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

Abstract

It is generally agreed that the eyes are the only perspicuous sense organs in Pycnogonids. Their structure has previously been described by the present authors when they were shown to be of the non‐inverted, prebascillar type. The present study investigates the photo‐response in Nymphon gracile (leach) and Pycnogonum littorale (Strom), two 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 behavioural biology of the two species. Both species exhibit similar optimum sensitivities between wavelengths of 530–545 μ. 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 favourable environment.