Effect of intermediate host size (Cerastoderma edule) on infectivity of cercariae of three Himasthla species (Echinostomatidae, Trematoda)

To understand prevalence patterns of parasites in marine host populations e xperimental infection studies an required. Bivalves are important host orga nisms to a variety of trematodes species and in our study area (Wadden Sea) three different Himasthla species co-occur in cockle populations. These sp ecies are morphologically very similar but differ with respect to various m orphometric dimensions. To study the possible functional importance of diff erences between Himasthla cercariae (the free-living stage shed from prosob ranch snails and encysting as metacercariae in bivalves), we experimentally measured the infectivity of the three congeners in regard to different siz e groups of juvenile cockles. The smallest species, N. interrupta, has a hi gh infectivity in small cockles (optimum around 4 mm), whereas the two othe r congeners H. continua and H. elongata exhibit low infection efficiencies in cockles less than 6 mm and higher efficiencies in larger cockles. Behavi oural experiments were performed to identify proximate causes underlying th e observed infection patterns. parasite avoidance behaviour of the cockle v aries in a host-parasite size-dependent manner so that a large cercaria ten d to provoke an avoidance response in a small cockle. The possible conseque nces of the observed host size preferences in relation to definitive host s pecies (waterbirds) are discussed and it is suggested that one or more of t he parasite species an adapted to other host species and that their sympatr ic occurrence in cockles in our study area is a result of a spinoff from th eir main cycle mediated through migratory birds. (C) 1999 Elsevier Science B.V. All rights reserved.