Plasticity of Feeding Habits of Two Plectroglyphidodon Damselfishes on Coral Reefs in Southern Taiwan: Evidence from Stomach Content and Stable Isotope Analyses

On the west bank of the intake bay of a nuclear power plant at Nanwan, southern Taiwan, a recent outbreak of sea anemones has eliminated most branching corals at the site. In this habitat currently dominated by sea anemones, the abundance of Dick's damsel, Plectroglyphidodon dickii Lienard 1983, has increased more markedly than the sympatric congener, P. johnstonianus Fowler and Ball 1924. To study whether the distribution patterns of the 2 Plectroglyphidodon damselfishes were underlain by food availability and feeding habit plasticity, the reef area in the embayment was divided into 2 different zones, zone A dominated by Acropora corals and zone B dominated by a sea-anemone, and both stomach content and stable isotope analyses were used to delineate feeding habits of these 2 damselfishes in these 2 zones. Stomach contents showed that the major food items for P. dickii in zone A were filamentous algae (33.4%) and coral polyps (22.3%), while in zone B, they were filamentous algae (35.8%) and sea anemones (28.2%). In contrast, coral polyps were the major food item for P. johnstonianus in both zones (accounting for 75.5% in zone A and 67.5% in zone B). Analyses of d super(13)C and d super(15)N values concurred that coral polyps were the major nutrient source of P. johnstonianus. In addition, the contributions of both coral polyps and sea anemones to P. dickii were verified. Overall, it was postulated that in the sea anemone-dominated habitat, sea anemones were substituted for coral polyps in the diet of P. dickii, but similar plasticity did not occur in P johnstonianus. Thus the higher feeding plasticity of P. dickii might have made it possible to more-successfully immigrate to the newly developed habitat.