Growth patterns of Amphiura filiformis support the hypothesis of organic enrichment in the Skagerrak-Kattegat area

Population size structure, abundance and biomass of the burrowing ophiuroid Amphiura filiformis were measured in the period 1970 to 1989 at three widely spaced sites at 55 to 100 m depth in the Skagerrak-Kattegat area. This species is a dominant component of the fauna in the area, and also one of the species accounting for a great part of a recent general increase in benthic biomass. Abundance at two of the sites and biomass at all three sites increased significantly with time from the 1970s onwards. Maximum individual size and growth of the cohort of adults showed a marked increase in the period 1980 to 1983 at the three sites. In the 1980s annual absolute growth increments of the recruits were clearly greater than in the 1970s, and size-specific growth rates were virtually unchanged. These results are consistent with the hypothesis that recent increases in benthic biomass in this area result from increased input of food to the benthos. A. filiformis has the potential for rapid growth; estimated maximum weight-specific growth rates were more than 1.5% d^-1 in the disc diameter interval 3 to 9 mm, which is comparable to rates found in shallow water filtering bivalves. The maximum disc diameter growth rate was ca. 0.4 mm mo^-1.