Complex interactions of a deposit feeder with its resources, roles of density, a competitor, and detrital addition in the growth and survival of the mudsnail Hydrobia totteni

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Abstract
Balanced experiments varying detrital input, snail density and presence of a compteing mudsnail demonstrate complex effects on the growth and survival of H. totteni (Prosobranchia,Hydrobiidae). Although addition of detritus derived from Ulva initially depressed diatom standing stock, an eventual subsidy is reflected in increased snail growth. Both H. totteni density and presence of the competitor decrease snail growth and survival; a significant proportion of this effect is transmitted via microalgal depression, but intraspecific and interspecific interference is also important. H. totteni populations are probably strongly affected by both interspecific and intraspecific competition, as diatom abundances and experimental animal densities in the laboratory are within the ranges observed in the field. The indirect effect of detritus addition on microalgal growth may be important in the economy of deposit feeders, due to the common winter abundance and late spring decay of the sea lettuce Ulva rotundata.