Opisthobranch mollusks feed on a wide variety of organisms that generally are not food for other animals. Most opisthobranchs are highly specialized predatory herbivores or carnivores; they have a very selective taste for the organisms that they prey on, which are often limited to a single genus of plant or animal. Generally, the species belonging to each family of opisthobranchs eat similar prey types.
Their food specialization is such that, for some species, the metamorphosis of the larval stage to adulthood is triggered by the presence in the environment of the species on which they prey. Thus, the life of an opisthobranch may be linked to that of its prey and, therefore, they are classified in two categories:
- Opisthobranchs that feed on ephemeral preys, such as hydrozoans, tend to be short lived and may produce several generations every year.
- Opisthobranchs that feed on non-seasonal preys, like sponges and dead man’s fingers, can live for a year or more, but little is known.
Most opisthobranchs have a teeth ribbon called radula. It consists of a very characteristic structure of chitinous teeth and it is extremely important from the taxonomical and ecological point of view.
Taxonomically, the radula shows species-specific differences in the number of rows of teeth composing the ribbon, as well as the specific form of each tooth. These features, once quantified, become the “fingerprint” of each individual species and it is expressed by a “radular formula”.
Unfortunately, the use of radular analysis is not always conclusive for species identification: Within a particular genus, even within a particular family, there can be a great variation in the number of teeth and/or morphology of each tooth. Therefore radular analysis is a very important taxonomic aid, but should not be used as the only tool for species identification.
Ecologically, the radula may be a species-specific tool adapted to the animal’s favorite food. Except in Anaspidea, Sacoglossa and some Cephalaspidea orders, all other opisthobranchs are grazing carnivorous. Usually food preferences are very specific, and the shape of the radula corresponds well with these tastes. Many opisthobranchs have a pair of jaws, preceding the radula, which allow them to capture grazed food.
Doridacean nudibranchs have a wide radular area, with many teeth to gnaw and swallow the bits of sponges they prey on. By contrast, the phanerobranch doridacean’s radula is somewhat narrower due to the filamentary and articulated nature of their food. With certain exceptions, no dorid has strong jaws. By contrast, most aeolidacean nudibranchs and some dendronotaceans feed on hydrozoans and bryozoans, so they have a narrow radula with strong jaws. Within any of these groups there are wide variations in the radula, generally linked to food preferences.
The opisthobranchs without radula feed in different ways: some eat their prey whole (as the genus Melibea), others suck the cell contents (as the genus Doriopsilla, Dendrodoris and Phyllidiopsis) with the help of a highly specialized “buccal pump”.
Within the suborder Sacoglossa, old, worn teeth fall and are stored in a space shaped like sac in the mouth, origin of the name of the group. It’s not yet understood what the advantage of saving the old teeth is. The radula of this group consists of a single row of teeth situated in a position that allows “puncturing” the capsules of other opisthobranchs eggs or algal cells.
To supplement their diet, some Sacoglossa (such as Elysia timida) retain chloroplasts from the algae they feed on (chloroplasts are intracellular organelles responsible for photosynthesis in plants) so they continue to operate within their digestive system (which usually has a transparent skin) and producing sugars that are absorbed and consumed by the animal. Some tropical aeolidaceans and dendronotaceans can have symbiotic algae inside their tissues (as do tropical corals), with a function similar to that of chloroplasts.
Some Cephalaspidea of the family Aglajidae have no radula, but instead have a very muscular buccal mass that allows fast attack and swallow their prey whole.
The Notaspidea have strong jaws and a small and fine radula, usually composed of a 200 teeth arranged in a single row. Little is known about their preferred prey, but some species have been found associated with certain tunicates and sponges.
The Anaspidea have a long and wide radula suitable for grazing on algae on which they feed.
Some genus of opisthobranchs and their food
- Hydrozoans – Doto, Hancockia, Flabellina, Facelina, Godiva, Caloria, Eubranchus, Cuthona, Embletonia, Tergipes
- Anemones – Aeolidia, Aeolidiella, Spurilla,
- Soft corals – Tritonia
- Sea pens – Armina.
- Gorgonians – Tritonia.
- Calcareous sponges – Aegires
- Non calcareous sponges – Cadlina, Chromodoris, Hypselodoris, Rostanga, Doris, Aldisa, Discodoris, Geitodoris, Platydoris, Jorunna, Thordisa, Peltodoris, Dendrodoris, Doriopsilla, Phyllidia.
- Bryozoans – Okenia, Onchidoris, Polycera, Tambja, Limacia, Crimora, Janolus
- Ascidians – Okenia
- Planctonic crustaceans – ¿Tethys?
- Fish and cephalopod eggs – Calma
- Opisthobranch eggs – Favorinus
- Other opisthobranchs – Roboastra, Gymnodoris, ¿Facelina?
- Kamptozoans (Entoprocta) – Trapania