Distribution and mobility of juvenile Polychaeta in a sedimentary tidal environment
It has been assumed that the distribution of marine benthic populations depends mainly on pre-settlement processes. Juvenile dispersal after settlement is here investigated whether it attains a key role in determining spatial patterns of adult populations. Distribution patterns of juvenile and adult polychaetes in the Wadden Sea near the Island Sylt are described. A total of 43 polychaete species is recorded. Juvenile abundance was significantly higher in structured habitats than in non-structured ones. Spatial separation of juveniles from adults across habitats was found in some species. This may indicate juvenile migration as a mandatory process in such populations. A second indication of dispersal by juvenile benthic stages was found, when small-scale distribution patterns (4 m2) were analyzed almost daily over a two months period. At high spatio-temporal resolution, distribution patterns for Spio martinensis, Typosyllis hyalina, Ophelia rathkei and Capitella minima exhibited a high variability explained mostly by ongoing immigration and emigration. In contrast to extensive drifting in juvenile bivalves, only few juvenile polychaetes were found in water samples or in nets above the bottom. With the hypothesis that juveniles crawl at or below the sediment-water interface, an in situ experiment was set up. A combination of drift nets and covered grooves placed level with the sediment surface were used in order to trace crawling performance in juveniles. No juvenile worms were found in the drift nets, while in the grooves small species were able to crawl actively at least one meter per day. It is assumed that post-settlement dispersal plays an important role in the population ecology of some polychaetes. Juveniles are able to settle in habitats favorable specifically for juveniles irrespective of habitats preferred by adults. Active migrations at the bottom may bring juveniles in due time to sites suitable for the completion of their life cycle.