24 UNDERWATER GUIDE TO MARINE LIFE 



The depth of water influences coral growth. Because of the symbiotic, 

 photosynthetic, oxygen-supplying flagellate protozoans (Chapter 7) that live in 

 coral polyps, growing reefs are not found in waters deeper than about 150 or 

 perhaps 200 feet. Clarity of water has a great deal to do with successful coral 

 growth. In fact, silting can kill coral by cutting down light or by smothering the 

 coral animals. 



Lowered salinity will eliminate corals. The mouths of rivers never have 

 coral growths and heavy tropical rains are a reef -destroying factor. 



With all these things taken into consideration, it is almost a wonder that 

 reefs are formed at all. Nevertheless, reefs are one of the most common char- 

 acteristics of all tropical seas, being common between 30° north latitude and 

 30° south latitude. The Gulf Stream pushes the northern limit up to Bermuda 

 at 32° north latitude. Probably the finest and most extensive coral formations of 

 the North American continent are those of the Bahama Islands. 



Coral grows at varying rates depending on the local conditions. A usual average 

 of one inch a year is given by Hesse, and others (1951). The growth of branched 

 coral is sometimes marked by yearly ridges. The American Museum of Natural 

 History in New York has a fine specimen of antler coral from the Bahamas 

 showing these ridges a good two to three inches apart. This is adequately 

 rapid to make nautical charts worth very little after a few years. In places like 

 the Red Sea, whole port towns have had to be abandoned because coral growth 

 made their harbors inaccessible. 



The heavy stone corals and brain corals are the foundation stones of reefs. 

 The branched Acro-pora shows great variety of form, being more delicate in 

 deep or quiet waters. All of these look like they are of a permanent nature, 

 rugged and durable, but the story of a reef is not one of mere construction. It 

 is also one of destruction, as is the whole cycle of life. Coral polyps die eventually. 

 Single coral colonies may live and grow over a period of a couple of hundred 

 years, but this process comes to a halt, and if the dead coral is not quickly 

 built upon by new colonizers, its lime will be etched by the waves and turned 

 to sand or redissolved in the sea. Waves are the greatest of the physical factors 

 which destroy reefs. Two more of these are winds and rains. But probably the 

 greatest overall destroyers are boring animals such as worms, sponges, clams, 

 crustaceans and others. Fishes which gnaw on coral, such as the common 

 parrot fishes, also add their bit in destroying coral formations. All of these 

 forces help reduce coral to sand, which sometimes becomes compacted into 

 coral limestone such as that seen on shores near many coral seas. The best 

 reefs to visit are those in which the forces of construction overbalance those of 

 destruction. The practiced eye soon learns to recognize the difference between 

 dying and flourishing reefs, which are sometimes close together. 



Three types of reefs are known (/ig. 5). These are (1) the fringing reefs 

 which closely follow coasts near shore, (2) the barrier reef which is separated 

 from shore by a lagoon, and (3) the coral atoll which is a ringlike island 

 formed by the sinking of a volcanic island at sea. The longest construction 

 ever built by living animals, including man, is Australia's Great Barrier Reef, 

 1,250 miles of coral formations. The deepest constructions of life are also of 

 coral origin. At Eniwetok Atoll in the Pacific, a boring of 4,000 feet was made 

 through limestone before striking a volcanic rock bottom (Barnett, 1955). 



