Loisette Marsh

Apartment-style living on a kebab sponge

Sponges are important three-dimensional habitats for other organisms such as polychaetes, crustaceans, echinoderms and fish that often live or breed on preferred parts of a sponge, thus reflecting functional relationships (e.g. Koukouras et al. 1995). Most Caulospongia spp. (Demospongiae: Suberitida) have morphologies supportive of epibiosis. They are characterized by an erect axis and basal stalk, with a reticulate body with lumina, or horizontal, disc-like lobes arranged along the axis (Fromont 1998). This tiered morphology led to our field name ‘kebab sponge’ (Fig. 1a–c). We sampled a specimen of Caulospongia pennatula (Lamarck, 1814) at Ningaloo Reef, northwestern Australia, that had wide horizontal surfaces, and on the edges underneath each tier, one or two—never more—porce la in crabs Pis id ia cf . streptochiroides (Johnson, 1970) were found (Fig. 1c–e). Porcelain crabs can be vividly coloured, mimicking the appearance of the surface they live on (Haig 1981). Here they were of a slightly lighter ochre colour than the sponge and thus well camouflaged (Fig. 1d–f). The hollow, lattice-like axis of the sponge was inhabited by Ophiothrix sp. brittlestars, whose bodies were lodged within, with their arms extending onto the lobes, also of a similar colour (Fig. 1g–k). Brittlestars are known to feed on detritus on benthic invertebrate surfaces (Stewart 1998), while porcelain crabs can scavenge or feed on host exudates, but mostly filter the water column using modified mouthparts (Kropp 1981). Presumably, the porcelain crabs colonise the sponge in order to be elevated above the seafloor to gain access to water currents, and may also benefit from the sponge’s feeding currents. We counted 19 crabs (seven males, seven egg-carrying females, five juveniles) and three brittlestars on this sponge. Thus, the morphology of a kebab sponge has microhabitats that offer shelter and possibly enhanced nutrition, while allowing epibionts to be elevated in the water column.