is often incomplete; there is a larval stage called “veliger”. The “veliger” hatches and becomes part of the plankton. Often the larvae will settle and transform into their adult form ONLY in the presence of the species they prey on. In some species the development is completed within the egg, so the “veliger” is never released. This is known as direct development and, as a result, minute individuals, identical to the adults, emerge from every egg.
In species with planktonic dispersion eggs are small and take a short time to hatch. However, in species with direct development, egg size tends to be large, as it is essential that the egg contains enough food to feed the embryo through the juvenile stage.