An important part of the bottom of a coral reef consists of dead coral reef framework cavities; this includes the spaces and surfaces under rubble, the undersides of skeletal organisms such as corals, the shaded undersides of overhanging dead or live coral, and deep framework cavities. Cavities are formed below protruding edges of stony corals in the coral reef framework and are often enlarged by bioeroding organisms. These cavities make up a major part of the volume of the skeleton of a reef. Estimates of the volume encompass 30–75% of total reef volume (Ginsburg 1983).
Cavities provide a surface area for colonization by organisms that may be greater than the horizontally projected reef surface area (Jackson and Winston 1982; Ginsburg 1983). The species composition of these cryptic habitats has been extensively studied (Kobluk and van Soest 1989; Meesters et al. 1991). Sessile groups such as sponges, crustose coralline and filamentous algae, ascidians, polychaetes, bryozoans, and foraminiferans usually dominate cryptofauna. Bioeroding organisms, such as clionid sponges and polychaetes, can make up a substantial part of the biomass in cavities (Scoffin et al. 1980; Hutchings 1983).
Cavities with their organisms can be considered as functional units, which have an important role in total reef metabolism (Gili and Coma 1998; Wunsch and Richter 1998; Richter and Wunsch 1999). There are indications that these functional units represent the major trophic link in organic matter transfer from the pelagic to the benthic compartment (Brock and Smith 1983; Linley and Koop 1986; Gast et al. 1998; Richter et al. 2001).
It is practically unknown what the influence of physiognomic characteristics of cavities is on the composition and cover of the cryptofaunal community. By physiognomic characteristics, we refer to cavity characteristics such as volume, hard substratum surface, morphology, orientation, and sizes of openings to ambient reef water. The geometry of a cavity will affect rates of exchange between cavity and overlying water, i.e., the residence time of water in cavities. Residence time is supposed to influence the water-borne food supply to cavities. We presume that these factors influence the composition and cover of the cryptofaunal community and the organic matter supply to cavities. Although qualitative observations supporting this hypothesis have been made, quantitative data are lacking because an easy instrument to measure cavity geometry was not available.
We developed a simple, inexpensive instrument, the so-called cave-profiler, which can be easily operated by a diver underwater to measure the inside morphology of normally inaccessible cavities (up to 1.5 m deep). Threedimensional processing of data, collected with the caveprofiler yields: (1) visualization of the highly irregular inner structure of framework and overhang cavities; and (2) accurate values for cavity volume, hard substratum surface area, and their aspect ratios.