Atrina fragilis (Pennant, 1777)
Orkney Islands, Irish Sea & English Channel to Mauritania, Mediterranean. The species lives deeply burrowed in sediments at depths from 30 to 600m.
Above and below: Málaga, Andalucia, S. Spain. 215mm.
Synonyms: borealis, elegans, laevis, truncata
Original taxon: Pinna fragilis.
Pinna fragilis drawn in T. Pennant: British zoology vol.4, London 1777, via BHL.
 
Pennant: «  Pinna with a very thin semi-pellucid whitish shell, most opake near the apex. Marked on the surface with longitudinal slender ribs, roughened with concave scales ; and the whole traversed by innumerable fine striae. In young shells the ribs and scales are almost obsolete. The valves of lesser transverse diameter. The largest about five inches and a half long ; and three and a quarter broad in the broadest part. The figure is of a broader specimen than usual. Dredged up at Weymouth. From the Portland cabinet. » – vol. 4 p. 114.
“Broad” specimen dredged at 150m deep, off Almería, Andalucia. 169mm.
Shells from Gulf of Biscay.
122m deep, south of La Rochelle, Charente-maritime, W. France. 227-244mm.
Formerly abundant in Scotland and southern Cornwall (Solandt, 2003), but victim of trawls and dredges, the species is now protected in many countries.
Off Doon Bay, southeastern coast of Rathlin island, north of Ballycastle, Antrim County, North Ireland. Original picture provided by B. Picton for iNaturalist – (CC BY).
Variations in specimens from British Isles.
From top to bottom: Pinna ingens from Hebrides, Devonshire and north Ireland; Pinna elegans from the Devonshire; Pinna fragilis from Plymouth, Dorsetshire and Sussex. This last one looks terribly like an Atrina pectinata Lin.
Drawings from a plate of T. Brown: Illustrations of the recent conchology of Great Britain and Ireland, Edinburgh & London 1844, via BHL.
“Pinna elegans”.
Marina di Ravenna, Emilia-Romagna, NE. Italy. 290mm.
« The name Pinna pectinata or Atrina pectinata refers to an Indo-Pacific species, but has long been misapplied to the European species now known as Atrina fragilis. All usages of “Pinna” pectinata or Atrina pectinata for a European species are erroneous. » (WoRMS). It is true that the angled and lengthened shells of young fragilis can be confused with some thin specimens of the Indo-Pacific pectinata Linné.

Juvenile from Leucate, Occitania, S. France. 90mm.
Original picture provided by S. Clanzig (FR).
(CC BY-NC-SA)
The byssus in fragilis.

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