ter that flows by, or that creep about on the branches 

 safe above the smothering sediments of the soft sea 

 floor. 



Dendrophylhds are also well known in the Medi- 

 terranean for their lovely yellow or red polyps. As- 

 troides (Plate 24) forms an orange-colored belt 

 below the water line. A similar coral is Tiihustrea, 

 a bright red dendrophyllid that is widely distributed. 

 On Jamaican reefs the red flesh of Titbastrea stands 

 out in sharp contrast with the reef corals that take 

 their soft green and golden-brown colorings from 

 plantlike cells contained within the transparent and 

 colorless tissues. 



Corals Restricted to Shallow Tropical Seas 



Nearly all shallow-water corals of warm waters — 

 and this includes all the true reef corals — are abun- 

 dantly filled with plantlike photosynthetic cells 

 (called zooxanthellae and thought to be modified 

 dinoflagellates). The chemical partnership that links 

 animal and plantlike cells apparently makes possible 

 the close spacing of the polyps of huge reef com- 

 munities, in which millions upon millions of individu- 

 als are crowded together in great honeycombs of 

 coral, further congested by myriads of hangers-on. 



Reef corals do not digest their plantlike cells even 

 though these always occur in the digestive lining. If 

 starved or placed in the dark, the corals eject the 

 little guests. Nor is oxygen usually in short supply on 

 the wave-beaten surface of a reef. The most in- 

 formed guess, that of C. M. Yonge, who during the 

 Great Barrier Reef Expedition in 1928 performed 

 many carefully controlled experiments on living reef 

 corals, is that the corals benefit most from the rapid 

 removal of their carbon dioxide, and especially of 

 their nitrogenous and phosphate wastes — and that 

 this rapid turnover of materials promotes the prolific 

 growth of tropical reefs. This means that true reef 

 corals are limited in their distribution to conditions 

 under which the plant-animal bond remains intact. 



Rate of growth varies with species, location, depth, 

 and other factors. Some corals measured in the 

 1 890's by Saville-Kent on Thursday Island, in the 

 Great Barrier Reef area, were measured again 

 twenty-three years later. A brain coral had increased 

 from 30 to 74 inches in diameter, and a specimen of 

 Porites (Plate 25), a very dense coral, from 19 feet 

 to 22 feet 9'/2 inches. The East Indian reefs, below a 

 depth of fifteen feet, grow upward as much as 4 

 inches a year. 



A map of coral-reef distribution reveals that the 

 reefs occur in a great warm belt that engirdles the 

 middle of the globe, roughly between latitudes 30^N. 

 and 30°S., so that the reefs of the western Atlantic 

 extend for about the same distance north and south 

 of the equator as do those of the Indo-Pacific. Within 

 this belt temperatures average 21 °C. (about 70 F.) 



Each polyp of the coral colony ( Astraiif^ia ) is about 

 % inch high. It produces new individuals by bud- 

 ding, thus enhirging the colony. Astran^ia tolerates 

 the cold waters off Cape Cod, Massachusetts, but 

 does not form reefs there. (American Museum of 

 Natural History) 



or higher, and never drop lower by more than a few 

 degrees or for very brief periods. The most flourish- 

 ing growth and the greatest variety of species is in 

 waters that average 25° to 29°C. (77° to 84°F.), 

 and most of the antler-like branching forms are in 

 this narrower zone. Below an average temperature 

 of 23.4°C. (74.3°F.), the species are dominated by 

 the more resistant rounded forms. 



A closer look at the map brings out great gaps in 

 the distribution of corals where cold currents from 

 the Antarctic stream north along the western coasts 

 of continents. The western coasts of Africa and South 

 America have almost no reef corals. Neither western 

 Mexico nor California have any true reefs. Warm 

 currents, on the other hand, make a small bulge in 

 the reef belt at Bermuda, where the northward-flow- 

 ing warm waters of the Gulf Stream support beauti- 

 ful if not typical reefs at 32°N. The southernmost 

 Atlantic reefs are those at the latitude of Rio de 

 Janeiro (23°S.). In the Pacific the reefs extend 

 northward to the southern shore of Japan, while in 

 the southern hemisphere the reefs farthest from the 

 equator are those of Queensland, Australia (about 

 24°S.). Other great gaps occur at the mouths of 

 rivers, where fresh water and silt are fatal to the 

 growth of reef corals. 



Thus the lesser of the two great centers of reef- 

 building, that of the Caribbean and adjacent waters, 

 includes the reefs of Bermuda, the Bahamas, the 

 West Indies, southeastern Florida, and parts of the 

 coast of Brazil. Diving expeditions have found some 

 reef growth on the western or Gulf coast of Florida, 

 at depths of 50 to 1 50 feet. But only the southeastern 

 shore of the peninsula of Florida has good reefs, and 

 these can be seen a few miles south of Miami. Some 



111 



