Density-dependent migration in an Amphiura filiformis (Amphiuridae, Echinodermata) infaunal population

The hypothesis that the semi-mobile brittle star Amphiura filiformis may have density-dependent migratory behaviour and that their dispersion may be. dependent on food availability was examined. Sediment with intact fauna dominated by A. filiformis (similar to 2250 ind. m(-2)) was brought to the laboratory in 0.22 m(2) plexiglass boxes. The experiment was a 2-factorial design with density (high or moderate) and food (fed in excess or starved) as factors. To investigate migration in the high and moderate densities, the abundance of A. filiformis was experimentally manipulated in the boxes at the start of the experiment to 'low' (100 ind. 0.11 m(-2)) numbers in one-half of all boxes and to 'high' (350 ind. 0.11 m(-2)) or 'moderate' (220 ind. 0.11 m(-2)) numbers in the other half. The experiment was run for 63 d. At termination of the experiment, a mean number of 90 and 36 ind. box(-1) had migrated into the low density side of the box from the high density and moderate density side, respectively. Significantly higher migration rates per individual were observed from the high density side compared to the moderate density side. The migration rate was estimated from a diffusion coefficient based on the 'Random Walk' theory. No significant differences in migration rates were observed between fed and starved boxes, indicating that dispersal was primarily density-dependent rather than food-dependent. Based on the diffusion coefficient, a total dispersal of all A. filiformis from the high density side was estimated at 47 m h(-1). Fed A. filiformis had a higher weight and larger gonads than starved brittle stars. A separate experiment was set up to study migratory behaviour. It appeared that A, filiformis can move both on the sediment surface and within the sediment. In conclusion, migration in A. filiformis may be a common feature, which may cause a more or less continuous displacement of sediment with significant ecological effects.