MEPS 394:125-136 (2009)  -  DOI: https://doi.org/10.3354/meps08189

ABSTRACT: The association between the Japanese scallop Patinopecten yessoensis and the scallop-shell-boring polychaete worm Polydora brevipalpa was investigated in the natural environment. Fatty acid analyses were performed on the scallop, polychaete, and potential food materials available in the environment, with the aim of determining the main food sources of these animals and the trophic relationships between species. The scallop and the polychaete showed a similarity in their fatty acid compositions. High concentrations of 20:5n-3 and 14:0, substantial concentrations of 16:2n-4, 16:3n-4, and 16:4n-1, and ratios of 16:1n-7 to 16:0 close to 1 indicated significant contributions of diatoms in the diets of both mollusk and polychaete. Detectable levels of 18:4n-3 suggest that dinoflagellates also contributed to the diet of the animals. The fatty acid markers showed that the scallop and polychaete feed extensively on a mixed, but probably diatom-dominated phytoplankton diet. Additionally, the elevated level of bacterially derived fatty acids and saturated (mainly 18:0) acids in polychaete lipids point to an additional dietary source, detritus and bacteria. Elevated amounts of 22:6n-3 and 18:1n-9 in the scallop digestive gland are indicative of zooplankton consumption. The similarity of the diets is a major determinant of the feeding competition between these species in the community. The decrease in the scallop internal volume concurrent with an increase in the degree of shell occupation by polychaetes suggests a reduction in water-filtering potential by the scallop. Linear regressions of the degree of polychaete occupation against scallop weight and shell height revealed negative relations in both cases. With high levels of shell occupation by polychaetes, the commensal relationship between scallop and polychaete acquires some elements of parasitism.

KEY WORDS: Community · Inter-specific relationships · Trophic biology · Fatty acids · Mollusks · Polychaetes