DENSITY-DEPENDENT MIGRATION IN AN AMPHIURA-FILIFORMIS (AMPHIURIDAE, ECHINODERMATA) INFAUNAL POPULATION

The hypothesis that the semi-mobile brittle star Amphiura filiformis m ay have density-dependent migratory behaviour and that their dispersio n may be. dependent on food availability was examined. Sediment with i ntact fauna dominated by A. filiformis (similar to 2250 ind. m(-2)) wa s brought to the laboratory in 0.22 m(2) plexiglass boxes. The experim ent 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 exper imentally 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 'hi gh' (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 i nto 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 mo derate density side. The migration rate was estimated from a diffusion coefficient based on the 'Random Walk' theory. No significant differe nces in migration rates were observed between fed and starved boxes, i ndicating that dispersal was primarily density-dependent rather than f ood-dependent. Based on the diffusion coefficient, a total dispersal o f 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 sta rved brittle stars. A separate experiment was set up to study migrator y behaviour. It appeared that A, filiformis can move both on the sedim ent surface and within the sediment. In conclusion, migration in A. fi liformis may be a common feature, which may cause a more or less conti nuous displacement of sediment with significant ecological effects.