Effects of winter severity on life history patterns and population dynamics of Hydrobia ventrosa (Gastropoda: Prosobranchia).

Abstract

Interannual variations in population structure and life history patterns of the mudsnail Hydrobia ventrosa were investigated in relation to winter seventy in a windflat of the Wismar Bay (southwestern Baltic Sea). The population structure of H. ventrosa was studied in the field from 1995 to 1999 and egg production of different year classes was measured in the laboratory during the reproductive season of 1997. Abundance of H. ventrosa ranged between 7,000 and 150,000 ind./m 2 with density fluctuations according to the abundance of juvenile snails. The abundance of adult mudsnails remained remarkably constant with a mean density of about 25,000 ind./m 2 . Recruitment occurred between June and November with two peaks of reproduction in 1995 and a single one in the following years. Accordingly, the year class born in 1995 consisted of two cohorts, whereas the following year classes consisted of a single cohort only. H. ventrosa reached a maximum life span of about two years and a maximum shell height of about 3.5 mm. Shell growth was similar between year classes. According to laboratory experiments, about 95 % of the total annual egg production was contributed by 1-year-old females and <5 % of the eggs were laid by females which had overwintered twice and by young-of-the-year. Egg production decreased from 1.0 eggs female -1 day -1 in spring to less than 0.5 eggs female -1 day -1 in autumn. The annual production of recruits was higher after the moderate winter of 1996/97 than after the mild winters of 1994/95 and 1997/98. The cold winter of 1995/96 resulted in a delay of the reproductive season in 1996 and it is suggested that this delay restricted recruitment.