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.
This study examined the behavioral characteristics of juvenile Japanese surf clam Pseudocardium sachalinensis in response to sand erosion and deposition under laboratory conditions, then estimated the fluid conditions critical to wash the clams out of the sand. The juveniles started burrowing down ward into sand when their burying depth decreased to less than 1.5 times the shell length due to erosion , and moving upward when the depth increased to more than 2.5 times the shell length due to deposition of sand. Burrowing and climbing actions of the juvenile continued until the depth reached 1.7-2.1 times the shell length once again. The burrowing and climbing speeds were about 1.8 times higher than the speed of erosion or deposition of sand. It was concluded that the juveniles would maintain their po sition actively in order to avoid being washed out or buried too deeply by adapting their behavior to the dynamic conditions of sand movement. The juveniles were washed out of the sand when the erosion speed exceeded 3.5 mm/min, and such erosion occurred when oscillatory water flow speeds over a sandy bottom with median particle diameter of 0.3 mm exceeded more than 45 and 55 cm/s at periods of 4 s and 8 s, respectively.
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