ABSTRACT
The ophiuroid Ophiura sarsii forms dense beds, i.e. high-density populations uniformly covering large area of the sea floor, in the upper bathyal zone widely around northern Japan. Size-frequency distributions of O. sarsii were polymodal, and were usually dominated by a peak of large individuals. Comparisons of size structure at various localities showed that the maximum and mean sizes of O. sarsii were negatively correlated to the population density, and this is probably due to some density-dependent effect on growth. Depth-related gradients in size structure were investigated in the area of Otsuchi, Pacific coast of northeastern Japan. In the shallower part of the dense bed where O. sarsii occurred in the absence of other species of ophiuroids, high seasonal variation and high proportion of small individuals were observed in size structure, suggesting relatively strong recruitment occurred.
