The Influence of Light on the Twilight Migrations of Grunts

Behaviors that precede the daily migrations of mixed-species schools of juvenile grunts (Pomadasyidae), from patch reefs to grass beds at dusk and vice versa at dawn, are defined and utilized to ascertain the precision of the migrations. Although premigratory behaviors differ at dusk and dawn, the migrations are precise twilight events which occur at the same light intensities during dawn and dusk. Histological sections of the retina reveal that both cones and rods are fully exposed to ambient light during the migrations. Under the difficult photic conditions that prevail during migration, the retina is structured photomechanically to maximize the absorption of ambient light. Body colorations of the grunts, which consist mostly of intense colored stripes during the day, are replaced at night by cryptic melanic patterns. The precision of migration, the photomechanical movements in the retina, and the changes in body coloration are considered adaptive because they reduce predation on grunts when they migrate and are most vulnerable to attack. In support of this conclusion, the migrations take place just before the evening and just after the morning \u27quiet period\u27 - thus they avoid that period during twilight when predation is highest in tropical fish communities

The Behavior of Heterotypic Resting Schools of Juvenile Grunts (Pomadasyidae)

Resting schools of juvenile grunts (less than 12 cm length), composed primarily of Haemulon flavolineatum and H. plumieri, were studied from 1972 to 1976 on a series of patch reefs surrounded by seagrass beds on the northeast coast of St. Croix, U.S. Virgin Islands. Juvenile grunts form large inactive multispecies schools in reef areas by day. Repeated censuses over several years reveal that certain parts of the reef, particularly those with large formations of the corals Porites porites and Acropora palmata, are traditional schooling areas. When the fishes reach a size of about 15 cm, the H. plumieri form homotypic schools which roam the patch reefs but are no longer associated with particular formations of coral. In contrast, larger H. flavolineatum are uncommon on the patch reefs and they may move offshore to deeper water. The schools of juvenile grunts migrate some distance away (often 100 to 300 m) into seagrass-covered areas to feed on invertebrates at dusk each day. The routes which are followed into the seagrass beds are precise and persist over years. At dawn the grunts return to the reef on the same routes. Mass marking and release experiments show that juvenile grunts will move over long distances (approximately 3 km) to reach a home reef