ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 27 



whereas in the less agitated water there is abundant evidence of a severe 

 struggle between various forms for mastery. Thus, in the mid-region of the 

 reef-flat Seriatopora crowds out a number of species which appear both shore- 

 ward and seaward of this region, and all other species are reduced in numbers 

 where Seriatopora is most successful. The surges can not destroy the small 

 heads which are sheltered in crevices, but on the contrary the agitated water 

 must bring to them a good supply of pelagic animals for food. Thus, Wood 

 Jones is perhaps somewhat misleading when he asserts that the region of the 

 breakers is relatively deficient in coral life. However, at a distance of about 

 1,725 feet from shore, where the lithothamnion ridge is most broken and 

 pounded by the surf, corals do not thrive well, possibly owing to the extensive 

 growth of a Spirogyra-like seaweed upon the rocks. It will be recalled that 

 in 1878 the corals in parts of the lagoon of Cocos- Keeling were destroyed by 

 water from a supposed volcanic vent, and Wood Jones observes that the 

 coral growth in these regions has not reappeared even after thirty-four years. 

 Wood Jones attributes this failure to the growth of a Spirogyra-like seaweed. 

 Curiously, a parallel case occurred at Tortugas, Florida, where Acropora 

 (which once grew in abundance in the lagoon) was killed by the " dark water " 

 of October 1878, 1 and has not yet (1917) renewed its dominance, only an 

 occasional small cluster being here and there found in the shallow waters of 

 the lagoon where once the patches covered acres in area. Thus corals which 

 were once the dominant species may, if destroyed, be unable to reassert 

 their supremacy even after thirty-nine years. But the author is inclined 

 to believe that causes other than the growth of seaweed have prevented 

 Acropora from reappearing in the lagoon at Tortugas. 



Where the corals struggle mainly against an adverse environment the 

 heads are widely separated but are of many species; but where the environ- 

 ment is favorable and the struggle is chiefly between coral and coral, the heads 

 are closely crowded, but the kinds that can survive are few. One seems to 

 see this law illustrated upon a grand scale in the many individuals of few 

 kinds which crowd the cold waters of temperate and subpolar seas where the 

 plant-food is abundant and the temperature near an optimum for vital 

 processes. In the tropical oceans, on the contrary, the plant-food is deficient 

 and the temperature is close to the danger-point, and here in this relatively 

 unfavorable environment individuals are rarer than in cold seas, but the 

 number of species is far greater. 



It will be recalled that Duerden, 1904, 2 found that corals require animal 

 food in considerable quantity, and Vaughan, 191 2, 3 showed that their food 

 is exclusively animal, and that they do not devour phyto-plankton. One 

 might suppose, therefore, that the gradual decline in the number of coral 



■Jefferson, J. P., J. Y. Porter, and T. Moore, 1878, Proc. U. S. Nat. Mus., vol. I, pp. 244-246. 

 J Duerden, J. E., 1904, The Coral Siderastrea radians, Carnegie Inst. Wash. Pub. No. 20, p. 5. 

 'Vaughan, T. W., 1912, Carnegie Inst. Wash. Year Book No. 11, pp. 159-161. 



