Eubranchus exiguus

Eubranchus exiguus (Alder & Hancock, 1849)

Taxonomic note: The phylogenetic analyses performed by Cella et al. (2016) revealed that the traditional Tergipedidae family is polyphyletic and belongs to a larger monophyletic clade including members of the traditional families Eubranchidae, Fionidae and Calmidae; this was an unexpected result, since the validity of these taxa and their distinctness from the Tergipedidae was never questioned before. They proposed to join the families Tergipedidae, Eubranchidae, Calmidae and Fionidae under the name of Fionidae. This decision has been reinterpreted and completed in the paper by Korshunova et al. (2017) because it obviated evident morphological and molecular aspects.

• Aeolis exigua (Alder & Hancock, 1848)
• Eolis exigua Alder & Hancock, 1848 (original)
• Galvina exigua (Alder & Hancock, 1848)
• Tergipes fustifer Lovén, 1846

Description
The specific name of this animal refers to its small size, usually not exceeding 6 mm. The body has a whitish colour, somewhat yellowish, which is masked by the numerous greenish spots of varying size that appear in the back and sides of the body and between the bases of the cerata, where they are somewhat darker. These spots can also be located on the back of the head in larger size specimens. The heart area is located on the back, between the second or third group of cerata, depending on the size of the animal. Immediately behind and slightly to the right there is the anal opening. Oral palps are short and have the same colour as the body and may also have some greenish spots. The rhinophores are smooth and semi-transparent and have some white spots, which also often occur between the bases of them. The end of the rhinophores may have a greenish-brown subapical ring. There are usually 6 to 7 groups of cerata in both sides of the back, but usually only the first two groups have more than one cerata, while the rest have just one. Cerata have the ability to contract, acquiring a more or less rounded shape, and to stretch, and then it can be observed having two expansions, a larger volume one in the centre of cerata and a smaller subapical one. The cerata are semitransparent and have greenish spots that sometimes are concentrically aligned; a subapical pale brown ring is also observed. The digestive gland in the cerata is coloured light brown and is usually wider at the greater expansion of the cera. The foot is somewhat broadened in its anterior zone, forming a pair of short, rounded propodial palps.

Biology
E. exiguus is a aeolidacean that usually lives on different species of algae (Codium spp) having hydrozoans colonies as epibionts. Because of its small size and mimicry, individuals of E. exiguus are mostly invisible on the algae. Numerous species of hydroids (Abietinaria abietina, Bougainvillia ramosa, Cordyophora lacustrus, Coryne sp., Eudendrium sp., Halecium halecinum , Hydrallmania falcata, Kirchenpaueria pinnata, several species of Laomedea, Obelia, Plumularia catharina and Tubularia indivisa) have been cited as a substrate for this species, which has also been observed on shells of Mytilus galloprovincialis with epibiontic hydroids of the genus Obelia and on Posidonia oceanica leaves. It is suggested that feeds mainly on polyps of Obelia (Schmekel & Portmann, 1982). The spawn of this small Eubranchus is kidney shaped and measures 1.5 mm. The eggs are white, have a size of about 90 microns, and are tightly placed within the organic matrix of the spawn.

Etymology

• “Eu” derives from Greek word for “good”, but also “original” or “primitive”. “Branchus” derives from “branchos”, gill.
• “exiguus” from Latin word for “insignificant”, “insufficient”.

Distribution
This species is distributed throughout all the coasts of Europe, from Norway to the Mediterranean Sea. There are doubtful reports from Greenland and the Atlantic coast of the United States that should be validated. In the Iberian Peninsula it has been observed in the Bay of Biscay, Galicia, Portugal, Straits of Gibraltar and the Catalan coast. In Catalonia it has been reported in Llafranc, Tossa de Mar, Blanes, Sitges and the port of Tarragona.

Known georeferenced records of the species: Eubranchus exiguus

Sources:
OBIS GROC 2010-2011 Enric Madrenas João Pedro Silva Bernard Picton GBIF	OPK VIMAR Manuel Ballesteros M@re Nostrum Altres fonts Marine Regions

Similar species
Eubranchus capellinii, with a trapezoidal spot behind the rhinophores.

More pictures

Eubranchus exiguus 5mm @ Sgeir Mhor, Skye, Scotland in 12 m by Jim Anderson

Eubranchus exiguus @ Thau, France by Pascal Girard

Eubranchus exiguus @ Ponton De La Bordelaise, Bassin De Thau by Pascal Girard

Eubranchus exiguus @ France by Pascal Girard

Eubranchus exiguus @ France by Pascal Girard

Eubranchus exiguus @ France by Pascal Girard

Eubranchus exiguus @ France by Pascal Girard

Bibliography

1246567 Eubranchus exiguus 1 eunomia-revista-en-cultura-de-la-legalidad 50 creator asc 1 579 https://opistobranquis.info/wp-content/plugins/zotpress/
Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

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