The ciliate Ancistrocoma pelseneeri was described by Chatton and Lwoff in 1926 as a symbiont of the clam Macoma balthica in France. The parasite was redescribed as Parachaenia myae, from the introduced Softshell Clam (Mya arenaria) in San Francisco Bay. Kozloff (1946) found that 'P. myae' was identical to Ancistrocoma pelseneeri and found it in Mya arenaria, Limecola petalum, (introduced, reported as Macoma balthica) and from the natives Macoma inquinata. M. nasuta, M. secta, and Cryptomya californica (California Softshell Clam). It is presumed to have been introduced from the Atlantic Ocean with the Softshell Clam. It has been reported in Gulf of Campeche, Mexico, the French side of the English Channel, the Baltic Sea (Poland), China, and the Bay of Bengal. It is possible that these records represent a species complex. Genetic studies of this species are desirable. Large numbers of the ciliates can occur in the gills of bivalves, but there is limited evidence of impacts on growth or survival.
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is a parasite of Mya arenaria (Softshell Clam); Macoma balthica, and other bivalves (Cohen and Carlton 1995).