Published online by Cambridge University Press: 11 May 2009
Breeding and population statistics were assessed from February 1993 to September 1995 in a mixed population of the mudsnails Hydrobia neglecta and H. ulvae in the intertidal zone of a macrotidal bay on the Atlantic coast of Brittany, France. The biology of H. neglecta in 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 of Hydrobia that occur in coastal lagoons. The population of the larviparous H. ulvae also 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 intertidal H. ulvae population 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 of H. ulvae deriving 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 of H. neglecta by H. ulvae in spite of received wisdom that H. ulvae is the competitive dominant under fully marine conditions; mortality of H. ulvae was more than three times that experienced by H. neglecta regardless of age of the snails.