Effects of grazing and damselfish territoriality on bioerosion of dead corals: Direct effects

An experiment was performed on Britomart Reef, Great Barrier Reef (central region) to determine the relationship between fish grazing, damselfish territoriality, and external bioerosion of dead coral substratum. The damselfish, Hemiglyphidodon plagiometopon Bleeker (Pomacentridae), was used because it excludes herbivorous fish from its territories, creating distinct undergrazed patches in the environment. Its territories were used to simulate conditions of naturally reduced grazing. Pieces of the freshly killed plating coral Pachyseris speciosa Dana were placed under four conditions: (1) within cages designed to exclude grazing fish; (2) within the territories of the damselfish H. plagiometopon; (3) beneath shade-tops to control for a decrease in light; and (4) outside damselfish territories, fully exposed to fish grazing.

Undeterred fish grazing was found to cause high levels of external bioerosion of dead foliose corals, causing a decrease in photographic area of plates by up to 50% within 18–24 months. H. plagiometopon effectively reduced rates of fish grazing and thus external bioerosion within their territorial borders. The degree of this effectiveness, however, varied with the individual. Fish grazing was found to greatly alter the microtopography of the substratum, almost completely removing the carinae and septa of the coral. This was concomitant with the loss of most microalgae and sessile invertebrates, suggesting that newly settled invertebrate larvae are probably disturbed by fish grazing, as has been found in related studies.

Time-lapse photography revealed that, in addition to larger scarids and acanthurids, small fish (Pomacentrus molluccensis Bleeker and blennies) graze diurnally and small invertebrates (pagurid and galatheid crabs) feed nocturnally on the coral substratum. These organisms were found to have negligible bioerosional effects. The guts of both P. molluccensis and the crab Galathea subsquamata Stimpson, however, were found to contain the spicules of boring sponges, suggesting that predation may influence internal bioerosion by affecting population dynamics within the boring cryptofaunal community.