The Pyramidelloidea which recently have been transferred to the lower heterobranchia are
briefly mentioned. The pyramideliids may be mistaken for prosobranchs but they are minute
parasitic snail-like molluscs with thick external shells but which lack an operculum (the lid blocking
shell entry). An external shell is also present in other members of the lower heterobranchia,
previously considered as members of the opisthobranchia: Acteonidae, Ringiculidae and
Rissoelloidea. A thin external shell is present in a number of other opisthobranchs most commonly
in the clade Cephalaspidea.
The Cephalaspidea consists of molluscs with external or internal
shells as well as species with no shell. In general the shells are thin
and often transparent. Some like Bulla can accommodate the entire
animal in the shell, whereas in others (Philine, Chelidonura) the shell is
reduced and internal. The cephalaspidean head is generally flattened
(cephalic shield) and foot often bearing parapodial lobes which may be
used for short swimming bursts. Many cephalaspideans can be found
in fine sand or close to sandy areas, where they burrow to hide. Many
species are carnivorous feeding on small polychaetes, bivalves and
The Thecosomata and Gymnosomata, commonly known as the pteropods, are a group of pelagic molluscs which are rarely
encountered and often lack the variety of bright colours normally associated with the opisthobranchs.
The Umbraculida consists of 2
families, the Umbraculidae and the
Tylodinidae. These species have a
large patelliform shell and live on
sponges. In Tylodina perversa the
colour is a bright yellow and the
mollusc spends its life burrowing
and eating on the bright yellow
sponge Aplysina aerophoba.
The Acochlidiacea group is a
small group of very small
opisthobranchs some of which are
the only freshwater
opisthobranchs recorded. These
mesopsammic animals are shell less
but may have minute spicules
present within the body. They are
carnivorous and feed on minute
organisms found in sand
The Aplysiomorpha are a small group but boast the largest members, the sea-hare or serduq
il-bahar in Maltese (Aplysia sp.) which can grow to around 30cm. Members of Aplysiomorpha are
herbivorous and are often found in very shallow water, often a few centimetres below the sea
surface feeding on brown algae, but they are also found in deeper water feeding on red algae. A
few species have a reduced internal shell.
The Sacoglossa is a large clade of medium to small opisthobranchs. The majority of species are
shell-less but a few have an external 'gastropod' shell (Oxynoe) or a 'bivalve' shell (in the
Indo-Polynesian Julia sp.). Sacoglossan species are also herbivorous, feeding mainly on green
algae, but an interesting aspect of a number of species is that they sequester chloroplasts from
the plant material and utilise their photosynthetic ability to generate sugars, replenishing the
chloroplasts from time to time. These opisthobranchs are green in colour by virtue of the trapped
chloroplasts inside their body.
The Runcinacea are a small group of minute shell-less opisthobranchs considered close to the
Cephalaspidea. Runciniid molluscs are probably herbivorous and are often found in shallow water
with other members of the Sacoglossa.
The Nudipleura is a large group of morphologically diverse opisthobranchs and can be divided further into 2 clades, the
Pleurobranchomorpha and the Nudibranchia.
The Pleurobranchomorpha is a small group with a reduced internal shell. They were previously
classified in the order Notaspidea and typically have a large notum under which a side-gill can be
seen. Some species (Berthella sp.) are known to secrete sulphuric acid and taste unpleasant to
fish, their main predators.
Members of the clade Nudibranchia are the best known
opisthobranchs, due perhaps to their intense coloration, variety of
form and the fact that they stand out when on their prey (aeoliids in
particular). The nudibranchs (= naked gill) are subdivided into several
families and the classification has undergone several changes
recently. The families are subdivided mainly on the external
The typical dorids (superfamily Doridoidea) feed on sponges aand are
often well camouflaged. Some have the ability to sequester chemicals
synthesized by sponges which render them unappetising to predators.
The superfamily Phyllidoidea also feed on sponges but unusually for a
mollusc they lack a radula (rasp like teeth) and eat by sucking on they
The subclade Aeolidida (the aeolids) is a large group of nudibranchs
that generally feed on hydroids. The aeolids digest the hydroids
except for the cnidocytes (or nematocytes) and store these at the
tip of the cerata using them as defence. The phenomenon of trapping
cnidocytes is called kleptocnidae is known only in this group of