THE EFFECT OF LIGHT AND DARKNESS ON HATCHING IN THE POMACENTRID ABUDEFDUF-SAXATILIS

Reports that pomacentrid embryos hatch after dusk are confirmed by pho tic manipulation of sergeant major eggs. Embryos placed in the dark fo r 20 minutes or longer prior to their normal hatching after sunset hat ched, whereas controls held in light did not hatch. Percent of hatched embryos correlated with increasing exposure to darkness up to one hou r after which no further improvement in hatching was observed. Embryos maintained in continuous light during their normal twilight hatching period did not hatch. Also, embryos exposed to 60 minutes of darkness, if interrupted by one minute of light every 10 minutes did not hatch. The percent hatch in dark treatments varied significantly between nes ts and, in some treatments, correlated negatively with the size of the egg clumps (number of eggs per clump) tested. To initiate hatching in the presence of light required intensities of 0.03 lux or less. These low intensities are not reached until about 20 minutes after sunset o n the reef where the embryos occur. We conclude that hatching for some embryos occurs about 30 minutes after sunset but for most is not comp leted until at least one hour after sunset. Hatching therefore takes p lace at a time long after potential diurnal fish predators have refuge d in the reef structure.