Effect of wave-induced oscillatory flow on grazing by a subtidal sea urchin Strongylocentrotus nudus (A. Agassiz)

Abstract

Oscillating-flow-tank experiments were conducted to evaluate the effect of wave-induced oscillatory flow on feeding by subtidal sea urchins Strongylocentrotus nudus (A. Agassiz). Feeding rates by two size groups (mean test diameter=53 and 80 mm) were measured for rehydrated dried blades and fresh live thalli of kelp Laminaria spp. Feeding rate of the larger sea urchins was markedly reduced at the peak velocity of 0.3 m/s and virtually ceased beyond ≈0.40 m/s. The peak velocities at which feeding rate began to decrease and reached almost nil were somewhat lower for the smaller sea urchins than for the larger ones. The velocity limit for feeding does not appear to be a function of temperature. Movement by sea urchins was reduced to half the rate in still water at the peak velocity of 0.30–0.40 m/s. Sea urchins could hardly move beyond 0.70 m/s. The depth variation in the mean of the wave-induced significant peak bottom water velocity, predicted from 6-yr offshore wave data available on the northeastern Pacific coast of Honshu, Japan, was consistent with depth distributions of S. nudus and kelp. The predicted frequency of the significant peak velocity <0.40 m/s, in which sea urchin grazing might be possible, was as high as 70% at 12 m depth but was only 3% at 2 m depth. Mirroring this, sea urchins were almost absent at 2 m depth and increased in density with increasing depth while the kelp decreased in density with increasing depth and were almost absent at 12 m depth.