The response of the PycnogonidsNymphon gracileleach andPycnogonum littorale(Strom) to light stimuli of different wavelength and intensity

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Abstract
It is generally agreed that the eyes are the only perspicuous sense organs in pycnogonids. Their structure was previously described; they were of the non-inverted, prebascillar type. The present study investigates the photoresponse in N. gracile (Leach) and P. littorale (Strom), 2 common British pycnogonid species. Various aspects of the photoresponse are described and comparisons are made between the responses to light of different wavelengths and also between animals of different ages. The light responses are discussed in the context of the general behavioral biology of the 2 spp. Both species exhibit similar optimum sensitivities between wavelengths of 530-545 .mu.. This is similar to many marine invertebrates which occupy a similar habitat. The response in each species is such that the animal tends to remain in a favorable environment.