Opisthobranchs: what are they and Mediterranean species

The opisthobranchs are one of the marine animal groups that most attract the attention of divers and lovers of underwater life. Do you want to discover what are they, some curiosities and some species of the Mediterranean?

Most photographs are courtesy of the biologist and oceanographer Marc Collell. Visit his Instagram (@mcollell) and enjoy!

OPISTHOBRANCHS: WHAT ARE THEY AND MEDITERRANEAN SPECIES

THE GASTROPODA MOLLUSCS

The opisthobranchs are a group of marine animals included in the gastropod molluscs, along with snails, limpets, slugs and periwinkles, among others.

gasterópodo, pulmonado
Snails and opisthobranchs are included in the same group of molluscs (Picture: Jürgen Schoner, Creative Commons).

The gastropods, which constitute the largest group of molluscs, with about 70,000 living species and 15,000 fossils, are characterised by a torsion process, a process that takes place in the veliger larva stage, whereby the visceral organs rotate up to 180º. As a result of this process, the anus and mantle cavity are located on the anterior side and are opened above the mouth and head.

However, some groups have reversed the process, so that the anus opens on the right or posterior side.

The gastropods have traditionally been classified into three subclasses:

  • Prosobranchia: include most marine snails and some freshwater and terrestrial gastropods. Upon suffering the torsion process, the mantle cavity is anterior and the gill or gills are placed in front of the heart.
  • Opisthobranchs: it is the group that occupies us.
  • Pulmonata: include land snails, freshwater snails and slugs. Instead of gills, they have lungs.

WHAT ARE OPISTHOBRANCHS?

The opisthobranchs include marine slugs, sea hares and marine butterflies, among others. They are popularly known as nudibranchs, but in reality nudibranchs are only a group of opisthobranchs.

The opisthobranchs have totally or partially reversed the torsion process, so that the anus and gill (if any) are on the right or posterior side. They have two pairs of tentacles, and the second is often modified and named rhinophore, so that it presents a set of folds that increases the chemioreceptor surface and that has an appearance of ears. They have shell or not.

Aglaja tricolorata, opistobranquio, nudibranquio
Opisthobranchs have suffered a total or partial process of reversed torsion, so they look as if they had a symmetric body (Picture courtesy of Marc Collell, @mcollell).

The opisthobranchs, at the same time, can be divided into 11 suborders, to emphasise: the nudibranchs, the Sacoglossa and the Anaspidea.

  • Nudibranchs have bare gills and never have shells. Gills may have different morphologies and can be located in different parts of the body. They are carnivorous and feed on other invertebrates and nudibranch eggs.
  • The Sacoglossa have much more distinct forms and may have shell or not. They normally feed on body fluids from seaweed.
  • Anaspidea are popularly called sea hares. They are characterised by having two large lateral expansions in the foot called parapodiums. In addition, they have a very thin inner dorsal shell. Its popular name is due to the fact that rhinophores develop so much that they look like ears.

THE COLOUR OF OPISTHOBRANCHS

If something characterises the opisthobranchia is its variety of colouration, some with very colourful and alarming colours (aposematic coloration) and others with more discrete colouration that are disguised with the substrate (cryptic).

Facelina annulicornis, cripsis, opistobranquio, nudibranquio
Facelina annulicornis is a nudibranch with cryptic colouration (Picture courtesy of Marc Collell, @mcollell)).

SOME CURIOSITIES OF OPISTHOBRANCHS

Some nudibranchs have the ability to incorporate the urticating cells they ingest from anemones and hydrozoa, such as the plumose marine algae (Aeolid). These have elongated papillae on the back, called cerata, in which the nematocytes of the cnidarians, which serve as a defense, accumulate.

Hermissenda crassicornis is a nudibranch with cerata (Picture: Magnus Manske, Creative Commons).

There are photosynthetic sacoglossal opisthobranchs. This is the case of Elysia chlorotica. This mollusc from North America has the capacity to perform photosynthesis because it incorporates the chloroplasts of the algae it consumes.

opistobranquio, sacoglossa, elysia chlorotica
Elysia chlorotica has the ability to perform photosynthesis (Picture: Karen N. Pelletreau et al., Creative Commons)

9 EXAMPLES OF MEDITERRANEAN OPISTHOBRANCHS

Aplysia depilans 

Aplysia depilans, opistobranquio, anaspidea, opistobranquio mediterráneo
Aplysia depilans (Picture courtesy of Marc Collell, @mcollell).

