Population Dynamics of Nassarius reticulatus (Gastropoda, Prosobranchia) in Gullmar Fjord, Sweden

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Abstract
The population dynamics of N. reticulatus (L.) (Gastropoda, Prosobranchia) was investigated in Gullmar Fjord on the Swedish west coast from 1973-1979. A fairly well-defined population on a shallow, sandy bottom in Kvarnbukten Bay (36,000 m2) was intensively studied. Settling of veliger larvae was very irregular and normally only occurred where the organic content of the substrate was fairly high, i.e., in deeper water. The snails, especially small ones (.ltoreq. 15 mm), were greatly attracted to detritus-rich substrates; larger ones congregated on carrion. The snails in Kvarnbukten Bay spent their first 3 yr in the settling area living mainly on detritus. The change in diet may be a factor influencing the intraspecific competition between small and large snails. When nearly 4 yr old (.apprx. 15 mm) they become sexually mature. From then on most of them participated in the yearly migrations: in April and May from winter quarters in deeper water (1-5 m depth) to the shallow part of the bay (0-1 m) where they spent the summer, and then back again in Sept.-Dec. Growth rate was fast during the first 3 yr with a length increment of about 5.5 mm yr-1, but then decreased considerably, becoming individually very variable. Maximum age was about 15 yr. Fecundity was high: .apprx. 6000 eggs female-1 yr-1; females spawned every year. Mortality rate during the planktonic phase was high but there were few predators on larger snails, parasitism in shallow water probably being the most important cause of losses at this age. Temperature was a main proximate factor affecting locomotory activity, migration, spawning and growth. Threshold temperatures for various activities were narrowly defined. Irregular settling of larvae and sporadic recruitment to the shallow parts of the bays seemed to be the main reason for population fluctuations in different bays. Advantages of inshore migration (temperature and availability of food) seemed to balance disadvantages (mainly parasitism) and this may explain the dimorphism in migratory habits. These snails show several special adaptations to the varying environment in this non-tidal, soft bottom littoral.