330 FOSSIL INVERTEBRATES/Corals and Other Cnidaria Figure 10 Representative Scleractinian corals (A) Fungia fungites, calical view of discoidal zooxanthellate corallum; Recent, Great Barrier Reef, Queensland, Australia; Â0.25 (B) Fungiacyathus symmetricus, discoidal azooxanthellate corallum; Recent, locality un known; Â3.5 (C) Cyclolites sp., calical view of zooxanthellate cupolate corallum with large numbers of closely spaced perforate septa; Cretaceous, Spain; Â0.8 (D) Favites sp., cerioid (scleractinian usage) colony in which intercorallite walls are formed by the interlock ing of the peripheral ends of the septa (a septotheca); Recent, Great Barrier Reef, Queensland, Australia; Â0.7 (E) Lophelia prolifera, azooxanthellate dendroid colony with well spaced corallites; Recent, coral bank on continental shelf, North West Atlantic; Â2.5 (F) Diploria sp., typical zooxanthellate meandroid colony (brain coral); Recent, West Indies; Â1 (G) Stylophora pistillata, ramose zoox anthellate colony in which each branch is made up of many plocoid corallites; Recent, Great Barrier Reef, Queensland, Australia; Â0.5 The role played by zooxanthellae is not entirely clear, but they make an important contribution to the energy budget of the coral, contributing up to 98% of its fixed carbon requirement They appear to accelerate calcification, which in the extreme case of the Stag’s horn coral Acropora can achieve a linear growth rate of 270 mm yr However, many massive Scleractinia appear not to have enhanced growth rates, and recent evidence suggests that, in this case, zooxanthellae may act to switch calcification on and off, rather than accelerate it, thus conserving energy The algae also remove waste from the coral and may thereby allow very large colonies (in excess of 10 m) of tiny individuals (approximately mm in diameter) to develop There is a striking contrast in coloniality between zooxanthellate and azooxanthellate corals The former are 95% modular by genus, whereas the latter are about 30% The symbiosis with algae appears to have developed in the early Jurassic A major factor in the success of scleractinian corals in building rigid structures is their ability to cement their skeletons securely to other skeletal material and to encrust hard substrates (Figure 11) Shallowwater scleractinian-dominated reefs became important from the Jurassic onwards, although deep-water banks built by azooxanthellate scleractinians extend back into the Triassic Palaeozoic Corals Rugose and tabulate corals enjoyed neither the advantages of algal symbiosis nor the ability to encrust securely to the extent seen in the scleractinians The first point has been contentious However, scleractinian zooxanthellate colonial corals show flattening of the skeleton and a significant reduction in growth rate with depth, as light levels decrease A recent study of a widely distributed tabulate coral, among those considered most likely to have harboured algal symbionts, showed no signs of these effects In addition, only 35% of rugosans (but all tabulates) are colonial, and massive growth forms of any Palaeozoic coral appear not to exceed about m in diameter Apart from some