ONTOGENY OF PHOTOTACTIC BEHAVIOR IN RED DRUM LARVAE (SCIAENIDAE, SCIAENOPS-OCELLATUS)

Phototactic behavior of red drum Sciaenops ocellatus larvae (age = 1 t o 17 d posthatch, size = 2.5 to 7.5 mm standard length) was examined. Numerical percent of test larvae phototactically responding per sample was determined after dark and light adaptation upon exposure to diffe rent stimulus light intensities and compared with dark or light contro ls. The relative spectral composition in these experiments was similar to that measured at mid-afternoon at 1 m depth in a coastal channel w here red drum larvae naturally occur. Day 1 larvae were not photorespo nsive; all tested older larvae showed positive phototaxis at higher st imulus intensities [greater than or equal to ca 10(-4) (Day 16) to 10( -1) (Day 3) mu Em(-2) s(-1)] and negative phototaxis at lower light le vels [as low as ca 10(-5) (Day 3) to 10(-7) (Day 5) mu E m(-2) s(-1)]. The light range for positive phototaxis generally increased with larv al age, as did the percent positive response when tested al the same l ight levels. These results indicate an ontogenetic increase in photore sponsiveness, beginning as early as Day 3 posthatch. Light-adapted lar vae as young as Day 4 exhibited a lower response than dark-adapted lar vae at the same stimulus intensities, indicating dark and light adapta tion. Estimated intensity thresholds for negative phototaxis were comp ared with near-bottom daylight intensities in 2 coastal channels where red drum larvae occur. Results indicate there is sufficient daylight available throughout the water column in the field for photoperception by red drum larvae. Using an estimate from the literature of a 3 to 4 log unit higher threshold for feeding than for negative phototaxis, w e conclude that natural intensities at depth are adequate for visual p redation during daylight hours.