EFFECTS OF VARYING IRRADIANCE ON FEEDING IN LARVAL WEAKFISH (CYNOSCION-REGALIS)

Weakfish larvae, Cynoscion regalis (Bloch and Schneider), were used in laboratory experiments, during May and June 1991-1993, to examine the effects of varying irradiance levels on capture and ingestion of zooplankton prey (rotifers). Treatments consisted of six different irradiance levels: no light, 5, 11, 15, 20, and 500 x 10(12) quanta.cm-2.s-1. These levels are typical of the irradiance range found in a 10-m water column during the late-spring, weakfish spawning season in Delaware Bay. Early-stage larvae (8 days post-hatching) did not feed in total darkness, and there was no difference in the incidence of feeding among the other treatment groups. Similarly, late-stage larvae (13 days post-hatching) showed no significant difference between the incidence of feeding in darkness and at 5 x 10(12) quanta.cm-2 s-1, though feeding within these two intensities was significantly lower than feeding in the other light levels. Results of a subsequent experiment indicated that the ability to feed in total darkness may depend on the abundance of available prey. Scanning electron microscope analysis of preserved weakfish larvae showed that neuromasts were not fully developed until larvae had reached at least 12 days post-hatching, and that younger larvae had only lateral line pores along the body trunk. There were no neuromasts evident on the head region, regardless of age. Thus, neuromasts may be involved in the capture of prey in darkness.