Abstract
The planktonic larval snow crab Chionoecetes opilio come up to near the sea surface induced by underwater light after hatching, and start descending when their weight, which increases with growth, exceeds the upward force produced by their swimming behavior towards the surface. Based on these activities, we developed a model representing the swimming pattern of the planktonic larvae until they acquire settlement. The larvae were put in the experimental tank with the same water conditions, and a monochromatic light of different wavelength, from 430 to 645 nm, was irradiated from above. The planktonic larvae in the zoea stage responded sensitively to blue light, showing positive phototaxis. Furthermore, experiments were made under various conditions with water temperatures from 3 to 24℃ and with salinities from 30.0 to 35.0 to examine the geotaxis of the larvae. A strong swimming inhibition was observed in a condition with a water temperature of 21℃ or more, but no influence by salinity was found. Based on these findings, we calculated the average upward swimming force of the larvae and compared it with their weight that produced a downward force. As a result, it was found that the sedimentation of the larvae, caused by their weight, starts at the megalopa stage.
