The Turbellarian Paravortex Parasitic in Scrobiculariid Bivalves: New Records from South-West England

Extract

The rhabdocoele turbellarian Paravortex scrobiculariae (Graff) was first described, but not named, by Villot (1878) who discovered this parasite living in the gut of Abra tennis (Montagu) in Brittany. In naming the species, Graff (1882) did not add to Villot's description. Wahl (1906) erected the genus Paravortex to include this form which he had found in Scrobicularia plana (da Costa) and Venerupis decussata (L.) at Trieste and Naples. The first record of P. scrobiculariae in British waters was supplied by Freeman (1957) who, working at Whitstable, Kent, found 8 specimens in 3 out of a sample of 40 S. plana he examined. No further British record appears to have been made.

A recent survey has shown Paravortex to be common and widespread in S. plana inhabiting the estuaries of south-west England. As noted by previous workers, Paravortex can be readily seen through the semi-transparent gut wall of its host on account of its pinkish colour; observation is facilitated if the bivalve is maintained in clean sea water for 3–4 days to allow evacuation of some of the gut contents and is most easily performed during the winter months when gametes are not present in quantity. In this study, counts of the Paravortex were made by removing the gut from the surrounding foot tissues and releasing the Paravortex by cutting the gut longitudinally. From each locality, 20–30 S. plana, all within the length range of 30–50 mm, were examined. Paravortex is viviparous, the newly-hatched young being 120–150 μm long; at this stage they are transparent and are easily overlooked amongst the gut contents; thus counts relate chiefly to adults 0·5–1·5 mm in length.