Fluid forces on short-spined sea urchin [Strongylocentrotus intermedius] and northern sea urchin [Strongylocentrotus nudus]

The drag coefficients and the added mass coefficients of short-spined sea urchin Strongylocentrotus intermedius and northern sea urchin Strongylocentrotus nudus were determind. Meanwhile adhesive forces exerted by the tube feet were also measured. The drag forces were measured in steady flow and the added masses were measured by torsion pendulum coating the sea urchins with paint. The drag coefficient Cd was greater for the northern sea urchin, while the added mass coefficient Ca was greater for the short-spinedsea urchin. The average values of Cd, Ca in the direction of the test diameter was 0.66, 0.64 for the short-spined sea urchin and 1.06, 0.34 for the northern sea urchin respectively. Adhesive forces on smooth glass substrates were 0.3 ?? 2.8kg for the short-spined sea ur-chin and 0.2 ?? 2.4kg for the northern sea urchin when they were more than 40mm in test diameter. The adhesive forces of both species were much larger than the calculated maximum wave forces using determined Cd, Ca on the same size of sea urchin in an estimated roughsea condition.