Large-scale patterns in trematode richness and infection levels in marine crustacean hosts

Little is known about the patterns of variation in parasitism in marine hosts. Trematodes, the dominant parasites in intertidal systems, are transmitted from their first intermediate hosts (snails) to a range of second intermediate hosts, including crustaceans. Using published studies of trematode infections in crustacean hosts, we investigated general patterns of variation in trematode species richness and infection levels (i.e. percentage of hosts infected and mean number of individual parasites per host). Since the production and release of infective stages in snails is strongly temperature dependent, we also investigated a potential decrease in trematode infection levels with increasing latitude (as a proxy for decreasing temperature). Trematode species richness in the crustacean hosts was generally low (mostly 1 or 2), and infection levels were moderate. However, there were differences among taxa in some groups, particularly among brachyuran crabs, which showed significantly higher values than in other groups. For amphipods, which were the only well-studied group across a large range of latitudes, we found negative correlations between latitude and the trematode species richness or measures of infection level considered here. These relationships persisted after correction of the potentially confounding effects of sampling effort, host body size and host generic identity (as a control for phylogenetic influences). We discuss these findings in light of environmental mediation of parasite transmission, in particular with respect to the probably fundamental role of temperature in driving the output of trematode infective stages in marine systems.