Wednesday 5 June 2019

Calliostoma bullatum: An extinct Pleistocene Snail from northern Sicily found alive on the coast of Mauritania.

The Calliostomatid Snail Calliostoma bullatum is known from Early-Middle Pleistocene (1.8-0.8 million-year-old) deposits around Messina on the northern coast of Sicily, Italy. It is notably larger than any living species in the genus Calliostoma, with a shell reaching about 34 mm in both height and width. Like living members of the genus it is interpreted as having been a deepwater species, associated with the deepwater mound forming colonial Corals Lophelia pertusa and Madrepora oculata, both of which are still extant, though Calliostoma bullatum has not been found in any of the numerous surveys of deepwater Coral Mounds carried out in the Mediterranean and Atlantic, and has therefore been considered to be extinct until now.

In a paper published in the journal Zootaxa on 4 June 2019, Leon Hoffman, Lydia Beuck, and Bart van Heugten of the Marine Research Department at Senckenberg am Meer, Marc Lavaleye of the Royal Netherlands Institute for Sea Research, and André Freiwald, also of the Marine Research Department at Senckenberg am Meer and of Bremen University, describe the discovery of Calliostoma bullatum living on deepwater Coral Mounds on the coast of Mauritania.

The species was found by a remotely operated vehicle from the Research Vessel Maria S. Merian, during a visit to the area in 2010. It was found at a number of locations on the Mauritanian coast; in the Tanoûdêrt Canyon, the Nouamghar Canyon, the Inchiri Canyon, Timiris Mound Complex, the Tioulit Canyon, the Banda Mound Complex, the Tamxat Mound Complex, and the Tiguent Mound Complex. All live specimens were found at depths of between 450 and 652 m, though dead specimens (shells) were observed at depths of as little as 414 m. It was found in association with mound reefs formed by the colonial Corals Lophelia pertusa and Madrepora oculata, the same environment as fossil specimens of the same species. Such Coral Mounds have been extensively surveyed along the Atlantic Coast of Europe, the Mediterranean, the Atlantic Coast of Morocco and Western Sahara, and the islands and seamounts of the northeast Atlantic, without Calliostoma bullatum having previously been identified, suggesting that it is absent from these areas, though the area to the south, along the coast of the Senegambia Region and beyond, is less well surveyed, so the species might be present here.

Calliostoma bullatum on various substrates. (9) Grazing on Hydroids. (10) Elongated, muscular foot supports feeding on polyp tissue (Madrepora oculata). Operculum and dorsal groove on foot indicated by arrow. (11) Muscular foot enables elevation of body above substrate (live Lophelia pertusa). Epipodial sense organs spread directly below the shell (arrow). (12) Feeding on apical portions of a live Madrepora oculata. (13) Two individuals feeding on Lophelia pertusa tissue; note the grazing tracks showing the bare, white coral skeleton devoid of polyp tissue in contrast to the orange to pale-pinkish live portions. (14) Three individuals of different sizes all feeding on epibionts of Lophelia pertusa framework; note the grazing traces on adjacent live Lophelia pertusa portions (see arrow). Tomas Lundälv/Sven Lovén Centre for Marine Infrastructure Tjärnö/University of Gothenburg in Hoffman et al. (2019).

Adult specimens of both Pleistocene and Modern Calliostoma bullatum have about seven slightly convex whorls (excluding the protoconch). These whorls have fine spiral striation, with Modern specimens having more striations than Pleistocene specimens, however striation number is known to vary between populations of other extant members of the genus Calliostoma, so this variation is not considered a bar to classifying the Modern and Pleistocene specimens as the same species.

(7) Specimen of Modern Calliostoma bullatum collected from the coast of Mauritania. (8) Fossil specimen collected from Pleistocene of northern Sicily in 1844, and illustrated by Rodolfo Amando Philippi. Hoffman et al. (2019).

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