A SHADOW RESPONSE IN A LARVAL CRUSTACEAN

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Abstract
A shadow response consisting of oriented movement to light and gravity was studied using a closed circuit television system for stage I zoeae from the crab Rhithropanopeus harrisii. If larvae are irradiated at an intensity that induces positive phototaxis and the light is extinguished, both light- and dark-adapted larvae descend. This response involves passive sinking, and is termed a sinking response. The minimum time that the light must be extinguished to evoke the response is 20-30 ms. If the light intensity is reduced to a level that should induce negative phototaxis, light-adapted larvae show a sinking response followed by a negative phototaxis. The time delay between the responses is related to the initial stimulus intensity and duration. The minimum decrease in intensity that induces the sinking response is equivalent to a reduction by a 0.5 O.D. [optical density] neutral density filter while the maximum response occurs at 1.0-1.1 O.D. and greater. These values are independent of stimulus time, intensity and direction. These behaviors are appropriate for avoiding zooplankton predators like ctenophores which do not visually sight and actively pursue their prey. The ctenophore Mnemiopsis leidyi is abundant in the Beaufort, North Carolina [USA] area and spectrophotometric determinations of this species'' apparent O.D. indicate that animals larger than a certain size attenuate light sufficiently to evoke the shadow response.