Aplysia depilans is an opisthobranch of the Anaspidea group that is distributed by all the Mediterranean. This sea hare, which is one of the largest in European waters, can reach 30 cm and weigh 1 kg. Its coloration may be brown or greenish, with white or yellowish spots. It lives in shallow seafloor of the coast, where there are abundant algae, from which it feeds.

Cyerce cristallina

Cyerce cristallina, opistobranquio, sacoglossa, opistobranquio mediterráneo
Cyerce cristallina (Picture courtesy of Marc Collell, @mcollell).

Cyerce cristallina is an opisthobranch of the Sacoglossa group that is distributed throughout the Mediterranean, in addition to the Atlantic. It has a maximum length of 3.5 cm. Its cream-coloured body may be white, brown or reddish. It is a herbivorous species, which feeds on algae. It has the ability to release the cerata if it feels threatened, which continue to move to distract the opponent.

Placida verticilata

Placida verticillata, opistobranquio, sacoglossa, opistobranquio mediterráneo
Placida verticilata (Picture courtesy of Marc Collell, @mcollell).

Placida verticilata is an opisthobranch of the Sacoglossa group mentioned in the coasts of English Channel, Canary Islands, Azores, Madeira, the Caribbean and the Iberian Peninsula. It measures between 0.7 and 1 cm in length. It has the body full of chloroplasts, which gives it an olive-green colour, with which it does the photosynthesis. It lives on top of the Codium seaweed, from which it feeds.

Peltodoris atromaculata

peltodoris atromaculata opistobranquio, nudibranquio, opistobranquio mediterraneo
Peltodoris atromaculata (Picture: Anders Finn Jørgensen, Creative Commons)

Peltodoris atromaculata is a nudibranch, being one of the most abundant species of the Mediterranean, although it also lives in other zones. Its colour recalls a cow. It can measure up to 12 cm in length. It usually lives on rocky bottoms with low lighting, at the entrance of caves and in coralline backgrounds. It feeds on the sponge Petrosia fisciformis.

Dondice banylensis

Dondice banyulensis, opistobranquio, nudibranquio, opistobranquio mediterráneo
Dondice banyulensis (Picture courtesy of Marc Collell, @mcollell).

Dondice banylensis is a nudibranch of the Mediterranean, although it has recently been observed in the Atlantic zone of the Strait of Gibraltar. This nudibranch, which can reach 7 cm in length, lives on a wide variety of substrates and habitats. When it feels threatened, it extends its cerata to the sides to defend itself.

Doto floridicola

Doto floridicola, opistobranquio, nudibranquio, opistobranquio mediterráneo
Doto floridicola (Picture courtesy of Marc Collell, @mcollell).

Doto floridicola is a nudibranch of southern Europe, although it has been observed in the British coasts and Ireland recently. With translucent body, can exceed a centimetre in length. It usually lives with colonies of hydrozoans such as Aglaophenia and Synthecium. The lays are shaped like a ribbon and placed on top of the hydrozoans.

Felimare picta

Felimare picta, puesta nudibranquio, opistobranquio, nudibranquio, opistobranquio mediterráneo
Felimare picta (Picture courtesy of Marc Collell, @mcollell).

Felimare picta is a nudibranch that, in addition to other areas, lives on the entire coast of the Iberian Peninsula, both Mediterranean and Atlantic. With a length that can exceed 12 cm, even 20 cm  in some specimens, this nudibranch lives on rocky walls with abundant algae, sponges and other invertebrates, especially dimly lit.

Polycera quadrilineata

Polycera quadrilineata, opistobranquio, nudibranquio, opistobranquio mediterráneo
Polycera qudrilineata (Picture courtesy of Marc Collell, @mcollell).

Polycera quadrilineata is a nudibranch that inhabits the waters between the Atlantic coasts of northern Europe to the Mediterranean. In the Mediterranean they measure about 2 cm, although outside of it they can reach 4 cm. It lives in walls of rocks with little illumination and abundant briozoa, of which it feeds.

Flabellina afinis

Flabellina affinis, puesta nudibranquio, opistobranquio, nudibranquio, opistobranquio mediterráneo
Flabellina affinis (Picture courtesy of Marc Collell, @mcollell).

Flabellina afinis is a very abundant nudibranch in all waters of Europe. It can measure up to 5 cm in length. This colourful nudibranch lives normally above the colonies of hydrozoans of the genus Eudendrium, from which it feeds. The laying, with the shape of an undulating cord, is laid on top of the hydrozoans.

If you have not had enough, watch this video:

Now it is high time you take your snorkel gear or dive and dare to look for opisthobranchs. Of course, you must have patience… What other species, besides those mentioned here, have you seen?

REFERENCES

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