Swimming Behavior of Cobia Larvae (Rachycentron canadum) Facing Prey and Predator

The swimming behavior (instantaneous speed and caudal fin beat frequency) of 4- to 5-day old cobia larvae was recorded with a horizontal optical system and a high-speed digital camera in small-volume vessels (50 mL) at 25 oC. Larvae were exposed to the following treatments: (1) rotifers (Brachionus sp.); (2) copepod nauplii (Temora stylifera and Corycaeus giesbrecht); (3) hydromedusae (Liriope tetraphylla); and (4) control (filtered seawater). Instantaneous swimming speeds in the rotifer treatment (maximum of 146.47 mm s-¹; mean and standard deviation of 28.99 ± 45.85) exceeded those observed in the nauplii (mean 8.95 ± 6.21 mm s-¹) and control (mean = 8.26 ± 4.23 mm s-¹) treatments, and were much higher than in the Liriope treatment (mean 3.44 ± 2.35 mm s-¹)  (ANOVA, p < 0.05). The caudal beat frequency was  higher in the controls (57.79 ± 5.85 beats s-¹) than with rotifers (47.32 ± 6.20 beats s-¹), nauplii (38.97 ± 14.69 beats s-¹) or hydromedusae (43.40 ± 16.45 beats s-¹) (ANOVA, p < 0.01). This study illustrates how swimming speeds and caudal beat frequencies of cobia larvae respond to mechanical or chemical clues associated with prey or predators.