Sediment-rejection mechanisms of 42 species of Australian scleractinian corals

Abstract

Mechanisms of sediment rejection by 42 species of Scleractinia from 31 genera, all with wide Indo-Pacific distributions, were investigated in situ and in the laboratory at Lizard Island, northern Great Barrier Reef. Rejection mechanisms of flat tissues (generally six replicates plus controls) were studied in response to a single rapid influx of 50 mg cm^-2 of 70/30% calcium carbonate/quartz sediment of each of four particle sizes: silt (<63 pm), fine sand (63-250 μm), coarse sand (500 μrn to 1 mm), and granules (1-3 mm). Additional observations were made of responses to variations in sediment loads (to a maximum of 1000 mg cm^-2) , to individual sediment particles, and to less-dense organic sediment and food, as well as of the effects of tissue angles, colony morphology, and in situ environmental conditions.

Ciliary currents, tissue expansion, and mucus entanglement occur in all of the species studied. Direct tentacle manipulation and pulsed partial contraction of the polyp or coenosarc also occur but were not observed in all species. Mesenteries may play a subsidiary or incidental role. Active-rejection mechanisms are consistent within species and, with the principal exception of some Faviidae species, are similar for the congeneric species studied.

Species are categorized according to their observed active-rejection capability. This capability is positively correlated with calice size: all species with large calices (> 10 mm in diameter) are capable of rejecting influxes of up to at least 50 mg cm^-2 of the tested sediment sizes with comparative ease; those species with small calices (<2.5 mm in diameter), particularly the two Porites species and the three Montipora species, are poor active rejectors; and other species, notably Acropora hyacinthus and Pocillopora darnicornis, though having some active-rejection capability, exhibit morphologies that make active rejection mostly redundant. Species with calices between 2.5 and 10 mm in diameter show more variation, but all very active rejectors in this size class have strong ciliary mechanisms.

There are differences in the area of a colony involved in the rejection of sediment influxes, depending on sediment size and density. Rejection of heavy influxes of all sediment sizes is principally restricted to flat or concave surfaces, whereas individual particles of silt and fine sand as well as light influxes of silt and almost neutrally buoyant particles of larger sizes frequently require active rejection from strongly inclined, and even near-vertical, surfaces. The significance of these findings in terms of energy budgets is discussed.