The enigma of gigantism: effect of larval trematodes on growth, fecundity, egestion and locomotion in hydrobia ulvae (pennant) (gastropoda:prosobranchia)

The effect of larval trematodes on growth, fecundity, egestion and locomotor activity in naturally infected Hydrobia ulvae (Pennant) was studied in the laboratory. Infected snails showed increased growth rates (shell height, body wet weight) compared with uninfected controls. C/N analysis of the snails suggested that the additional growth of infected specimens included shell material as well as dry soft tissue. Infection caused a significant reduction in penis size and an almost complete cessation of oviposition. As judged by their egestion rates, food consumption rates of infected and uninfected snails were roughly similar, but infection caused a significant reduction in locomotor activity. It is suggested that the energetic basis for parasite metabolism and excess host growth is in part reallocated reproductive energy following castration, and in part energy saved by reduced locomotor activity. Some previously reported hypotheses explaining the equivocal evidence on parasite-induced growth enhancement are discussed. It is argued that the phenomenon is not necessarily species-specific, but should also be addressed at the level of subpopulations both in and outside the snails' reproductive season.