EFFECT OF TEMPERATURE ON PHOTOTAXIS AND GEOTAXIS BY LARVAE OF CRAB RHITHROPANOPEUS-HARRISI (GOULD)

Investigations of the effect of sudden temperature change on the phototaxis of Stage I and IV zoeae upon stimulation from horizontal and vertical directions with 500-nm light indicate a temperature-induced geotactic response in larvae of the crab R. harrisi. For the horizontal tests both zoea stages were reared at 20.degree. C. Stage I showed positive phototaxis at temperatures between 15.degree.-35.degree. C, while Stage IV responded over the range of 10-30.degree. C. For the vertical tests, larvae reared at 25.degree. C were stimulated with overhead lights. Stage I zoeae ascended at 15.degree., 20.degree. and 25.degree. C and descended at 5.degree., 10.degree., 30.degree. and 35.degree. C. Stage IV zoeae ascended at 20.degree. and 25.degree. C and descended at 5.degree., 10.degree., 15.degree., 30.degree. and 35.degree. C. Although the descent at high temperatures could result from a negative phototaxis, a reversal in phototactic sign at high temperatures was not found in the horizontal experiments, and the same vertical movement pattern is observed in total darkness. Upon exposure to high temperatures near the water surface, larvae would descend by means of a positive geotaxis rather than a negative phototaxis. This response involves active swimming by Stage IV larvae and passive sinking by Stage I.