Coral reefs of Gorgona Island, Colombia, with special reference to corallivores and their influence on community structure and reef development

Abstract

This study offers an overview of the distribution and structure of coral reefs and coral communities at Gorgona Island, off the1 Pacific coast of Colombia. Biotic interactions, mainly the effects of coraliivores, are examined and compared with coral communities elsewhere in the eastern Pacific in order to provide a basis for understanding regional differences in community structure. Located at the south end of the Panamá Bight and ca. 30 km offshore, Gorgona is largely removed from the influence of up welling and freshwater dilution. The key corallivore Acanthaster, which is widely distributed in the tropical eastern Pacific, is absent from Gorgona, probably a result of unfavorable physical conditions (low temperatures and low salinités) for larval development in the Panama Bight area. True fringing coral reefs are present at Gorgona and are best developed on the eastern or leeward side, but they also occur in one area on the western and exposed side of the island. The La Azufrada reef, the largest of the fringing reefs, was about 1 km long and 150 m wide, had a minimum thickness of 8 m and supported live corals to a depth of IS m. Pocilloporid corals predominated on the reef crest and upper to mid forereef slope, and massive and small species increased in relative abundance in the deep forereef and sand plain zones. A corallivorous pufferfish and ovulid gastropod were abundant, but other known coraliivores (e. g., AcanfhOster, hermit crabs and Eucidaris) were either absent, uncommon or not feeding on live coral. Damage inflicted on live massive colonies by a damselfish, in order to "farm" algae, could play an important role in altering the relative abundances of corals in shallow reef zones. A comparison of coral reefs at Gorgona and Panama, in areas with and without Acanihaster, show that nonpocilloporid corals, which are preferred prey items, have a higher relative abundance on reefs where the predatory sea star is absent.