Population dynamics and growth of the gastropod Oxystele variegata (Anton) on an exposed rocky shore

Abstract

Oxystele variegata (Anton) was found to exhibit a gradient of increasing size up the shore with settlement of veliger larvae occurring primarily in the lower balanoid zone. This has a marked effect on biomass in different regions of the beach and, in combination with zone-dependent density, may be expected to affect the impact of the species as a grazer in different zones. Settlement was continuous during the sampling period but with a marked peak of recruitment in summer. Juvenile cohorts appear at 2-4 mm and grow to approximately 14 mm in their first year, after which growth declines, a mean size of 18 mm being reached after the second year. Growth of all size classes in the mid balanoid zone was markedly slower than in the lower balanoid zone because of greater snail densities and lower microphyte productivity. Low primary productivity in the upper balanoid, where large animals occur, may also be partially responsible for the slow growth rates recorded for these animals. Mean monthly mortality of adults was found to be 4.04% and that of juveniles 12,44%. Total population showed a decline of 30,80% over a period of 13 months. Decrease in numbers, however, occurred only on the lower shore and was related to high juvenile mortality rates