Document Type



Virginia Institute of Marine Science

Publication Date



American Malacological Bulletin


Special Edition No. 3

First Page


Last Page



The seasonal change in depth distribution of Arctica islandica (Linne) larvae at a station on the Southern New England Shelf for the period April-December 1981 is compared with the output of a numerical model designed to predict distribution in a scenario where active depth regulation predominates. Larvae in excess of 200 J.Lm length were present in the field in May at 1-30 m depth and, at depths of 20-40 m from late July through November. The majority of larvae captured in November were shelled veligers of 110 J.Lm length. Good agreement of the model with field data exists with respect to absence of A. islandica larvae in the warm (> 18°C) shallow (0-20 m) waters between July and early September, and the abundance of larvae throughout the depth range 20-40 m from July through October. The model predicts occurrence of larvae in June; however, they were not seen in the field . The discrepancy can be due to the combination of reduced spawning by adult A. islandica (which is not included in the model) and less than optimum conditions for larval development. The model predicts aggregation of the negatively geotactic larvae at the surface following decay of a seasonal thermocline. Such aggregations were not seen in the field indicating that vertical mixing of the water column in the fall months is sufficient to negate distribution patterns dominated by active depth regulation . Depending upon the nature, intensity and stability of stratification of the water column, it is evident that depth distribution of A. islandica larvae can be dominated by either active depth regulation or passive movement at the mercy of physical mixing. The conditions of transition from active to passively dominated dispersal and distribution are poorly defined.