Opisthobranchs

The opisthobranchs are a group of marine molluscs with a very diverse external morphology. These molluscs are hermaphrodites and reproduce by laying an egg mass, often near or on their food, which in the majority of cases develops into a planktonic veliger larva. Eventually the veliger larvae settle on their favourite food and start development into the respective species. In a few species direct development occurs in which a minute opisthobranch emerges from the egg (e.g. in Cadlina laevis). In a few cases and under specific conditions (poor oxygen concentration) poecilogonus development occurs in which both larvae and juveniles hatch from the same egg mass (e.g. in Berghia verrucicornis). Many species of opisthobranchs exhibit a variety of colours and shapes and these features usually attract attention to these interesting molluscs.

The classical Opisthobranchia, created by Milne-Edwards, has in the last five or so years under some major revisions and the classification is no longer valid. The subclass opisthobranchia was divided into 9 orders: Bullomorpha/Cephalaspidea (including the Runcinacea), Pyramidellomorpha, Thecosomata, Gymnosomata, Aplysiomorpha/Anaspidea, Pleurobranchomorpha/Notaspidea, Acochlidiacea, Sacoglossa and Nudibranchia. However DNA analysis and comparison between molluscan taxa has revolutionised the classification. According to Bouchet & Rocroi (2005) the 'opisthobranch' molluscs are currently divided into 7 clades (Cephalaspidea, Thecosomata, Gymnosomata, Aplysiomorpha, Sacoglossa, Umbraculida and Nudipleura) and two groups (Acochlidiacea and Cylindrobullida). Later Jörger et al. (2010) has modified some groups, but expect further modifications as more studies are carried out. The classification adopted here is different and convenient but classification higher than family (and occasionally genera) are to be checked with other more appropriate sources.

In the following pages you will find some information on the different clades and groups that make up the opisthobranch molluscs. Many of the identifications have been made by Dr. Antonio Perrone and to him I am indebited for the enormous help he has given during the years I was studying the opisthobranchs. I have made a few tentative identifications of a few other species. Whilst all care has been taken to avoid misidentification any errors present are solely mine.