Is the lagoonal mudsnail Hydrobia neglecta rare because of competitively‐induced reproductive depression and, if so, what are the implications for its conservation?

1. Females of the rare lagoonal mudsnail Hydrobia neglecta respond to increasing population density by a lowered output of eggs. The effect is even more marked when the widespread and abundant congener H . ulvae comprises part of a joint population. Thus, increasing the density by 50% from 18 000 H. neglecta per square metre decreases egg production by 30%, but when H. ulvae comprises 33% of the higher mudsnail density the decrease is 63%. 2. Mortality of both male and female H. neglecta is also greater in the presence of H. ulvae than in its absence, whilst mortality of H. ulvae in mixed populations is minor. 3. Since an allopatric female H . neglecta produces only 11–14 eggs during her expected reproductive lifetime, such a reduction of fecundity in the presence of moderate numbers of H. ulvae could prove disastrous for populations of H. neglecta closed to intraspecific immigration but open to invasion by H. ulvae. Such is the position in several areas, including the semi‐isolated, landlocked lagoons that are the habitat of H. neglecta outside the Baltic Sea. 4. This probably partly explains why H. ulvae and H. neglecta are rarely sympatric in such lagoons, and why H. neglecta is only common in Britain where H. ulvae is absent from the adjacent intertidal zone and unable to be washed into lagoons by overtopping. This has important repercussions on attempts to conserve this rare hydrobiid, and probably on other rare and legally‐protected lagoonal species. © 1998 John Wiley & Sons, Ltd.