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

Abstract

ABSTRACT The oceanlc copepods Pleuromamma gracdis and Pleuromamma xiphlas undergo del 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 granhs 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 negat~ve phototaxis to hght of 1 4 X lOI4 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 wth a submersible spec- tiorad~orneter to daytlme depth distnbution of P gracjlis and P xiphias captured near mldday w~th a h4OCNESS net system lt was found that the daytime depth distribution of both specles was consider- ably 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~th more hght penetration INTRODUCTION