Atagema gibba

Atagema gibba  Pruvot-Fol, 1951

• Doris setigera Delle Chiaje, 1836

Description
This is a large dorid nudibranch that can grow to at least 68 mm in length (Thompson & Brown, 1984). The body is broad and flattened, with a velvety texture due to the presence of numerous small, spiculated tubercles on the mantle. These tubercles are mostly whitish and form a distinctive reticulate (net-like) pattern against the chocolate-brown background. Some translucent brown tubercles are also scattered across the darker regions of the mantle (Wilson, 2004). A key feature of this species is the presence of a conspicuous elevated, zigzag pallial ridge running down the midline from between the rhinophores to the gill pocket. The mantle margin is loosely crenulated. The rhinophores of A. gibba are short and encased in trumpet-shaped sheaths. The gill pocket, located posteriorly, is 5-lipped and houses a retractable branchial plume. The head bears finger-like oral tentacles used for sensory perception (Picton & Morrow, 1994).

Biology
This species is typically found on steep rock faces at depths of 8–20 meters. It is associated with sponge-covered substrates, though its exact diet remains poorly understood. Observations in Ria de Arousa, Galicia, Spain (Almón et al, 2014), found specimens on the sponge Haliclona cinerea. Additionally, the nudibranch appears to exhibit a camouflage patern that often matches the color of the sponge, suggesting the sponge is a probable feeding source for the nudibranch (Almón et al., 2014). The egg mass consists of a ruffled yellow ribbon arranged in a spiral of approximately 1.5 turns. The large size of the spawn relative to the body is due to the swelling of the protective protein matrix in seawater (DORIS database).

Etymology

• Atagema, derived from Greek word “atagêma,” meaning disorderly or confused, possibly referencing the species’ cryptic dorsal morphology.
• Gibba. From Latin, hunch, because of the protuberance that runs along its dorsum and that differentiates it from other species. Colloquially it is called a “dromedary sea slug”.

Distribution
This is considered a rare species with a limited known distribution, as there are very few reports of this species since its description in 1951: It has been recorded from the Lizard Peninsula and the Isles of Scilly in southwestern England, as well as from Banyuls on the Mediterranean coast of France (Thompson & Brown, 1984). Additional records exist from the Channel Islands (Jersey), Normandy (Cherbourg), and Brittany (Roscoff and Trégastel). Two specimens in the island of Sálvora (Ria de Arosa, Galicia, Spain) and numerous specimens in the island Rúa (Ria de Arosa, Galicia, Spain) suggest a stable population in the area (GEMM; Almón et al., 2014). There are very few reports in the western Mediterranean: one in Sardinia and another two from Banyuls-sur-Mer, on the Mediterranean coast of France (Pruvot-Fol, 1951).

More pictures

Atagema gibba @ Poulziorec, France 14082010 by Jean Michel Crouzet

Atagema gibba @ Jersey by Nicolas Jouault

Atagema gibba @ Jersey by Nicolas Jouault

Atagema gibba @ Ouessant, baie de Lampaul, France by François Roche

Atagema gibba @ Jersey by Nicolas Jouault

Atagema gibba egg spawn @ Galicia, Spain by Almón et al (2014)

Atagema gibba @ Sálvora by Jacinto Pérez Dieste

Bibliography

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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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