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

Abstract

The oceanlc copepods Pleuromamma gracdis and Pleuromamma xiphlas undergo d e l vertlcal migrations over hundreds of meters in the Sargasso Sea Near mldday during the April 1985 BIOWATT cruise the peak depth distribution of adult P g r a n h s was centered near 250m whlle the peak depth distribution of adult P xlphlas was centered near 450111 These depths have a daytime photic environment character~zed by very low hght lntensltles of narrow spectral composition Photo-sensihv~ty of each species was estimated by producing an actlon spectrum for phototaxis Peak sensibvlty of both species was to llght of 480nm with 50 % of P graalls showing n e g a t ~v e phototaxis to hght of 1 4 X l O I 4 photons m-2 S -' , and 50 % of P xiphias showlng negahve phototans to light of 7 2 X 10" photons m-2 S-' By comparing underwater light intens~tles measured w t h a submersible spec-tiorad~orneter to daytlme depth distnbution of P gracjlis and P xiphias captured near mldday w ~t h a h4OCNESS net system lt was found that the daytime depth distribution of both specles was considerably shallower than the depth at whlch the underwater hght lntensity matched thelr photosens~tivlty thresholds Daytime depth distnbut~on was significantly correlated wlth light Intensity, however with populations being deeper on days w ~t h more hght penetration