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Journal of Experimental Biology 39,589-602 (1962)
Published by Company of Biologists 1962

The Effect of Light on the Shadow Reaction of the Sea Urchin, Diadema Setosum (LESKE)

M. YOSHIDA ¹

¹ Tamano Marine Laboratory, Shibukawa, Tamano, Okayama, Japan

1. The shadow reaction of the spine jerk in Diadema setosum has been studied with an improved optical system and a timing device.

2. Studies on the variation of the inhibitory threshold with time at which light is admitted after cessation of the preceding light, have differentiated two distinct periods. The inhibitory threshold stays constant during the initial period and then rises sharply. The length of the initial period and the rate of rise in the later phase are found to be temperature-dependent, the former being affected, in addition, by the intensity of the preceding light.

3. The reciprocity of area and intensity of light is proved for both the preceding and the inhibitory light.

4. A new approach is made to study the Weber-Fechner law in shadow reactions and it is found that the reaction in D. setosum obeys the law. In connexion with this it is found that a light, sub-threshold for eliciting a reaction, can still be effective in inhibiting it.

5. The mechanism involved is discussed in relation to our earlier ideas (Millott & Yoshida, 1960b).