THE ROLE OF PHOTORECEPTION IN THE SWARMING BEHAVIOR OF THE COPEPOD DIOITHONA-OCULATA

The copepod Dioithona oculata forms dense swarms near mangrove prop ro ots that are centered around shafts of light penetrating the mangrove canopy Swarms can be created in the laboratory within light shafts cre ated with a fiber optic light pipe. Laboratory observations of swarmin g behavior were recorded using video cameras, and the swimming behavio r of the copepods and density of the swarms were quantified using vide o-computer motion and image analysis techniques. Swarm formation resul ts from a combination of phototactic and klino-kinetic behavior Dark a dapted copepods initially exhibit a photophobic response to a light sh aft, but become positively phototactic within 3-5 min after exposure t o the light. Copepod aggregation rates under the light fit a saturatio n model, suggesting that copepods are attracted independently to the s warm marker. Copepods reverse their swimming direction when they encou nter light intensity gradients near the edge of a light shaft, which a ids in maintaining the swarm. Swarm formation can occur in the laborat ory at light intensities as slow as 0.1 mu M photons m(-2) s(-1),which is similar to light intensities at dawn when they are first observed to form in nature. Swarm formation appears to have an endogenous rhyth m, as copepods will not form swarms at night under a light shaft.