Opistobrachs, commonly called “sea slugs”, are one of the most attractive underwater animals for photographers and researchers. Why are they so special? They have been seen from pole to pole and from subtidal down to the abyss. Their morphological and biological variations seem unlimited. Sea slugs have specialized their feeding on organisms that are generally avoided by other animals, including algae, sponges and cnidarians (anemones, hydrozoa, etc), using their prey´s defense system for their own.
Almost all opisthobranchs are hermaphrodite with very complex and elaborated genital systems, which are used for identifying species. Specimens join their right flanks in a head to tail position, as shown in the picture above. In this case, Thecacera darwini has a long eversible ejaculatory duct. Generally, each individual may copulate severals times and store the sperm of many individuals. Why do they store the sperm? Imagine you are the size of a sea slug (12 cm long), you are underwater moving as fast as a garden slug. Is not easy to find another slug in the vast space of the Ocean, even more if it needs to be of the same specie and hypothetically, different sex. This is where their hermaphrodite condition and need to store sperm stems from. Just one encounter in a life with another individual of the same specie, can guarantee offsprings during several times.
The picture above shows something very interesting, because the pattern of the black spots over the body does not match between both individuals. This is usually an indicator of different species, however only equal individuals can breed and generate viable life. Therefore, we can be fairly sure that both belong to the same specie, without doing a taxonomic key.
-For the text: Marine Benthic Fauna of Chilean Patagonia (Vreni Häussemann & Günter Försterra editors).