Transport of Owenia fusiformis larvae (Annelida: Polychaeta) in the Bay of Seme. I. Vertical distribution in relation to water column stratification and ontogenic vertical migration

The vertical distribution of Owenia fusiformis larvae was studied from pump samples in the eastern part of the Bay of Seine, English Channel. between 9 May and 9 June 1987. Only 1 massive larval cohort was present. Four planktonic stages of different duration were distinguished. The duration of total larval life was estimated to be about l mo. Larvae were located preferentially under the pycnocline. Larvae showed an ontogenic migration with an increase of the larval center of mass (ZCM) for each stage according to age. This migration was associated with the 2-layer circulation present in the study area. Stage 1 was principally present in the diluted surface layer and carried downstream. Stage 2 was located just above the depth where the seaward residual flow is null. Downstream transport of these larvae should have been lim~ted. Stages 3 & 4 were located below the level of no net motion and consequently were transported upstream. However, the vertical distribution of Stages 1 & 2 could be changed in relation to water column stratification. When the stratification was slight, these larvae were evenly distributed over the water column, whereas a strong stratification generated a larval concentration under the pycnocline. The depth of larvae in the water may have resulted from interactions between behavior and water column structure. Ontogenic migration and aggregation of Stage 2 larvae just above the depth where net residual flow is null played a basic role in larval retention in the eastern part of the Bay of Seine and should partly explain the spatio-temporal stability of the adults.