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

Interannual Variations in population structure and life history patterns of the mudsnail Hydrobia ventrosa were investigated in relation to winter sev erity in a windflat of the Wismar Bay (southwestern Baltic Sea). The popula tion structure of H. ventrosa was studied in the field from 1995 to 1999 an d egg production of different year classes was measured in the laboratory d uring the reproductive season of 1997. Abundance of H. ventrosa ranged betw een 7,000 and 150,000 ind./m(2) with density fluctuations according to the abundance of juvenile snails. The abundance of adult mudsnails remained rem arkably constant with a mean density of about 25,000ind./m(2) Recruitment o ccurred between June and November with two peaks of reproduction in 1995 an d 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 simi lar between year classes. According to laboratory experiments, about 95% of the total annual egg production was contributed by I-year-old females and <5% of the eggs were laid by females which had overwintered twice and by yo ung-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 pr oduction of recruits was higher after the moderate winter of 1996/97 than a fter the mild winters of 1994/95 and 1997/98. The cold winter of 1995/96 re sulted in a delay of the reproductive season in 1996 and it is suggested th at this delay restricted recruitment.