Breeding, recruitment and survival in a mixed intertidal population of the mudsnailsHydrobia ulvaeandH. neglecta

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Abstract
Breeding and population statistics were assessed from February 1993 to September 1995 in a mixed population of the mudsnailsHydrobia neglectaandH. ulvaein the intertidal zone of a macrotidal bay on the Atlantic coast of Brittany, France. The biology ofH. neglectain the mixed population was identical to that found earlier for the same species when living allopatrically elsewhere in the bay, and was also that generally characterizing directly-developing species ofHydrobiathat occur in coastal lagoons. The population of the larviparousH. ulvaealso showed various features otherwise typifying lagoonal populations of this species: very high juvenile mortality, repeated failure of whole cohorts to recruit, and an age structure dominated by large and relatively old individuals. The number of eggs per capsule was normal for an intertidalH. ulvaepopulation but the number of capsules laid was remarkably low. Why both species should behave in this intertidal marine habitat as if it were lagoonal is unknown. The failure of the cohorts ofH. ulvaederiving from spring eggs in the bay may help explain why the majority of lagoonal populations of this species are dominated by old mudsnails and show negligible recruitment. There was no evidence of competitive displacement ofH. neglectabyH. ulvaein spite of received wisdom thatH. ulvaeis the competitive dominant under fully marine conditions; mortality ofH. ulvaewas more than three times that experienced byH. neglectaregardless of age of the snails.