Berthella stellata
Philine quadripartita
Tylodina perversa
Odostomella doliolium
Bulla striata
Haminoea hydatis
Aplysia depilans
Cuthona sp.
Hypselodoris sp.
Pseudoceros maximus
Pseudoceros maximus
Prostheceraeus giesbrechtii
Opisthobranchs - Identifying Sea Slugs
Ideally the sea slugs should be photographed in their habitat, this will help in identifying their food and also as a record of the specimen, its shape, colour and form. For small specimens or mesopsammic species collection is the only way of identifying the specimens with certainty. A good eye for detail is necessary so that small differences between specimens can be noted. Above all a small lens and a lot of patience are necessary. Take notes during observation of your material, even if scuba diving a pencil goes a long way and information written down will not be forgotten. Note depth, other identifiable species in the vicinity and spawn deposits.
The following are tips intended to help you narrow the possible suspects. The information given on this and other pages refer to Mediterranean opisthobranchs and the descriptions/photos refer to Maltese material.

The following external features should be noted:

> Is a shell present or not? If present is it external or internal (visible through the transparent body)? If absent, is it possible that the shell is small, internal and not visible through a dark coloured body?
> What is the general shape? Is it flat, elongated, rounded?
> Are there external appendages like cerata, tubercules, rhinophores?
> If rhinophores are present are they retractable, rolled, divided, with some form of ornamentation (e.g. lamellae)?
Solid and robust
Solid but light
Thin and fragile
with flaps
with cerata
Examples of species which are NOT opisthobranchs but may be mistaken for one (platyhelminths including polyclads) are shown below. These species are usually flattish and in water move differently than molluscs, one can say that the movement of the opisthobranchs is more muscular and controlled rather than the 'worm-like' movements of these species.
Other molluscs like Lamellaria sp. may be mistaken for opisthobranchs.
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