Selective feeding in the hawksbill turtle, an important predator in coral reef ecosystems

We evaluated selective feeding in hawksbill turtles Eretmochelys imbricata by comparing ingested prey species with their availability at 2 sites in the SW Dominican Republic. Hawksbills fed on 6 benthic species: 5 demosponges (Chondrilla nucula, Geodia neptuni, Myriastra kalitetilla, Spirastrella coccinea, and Tethya crypta) and 1 corallimorpharian (Ricordea florida). Hawksbills showed positive selection for 4 species (from highest to lowest): S. coccinea, R. florida, and C. nucula at Bahía de las Aguilas, and M. kalitetilla and C. nucula at Cabo Rojo. S. coccinea and M. kalitetilla are rare in the environment and highly selected by hawksbills, which supports a previous observation that their distribution on reefs could be greatly affected by spongivores. The 2 remaining selected species were the dominant prey species in lavage samples (R. florida = 59% and C. nucula = 34% of total volume). Since they were the most abundant species at each site, this illustrates that diet choice is based on a combination of selectivity for certain species and local abundance. The dominance of R. florida in the diet challenges the prevailing view that Caribbean hawksbills are strict spongivores. Finally, our results indicate that hawksbills can have a positive indirect effect on corals by grazing on coral competitors, as well as affect overall reef benthic biodiversity. Both C. nucula and R. florida harbor photosynthesizing symbionts and are aggressive competitors for space on tropical reefs. Thus, at natural population levels, grazing by hawksbills may well have played an important role in Caribbean reef structure and dynamics. Because hawksbill populations have been substantially reduced (to at most 10% of pre-Columbian population levels) their effect has been considerably diminished.