REPRODUCTIVE STRATEGIES IN CONTRASTING POPULATIONS OF THE COASTAL GASTROPOD HYDROBIA-ULVAE .2. LONGEVITY AND LIFETIME EGG-PRODUCTION

Norfolk Hydrobia ulvae show 1. (a) a late summer peak of egg production, coincident with the attainment of maturity by snails spawned the previous late summer (but not restricted to those first-time spawners) and 2. (b) spring spawning of overwintering adults, the latter accounting for 60-85% of the total annual egg production during the study period. Most adult snails derived from late-summer eggs and from larvae that metamorphosed in September-October, maturity being reached after 9-10 months of benthic life. Spring-spawned marine cohorts failed to survive to reproduce during the study period, as did the summer 1988 and all spring-spawned cohorts in the lagoon. Although potentially capable of producing ≈ 412 eggs · annum-1, a female lagoonal H. ulvae can expect to produce only 175 eggs during her 5 months of reproductive life (equivalent to 93 and 46% of her own AFDW, respectively). In contrast, although capable of a potential 2927 eggs · annum-1 (equivalent to 9.8 × female AFDW), an intertidal marine female (summer 1987 spawned cohort) can expect to produce 446 during her expected 2.75 months of reproductive life (equivalent to 157% of her AFDW). ≈ 13% of lagoonal adults, however, live through 5 breeding seasons, whereas only 10% of intertidal adults survive even for 1 yr after reaching maturity. The differences between these lagoonal and marine populations are broadly appropriate to the likely patterns of mortality at the two sites, and the well-known variability in life-history and reproductive characteristics of this species may therefore represent adaptation to differing expectations of survival. It is argued that H. ulvae may biologically be an iteroparous species that is normally rendered functionally semelparous by high rates of mortality, and that considerable evolution towards a true semelparous reproductive strategy has occurred. © 1990.