Effect of sediment particle size on recruitment of Owenia fusiformis in the Bay of Blanes (NW Mediterranean Sea) : an experimental approach to explain field distribution

The spatial distribution of the tubicolous polychaete Owenia fusiformis Delle Chiaje in the Bay of Blanes was studied during summer 1996. Populations were located on sandy sediments and a heterogeneous distribution of specimens was found. Three areas with different densities were observed: the vicinity of the harbour, the mouth of the Tordera River, and the open bay. Although adult individuals were present all over the bay at low densities, large numbers were mainly found near the harbour. Differences in mean individual biomass were also observed among stations. The largest individuals were found at the mouth of the river, where the fluxes of suspended matter were maximal. Juveniles of O. fusiformis also showed a regular pattern of spatial variation along the bay. The highest density of juveniles was found in the area where high densities of adults occurred. The consequences of this spatial variability were analyzed in relation to the effect of sediment grain size on 3 successive processes of recruitment (metamorphosis, juvenile transformation and tube construction) in running still-water experiments. The influence of sediment particle size on the disappearance of larvae in the 3 processes was analyzed. The large grain size of the substratum had a significant negative effect on the construction of O. fusiformis tubes. After settlement, O. fusiformis needs a high percentage of fine sediment to construct the tube. Our results support the hypothesis that the spatial distribution pattern of the species in the field is influenced by sediment particle size. The importance of the presence of O. fusiformis adult tubes and seagrass shoots in stabilizing the sediments, and thus determining the pattern, is also discussed.