Free-living corals: distributions according to plant cover, sediments, hydrodynamics, depth and biological factors

in 1974, 8 free-living coral species were found to inhabit the sandy sea floor adjacent to Lizard Island, Great Barrier Reef (14°40′S; 145°78′E). They fall into two groups which colonize two dissimilar sediment types. Plant cover increases with depth and, because of its effect on sediment characteristics, is thought to be a significant factor affecting coral distributions. The shallower coarse to medium grain sediments (0.5 to 0.125 mm) are mainly colonized by Heteropsammia cochlea, Heterocyathus aequicostatus, Diaseris distorta, and to a lesser extent by Cycloseris cyclolites. The deeper sediments are made up of a biogenically derived coarse fraction (larger than 0.5 mm) combined with an equally high proportion of fine-grade material. Corals typically found on these sediments are: Trachyphyllia geoffroyi, Catalaphyllia jardinei, Cynarina lacrymalis, and Cycloseris patelliformis. The content of non-carbonate material in the sediments reflects the hydrodynamics of the area and hence the degree of sedimentation, i.e., traction, saltation, or suspension loads, the corals have to cope with. Depth of occurrence was found to predict local coral distributions but was not applicable to other regions. Other factors which are discussed in relation to coral distributions include: coral mobility, coral shape, the effect of an obligate sipunculan associate, Aspidosiphon jukesii, in Heteropsammia cochlea and Heterocyathus aequicostatus, and settlement requirements