Density and growth of juvenileAmphiura filiformis(Ophiuroidea) in the Øresund

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Abstract
By regular, quantitative sampling during two years 13 050 specimens of Amphiura filiformis were collected at a locality in the Øresund (27 m depth, muddy sand). The material included all stages from newly settled to adults. Three settling periods were registered with maximum numbers of newly settled from 6200 to 6800 · m−2. The mean disc diameter at settling was 0.3 mm, after 1 year 0.6 mm and after 2 years 1.6 mm. The adults had disc diameters of 4–9 mm. Consequently, the juvenile life lasted at least 3 years.} During all seasons there was a remarkable paucity of individuals with disc diameters between 2 and 4 mm. No age groups were found among the adults. Altogether, there is every possibility of a long life span — maybe 25 years. The density of adults has been stable for at least 20 years and was during the study period 575 · m−2 (mean), max. 1050 · m−2. Compared with other European populations the density was high and, therefore, I conclude that conditions are favourable for Amphiura filiformis. At a disc diameter of 0.6 mm two arms start outgrowing the others. This enables the animal to burrow in the sediment while feeding at the surface. Stomach contents of the juveniles included prey (foraminiferans, ostracods) the size of the animal itself.