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 zoop lankton prey (rotifers). Treatments consisted of six different irradia nce 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 wate r column during the late-spring, weakfish spawning season in Delaware Bay. Early-stage larvae (8 days post-hatching) did not feed in total d arkness, and there was no difference in the incidence of feeding among the other treatment groups. Similarly, late-stage larvae (13 days pos t-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 t he other light levels. Results of a subsequent experiment indicated th at the ability to feed in total darkness may depend on the abundance o f available prey. Scanning electron microscope analysis of preserved w eakfish larvae showed that neuromasts were not fully developed until l arvae had reached at least 12 days post-hatching, and that younger lar vae had only lateral line pores along the body trunk. There were no ne uromasts evident on the head region, regardless of age. Thus, neuromas ts may be involved in the capture of prey in darkness.