Photosensitivity of the oceanic cope-pods Pleuromamma gracilis and Pleuromamma xiphias and its relationship to light penetration and daytime depth distribution

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Abstract
The oceanic copepods Pleuromamma gracilis and Pleuromamma xiphias undergo diel vertical migrations over hundreds of meters in the Sargasso Sea. Near midday during the April 1985 BIOWATT cruise, the peak depth distribution of adult P. gracilis was centered near 250 m while the peak depth distribution of adult P. xiphias was centered near 450 m. These depths have a daytime photic environment characterized by very low light intensities of narrow spectral composition. Photosensitivity of each species was estimated by producing an action spectrum for phototaxis. Peak sensitivity of both species was to light of 480 nm, with 50% of P. gracilis showing negative phototaxis to light of 1.4 .times. 1014 photons m-2 s-1, and 50% of P. xiphias showing negative phototaxis to light of 7.2 .times. 1011 photons m-2 s-1. By comparing underwater light intensities measured with a submersible spectroradiometer to daytime depth distribution of P. gracilis and P. xiphias captured near midday with a MOCNESS net system, it was found that the daytime depth distribution of both species was considerably shallower than the depth at which the underwater light intensity matched their photosensitivity thresholds. Daytime depth distribution was significantly correlated with light intensity, however, with populations being deeper on days with more light penetration.