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classification is no longer valid.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.