Effects of depth on the feeding capabilities of two octocorals

Feeding abilities of Plexaura homomalla and P. nina were measured during a saturation dive in the underwater habitat Hydrolab at Salt River Canyon, St. Croix, US Virgin Islands. Feeding rates were measured as the number of Sephadex beads ingested by polyps exposed to a known density of beads for 15 min. At Salt River, P. homomalla is found above 20 m and P. nina below 20 m. Feeding abilities were measured in each species' own habitat. At both 17 and 29 m P. nina had greater feeding rates than P. homomalla. These differences can be attributed to P. nina's larger polyps. Feeding rates of both species tend to be lower at 29 m than at 17 m, but feeding rates of P. homomalla at 17 m are equal to those of P. nina at 29 m. Habitat related differences in feeding rates correlate with light availability, but the distribution pattern of P. nina and P. homomalla at Salt River cannot be explained by their heterotrophic and photoautotrophic abilities.