Cercaria milfordensis nov. sp., a Microcercous Trematode Larva from the Marine Bivalve, Mytilus edulis L. with Special Reference to Its Effect on the Host

Abstract

Cercaria milfordensis was found by the writer in April, 1951 in 4 of 93 (4.3%) intertidal specimens of the common mussel, Mytilus edulis, from Mill Neck, Long Island, New York. Subsequently, this larva was found in Mytilus from Milford and Bridgeport, Connecticut, in both intertidal and subtidal areas where 30 of 454 (6.6%) mussels collected in 1951 and 44 of 567 (7.7%) in 1952 were found to be infected. Consultation with Dr. V. L. Loosanoff, Director of the U. S. Shellfish Laboratory at Milford, revealed that in 1936, he also had observed the parasite in M. edulis from Milford Harbor. Dr. Loosanoff kindly made available a collection of slides of adult Mytilus gonad tissue collected at biweekly intervals during the period June 1936-June 1937. Examination of this material showed that 12 of 174 (6.9%) specimens of Mytilus were infected with the microcercous larvae. Previous records of cercariasis in Mytilus spp. are European in origin and pertain to the larvae of three trematode species. Dubois (1901, 1903, 1907, 1909) described larval stages of Gymnophallus margaritarum [= Distomum margaritarum Dubois, 1901] from M. edulis and M. gallo-provincialis. The metacercaria of this species has held particular interest as a cause of formation in Mytilus and other bivalve species, and many conflicting accounts were rendered concerning its biology and identity. Palombi (1924) re-studied the species in M. gallo-provincialis in an attempt to clarify the conflicting opinions of the time and concluded that the pearl trematodes of Jameson (1902) and later authors were identical with the species of Dubois. Cole (1935) described two cercarial forms, Cercaria tenuans and Bucephalus mytili, from M. edulis in British waters; infection with the former species was termed orange-sickness from the orange color imparted to the mantle lobes of infected mussels by masses of orange-pigmented sporocysts. A similar gross appearance is characteristic of mussels heavily infected with the trematode larvae discussed here.