How large is the hand in the puppet? Ecological and evolutionary factors affecting body mass of 15 trematode parasitic castrators in their snail host

Abstract

Parasitic castration is an adaptive strategy where the parasite usurps its host’s phenotype, most notably the host’s reproductive effort. Though castrators are loosely known to be large relative to their hosts (compared to typical parasites), their mass has rarely been quantified and little is known about size variation, even if such variation exists. By cross-sectioning snails, we examined intra- and inter-specific variation in the parasite/host mass of 15 trematode species that castrate the California horn snail, Cerithidea californica. Trematode species occupied 14–39% (mean = 20.3%) of an infected snail’s soft tissue mass. Intraspecific variation in castrator mass fluctuated with variables that covary with energy available for host reproduction. Specifically, trematode mass was 24% higher in summer than in winter, 15% greater in snails from intertidal flats than from tidal channels, and increased with host mass to the 1.37 power (a finding contrary to that previously documented for other types of parasites). Relative body mass differed across trematode species, varying interspecifically with: (1) taxonomic family, (2) host tissue use (larger species used more types of host-tissue), (3) position in the trematode interspecific competitive dominance hierarchy (the two most subordinate species were the largest, otherwise size tended to increase with dominance), and (4) type of host used by offspring (species whose offspring infect relatively predictably occurring benthic invertebrates were larger than those infecting transient vertebrates). Our findings suggest that ecological constraints and life history trade-offs between reproduction and survival influence the mass of these very large parasites.