Turbonilla elegantissima (Montagu), a parasitic opisthobranch

Abstract

Turbonilla elegantissima (Montagu), though one of the larger and more familiar members of the family Pyramidellidae, is known only by the shell and the appearance of the animal when crawling. Forbes & Hanley (1853) and Jeffreys (1867) state that it is common around the British Isles, and that its foreign distribution is from Tromsö to the Mediterranean. In the fauna list of the Marine Biological Association (1931) it is recorded as T. lactea (L.) ‘occasionally under stones, particularly where there is a certain amount of silt’ from low water to a depth of 10 fathoms, and, in the Salcombe estuary, ‘very common on the Zostera south of Pilworthy Pt., (Allen & Todd, 1900)’. Actually this opisthobranch has its own extremely specialized habitat, as do, apparently, all other members of its family (Fretter & Graham, 1949): like them it is an ectoparasite, restricted to the vicinity of its host. Specimens have been collected from the laminarian zone along the south coast of Devon and Cornwall, the largest number from Church Reef, Wembury, where silt and sand around boulders and between shale ledges provide suitable niches for such sedentary worms as Audouinia tentaculata and Amphitrite gracilis. When covered by the tide the tentacles of these polychaetes extend through the silt and sand and spread over the surface of the rocks for a considerable distance, and it is in the vicinity of these tentacles that Turbonilla lurks to suck liquids from them by means of a long proboscis (Fig. 1). Audouinia and Amphitrite were also recorded at Pilworthy Pt., but their association with Turbonilla was then unknown.