A 6-month study consisting of collections and observations revealed that a diverse fauna of reef-fishes inhabit the rubble mounds constructed by the sand tilefish Malacanthus plumieri (Perciformes: Malacanthidae). In the Santa Marta region, on the Caribbean coast of Colombia,
M. plumieri occurs on sandy areas just beyond the coral zone. The population density is correlated with the geomorphology of the bays; the composition of the material utilized depends on its availability. Experiments showed that debris was distributed over a distance of 35 m. Hard substrate
must be excavated to reach their caves. In the area around Santa Marta the sponge Xestospongia muta was often used by the fish as a visual signal for suitable substratum. The rubble mounds represent a secondary structure within the “coral reef” ecosystem. These substrate
accumulations create structured habitats in the fore reef, which are distributed like islands in the monotonous sandy environment and where numerous benthic organisms are concentrated. The tilefish nests attract other organisms because they provide shelter and a feeding site in an area where
they would not normally be found. At least 32 species of fishes were found to be associated with the mounds. Some species lived there exclusively during their juvenile stage, indicating that the Malacanthus nests serve as a nursery-habitat. M. plumieri plays an important role
in the diversification of the reef environment.
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