220 ANNUAL BEPORT SMITHSONIAN INSTITUTION, 1917. 



spines of the sea urchin, Diadema setosum (now called C entrecMnus 

 setosus) ; look warily into crevices, and step carefully into pools, 

 otherwise sore feet, legs, arms, and hands for days or weeks to come 

 may be the penalty ! 



GEOGRAPHIC DISTRIBUTION OF CORAL REEFS. 



Having given a definition of " coral reef " and having eliminated 

 from the category of coral reef and coral rock, limestones whose for- 

 mation is independent of the activities of corals, a few words will be 

 devoted to stating where coral reefs occur. They are found in those 

 parts of the ocean where the conditions summarized on page 215 of 

 this article prevail. As the proper conditions of depth, salinity, and 

 purity of water, intensit}'' of light, and character of bottom, are wide- 

 spread in the ocean, the temperature factor is critical in restricting 

 coral reefs to certain areas in tropical and subtropical seas. Coral 

 reefs thrive only where the average temperature of the caldest month 

 of the year does not fall below about 21° C. (70° F.), and where the 

 usual temperature is between 25° and 30° C. (77° to 86° ¥.).^ A well- 

 known oceanographic fact is that the waters along the western shores 

 of continents are colder than those on the eastern sides. The great 

 living coral reefs are therefore in the tropical western Pacific Ocean, 

 around the tropical islands of the mid-Pacific, in the Indian Ocean 

 and the Eed Sea, and in the tropical and subtropical western Atlantic 

 Ocean. Eeef corals are weakly developed on the Pacific side of Cen- 

 tral America and Mexico and on the Atlantic coast of Africa. 



Some features of the Atlantic (Caribbean and Floridian) reefs 

 will now be compared with thoge of the Indo-pacific reefs. There 

 are at present two great biogeographic divisions of reef-coral 

 faunas: one is the Atlantic, the other is the Indo-Pa(^ific, separated 

 from each other by the land area of Central America. In their 

 ecologic relations the reef corals of the two regions arc identical, but 

 there are important systematic differences, and the Pacific corals are 

 more luxuriant in growth and more numeroug in species than the At- 

 lantic. That Pacific corals appear to grow more rapidly than those in 

 the Gulf of Mexico and the West Indies was pointed out on page 214 

 of this article. The number of species on a section of an Indo- 

 Pacific reef usually ranges between about 55 and something over 70. 

 Von Marenzeller records 71 species from the Red Sea ; Bedot lists 74 

 species and 5 varieties from Amboina, but I believe 4 of his specific 

 names are synonyms, leaving 70 valid species. I have identified 63 

 species in Mayer's collection from Murray Island, Australia, and 51 



1 For detailed information on this subject see as follows : Vaughan, T. W., Tempera- 

 ture of the Florida coral leef tract: Carnegie Inst. Washington Pub. 213, pp. 319-339, 

 1917 ; and Mayer, A. G., in his " Ecology of the Murray Island coral reef," ibid., pp. 

 1-48, pis. 1-19, gives the temperature records for the Murray Island reof for the period 

 while he was there. 



