Temporal variations of macroborers in massive Porites lobata on Moorea, French Polynesia

Abstract

Massive colonies of Porites lobata on the barrier reef of Tiahura, Moorea, can be divided into four categories: living colonies, colonies consisting of 50% live coral and 50% dead skeleton, 100% dead coral and colonies which have been reduced to a basal plate. Replicate samples of each of these colony types were collected in the same vicinity of the barrier reef during October 1987. The macroborers were extracted, identified, counted and their volumes determined by displacement. Kruskal-Wallis tests showed that three different boring communities occur within these four categories of Porites colonies. Live colonies are characterised by only 3 species, the bivalve, Lithophaga laevigata; the vermetid Dendropoma maximun; and the non boring serpulid polychaete Spirobranchus. The completely dead colonies contain up to 17 boring species, with five to six individuals per 100 cm. Sipunculans are the dominant bioeroders with the most abundant species being Aspidosiphon elegans, sp A and sp B. Colonies of Porites which have been reduced to basal plates contain up to 18 boring species of which the bivalve Lithophaga hanleyana and the sipunculan Aspidosiphon sp. B are the most abundant.

The cumulative volume of CaCO₃ lost by boring activity increases from 0.1 cm³ per 100 cm³ in a completely dead Porites colony to 1.4 cm³ per 100 cm³ in the residual basal plates of Porites. These can be extrapolated to minimum losses of 14.2 kg m^-3. We suggest that rates of boring increase with the time which has elapsed since the death of the colony and the dominant agents of boring also change with increasing age of the coral structure. There are significant additional losses of 5.25 kg m^-3 CaCO₃ caused by grazing echinoids and scarids.