Bioerosion and availability of shelter for benthic reef organisms

Abstract

Bioerosion was examined in 5 different reef habitats on the northeast of St Croix, USVI, and the hypothesis tested that cavities provided by borers govern the numbers and body sizes of an important motile cryptobiont, stomatopod crustaceans Bioeroders excavated significant percentages of material from beachrock or dead coral substrates In the intertidal zone (0 m), fringing reef (-1 m), lagoonal patch reef (-3 m), and the back reef (-3 m) and fore reef (-12 m) of the offshore barrier reef. Mean volumes removed were 8.4 to 18.3 % among habitats (range 0.1 to 32.9 % for individual rocks). Overall bioerosion was not correlated with depth, and was greatest on the patch reef. Results confirm that bioerosion is more intensive in Caribbean than western Pacific waters, and are consistent with the hypothesis that primary productivity governs the biogeographic impact of bioeroders. Average hole sizes were largest in the intertidal and on the fringing reef. Total numbers of cavities appropriate for stomatopods were greatest on the patch reef, intermedate on the fore, back and fringing reefs, and least in the intertidal. Stomatopod body sizes were largest in the intertidal and declined toward the offshore reef, congruent with the sizes of holes in these habitats. Also, the interbdal zone was inhabited by significantly fewer stomatopods than the fringing, patch and back reefs. There was no tendency, however, towards unusually high densities in the particularly abundant small holes of the patch reef. We conclude that sizes and to a lesser extent numbers of stomatopods are generally correlated with sizes and numbers of cavities available, although other factors (predation in subtidal habitats, storms in shallow habitats) sometimes influence population characteristics (especially densities). Consequently, the sizes of cavities available likely limit the body sizes of reef stomatopods, in contrast to those that excavate enlargeable burrows in soft bottoms, and biogeographic trends in their body sizes may be z t t ~ b ~ t a b l c to :he sizes sf cavtties p~uv ided by bioeroders. The sessile and particularly the motile cryptofauna represents an understudied but critically important component of the coral reef community, and boring organisms are prerequisite for its development. Fast and efficient predators evolved in the Mesozoic concurrently with scleractinian corals, large carbonate reefs and bioeroding organisms. We suggest that boring habits protected early bioeroders from predators and provided crypts, into which radiated a vast assemblage of benthic reef biota. This crytofauna was in turn molded by predation and thence competition for refuges. Thus, predation likely exerted a cascading effect upon the diversity and structure of coral reef communities via bioerosion and the availabhty of shelter for b e n b c reef organisms.