Life history trade-offs affect trematode parasites reproducing inside their 1st intermediate hosts. Within the constraint of the effect on host survival, parasite production of cercariae is subject to a size–numbers trade-off. Within each cercaria, resources must be partitioned between host-seeking and subsequent developmental functions. Three species of microphallid trematodes with the same 1st intermediate host (the gastropod Littorina saxatilis) were investigated. Maritrema arenaria periodically released many small cercariae. Microphallus similis released fewer, 15% larger, cercariae without periodicity. Microphallus similis cercariae were strong swimmers, moving toward the dark and downward in turbulent water, whereas Ma. arenaria cercariae remained suspended. Maritrema arenaria cercariae, although smaller in body and tail size, were produced at an average daily volume nearly twice that of M. similis. These differences are interpreted as transmission adaptations related to mobility and predictability of the 2nd intermediate host. Microphallus similis, with a mobile and less predictable crab host, adopted a ‘bet-hedging’ prolonged production of fewer cercariae by less intensive host exploitation, each cercaria having a high allocation to host-seeking behavior. Maritrema arenaria, with predictable sessile barnacle hosts, produced less mobile but potentially longer-lived cercariae in larger numbers. Microphallus piriformes metacercariae remain in the gastropod host. The number of M. piriformes metacercariae increased in larger hosts. The 3 species differed in the number of sporocysts and (meta)cercariae per sporocyst within the gastropod but not in the within-host volume of parasites. Variation in host exploitation and life history appeared adaptive for transmission to the next host.

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