Gastrointestinal helminths of loggerhead turtles (Caretta caretta) from the western Mediterranean: constraints on community structure.

Richness and composition of gastrointestinal helminth communities of 54 loggerhead turtles, Caretta caretta, from the western Mediterranean were interpreted from patterns of helminth exchange at 2 host taxonomic scales: exchange between marine turtles and other marine hosts and exchange within turtles. We predicted exchange of the former to be unimportant ecologically and evolutionarily because of the host phylogenetic distance. The absence of records of successful exchange at this host taxonomic scale confirmed that host physiological barriers seem to prevent contemporary parasite transfer between marine turtles and other sympatric hosts. Marine turtles also seem to exhibit an evolutionary association with their parasites largely independent from that of other marine hosts. Therefore, the composition of gastrointestinal helminth communities of marine turtles is predictably restricted to digeneans, nematodes, and aspidogastreans specific to these reptiles. At the scale of host species, helminth exchange among marine turtle species was expected to be significant. This was suggested by reports indicating a high proportion of parasites common to all sea turtle species. Mediterranean C. caretta harbored only 4 digenean species typical of marine turtles, and immature individuals of 1 digenean species accidentally acquired. Further, no relevant parasite exchange with other marine turtles was expected to occur because C. caretta is the only sea turtle species abundant in the western Mediterranean. Therefore, a predictable composition coupled with low species richness in infracommunities might be accounted for partly by constraints on parasite acquisition at both host scales. Host factors limiting parasite recruitment (mainly ectothermy and a wandering behavior) may also contribute to depauperate and isolationist infracommunities.