THE EFFECT OF SALINITY UPON PHOTOTAXIS AND GEOTAXIS IN A LARVAL CRUSTACEAN

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Abstract
Experiments were conducted to determine the effect of salinity on phototaxis and geotaxis in stage I and IV zoeae of the crab Rhithropanopeus harrisii. Larvae were exposed to sudden salinity changes and stimulated with various intensities of 500-nm light in the horizontal plane. Although the pattern of phototaxis of larvae exposed to 40.permill. was unchanged from that at 20.permill. (acclimation salinity), the level of positive phototaxis to higher intensities was significantly greater and the level of negative phototaxis to low intensities significantly lower at 40.permill.. Exposure to low salinity sea water (5.permill.) generally reverses the sign of phototaxis, since a significantly higher level of negative phototaxis and lower level of positive phototaxis occurs at light intensities above 10-2 Wm-2. The minimum amount of salinity decrease from the acclimation salinity that induces a reversal in phototactic sign from positive to negative phototaxis at 0.10 Wm-2 ranges from 1-2.permill., and appears independent of acclimation salinity and developmental stage. Total recovery of a positive phototaxis occurs in .apprx. 5.5 min for both zoeal stages, with a 50% return apparent in 1.5-3.2 min. Larvae stimulated from above with light of 0.15 Wm-2 or maintained in darkness in a vertical column exhibited salinity-dependent and age-dependent vertical distributions. At each of 7 test salinities (5-40.permill.), stage IV zoeae maintained a lower position in the column than stage I. Stage I had similar vertical distributions in darkness and overhead light. At salinities below the acclimation salinity larvae moved downward due to a positive geotaxis and negative phototaxis. Upon exposure to higher salinities, an upward movement due to a negative geotaxis and positive phototaxis occurs. Phototaxis was the dominant behavioral response in light. Stage IV zoeae migrate down in overhead light and darkness upon a decrease in salinity. The dominant behavioral response is a positive geotaxis. With an increase in salinity, an ascent only occurs under overhead light, indicating movement results from a positive phototaxis. Anesthetized larvae sink in both high and low salinity water. Thus, the ascent in high salinities does not result from floating due to the increased density of the water. A comparison of descent rates by anesthetized and unanesthetized larvae in 0.15 Wm-2 light directed from above and in low salinity water indicates that the normal descent results primarily from active downward swimming, although some larvae exhibit passive sinking. These behavioral responses to increases and decreases in salinity can act as a negative feedback system to keep larvae within the region of acclimitization salinity in the vertical water column.