Berthella stellata
Philine quadripartita
Runcina capreensis
Aplysia depilans
Tylodina perversa
Thuridilla hopei
Cuthona sp.
Odostomella doliolium
Bulla striata
Hypselodoris sp.
Trapania sp.
Opisthobranchs - The Sea Slugs 2
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 foraminiferans.
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 morphologies.
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 prey.
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 nudibranchs.
Shortcut to Alphabetical list
©2011-2014 All Rights Reserved. Design by Carmel Sammut.
This site is best viewed with Mozilla Firefox
Disclaimer & Privacy policy