ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 29 



from the mud which pours into the ocean from the Fly River region and 

 which prevents the growth of corals and keeps open the wide Bligh Entrance. 

 Moreover, being within 6 miles of the outer edge of the Great Barrier plateau, 

 the southeast trade wind drives water from the open Pacific upon their 

 shores. We know of no richer region than the Murray Islands for littoral 

 Echinoderms,' and man}^ types of moUusks are abundant among the coral 

 reefs; yet there are no large coral heads to be found upon the reef-flats which 

 surround these islands. 



At Thursday Island, where silt interferes seriously with the coral reefs, 

 the corals are more widely separated, but the heads are larger than at the 

 Murray Islands, and this is also true of the reefs between Townsville and 

 Cairns, where hurricanes are a disturbing factor; yet at the Murray Islands, 

 where conditions appear to be ideal for coral growth, we find only small 

 heads thickly clustered over the reef-flats instead of large ones more or less 

 widely isolated, as in other parts of the Great Barrier Reef. It seems 

 possible that in a hurricane region the large and firmly anchored stocks may 

 gain an advantage over the smaller ones in that they alone can survive the 

 eff^ects of such storms and may thus be free to fill the space once occupied by 

 the many small coral heads which formerly grew around them; also on reefs 

 subject to silting, the small heads must suff^er more seriously than the large. 



Small living stocks of organ-pipe coral, Tubipora, were found on the 

 southeast reef of Maer Island between i,ooo and 1,400 feet from shore, but 

 they were rare both shoreward and seaward of this region, and being every- 

 where few in number were not an important constituent of the reef. 



No Millepores were seen upon the southeast reef-flat, and they were 

 rare elsewhere among the Murray Islands. The fleshy Alcyonaria, such as 

 Sarcophyton, appear to be less sensitive to the injurious influences of silt than 

 are the Madreporean corals, and are the dominant forms ofi^ the western corner 

 of the island, being also common off the sandy northwest shore, but rare on 

 the wide southeast reef-flat, where there is little silt and the pure ocean water 

 is being constantly driven in over the reef by the southeast wind. 



When we come to study the distribution and association of the various 

 species of corals found on the southeast reef, we find that each species thrives 

 best at some definite distance from the shore. Some, indeed, are restricted 

 to zones extending neither to the shores nor to the outer edge of the reef. 

 Such for example are Pontes andrezusi Vaughan, which is found only between 

 450 and 1,100 feet from shore, and Seriatopora hyslrix, which is confined to a 

 region between 600 and 1,650 feet from shore. 



An inspection of table 3 shows that of the 24 genera of corals from the 

 squares across the southeast reef-flat only 3 extend entirely across the flat 

 from 500 feet out to the tide-pools on the crest of the lithothamnion ridge, 

 1,775 fs^t from shore. These 3 are nodular forms of Pontes, Pocillopora, 



'Dr. H. L. Clark collected 151 species by wading in the shallow waters near the shore of Maer Island. 



