44 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



flat actually rises about 6 inches above the level of mean low tide. But the 

 limestone of the fringing reef-flat between the lithothamnion ridge and the 

 shore has disappeared, leaving a lagoon about i8 inches deep at low tide, 

 and this despite the growth upon it of one of the most densely clustered coral 

 colonies the writer has ever seen. Were it not for the growth of this coral 

 the reef-flat would have suffered even more serious disintegration, and in 

 hurricane regions where the corals on top of the reef-flats are periodically 

 destroyed it seems possible that such a fringing reef as that of Maer Island 

 might gradually become converted into a barrier reef by the solution of the 

 limestone, due to "sand feeders" and to scouring due to currents. 



In this fringing reef there seems reason to support the belief that the 

 lithothamnion ridge was once close to the shore, but that it has advanced 

 seaward as the reef-wall grew outward, while at the same time its shoreward 

 side disintegrated, thus forming the present shallow lagoon of the reef-flat 

 which is prevented from becoming deeper only by the densely clustered coral 

 heads which grow upon its floor. In this reef-flat, I believe, we have an 

 example of a fringing reef which, were it not for the luxuriant growth of corals 

 over its floor, would change into a barrier reef. We call it a "fringing reef" 

 because its lagoon is only 1.5 feet deep. If the lagoon were 15 feet deep the 

 reef would be called a "barrier." 



Reef corals are, however, not the only organisms which tend to build 

 up limestones, for Cary' has shown that at Tortugas, Florida, the Alcyonaria 

 are more important than the stony corals, in this respect. 



SUMMARY OF CONCLUSIONS. 



The factors which chiefly determine the mode of distribution of the 

 various species of corals over the southeast reef-flat of Maer Island, in the 

 order of their importance, are (i) temperature, (2) silt,^ (3) mechanical effects 

 of moving water, and (4) the struggle for existence between the various 

 species of corals. 



For example, Seriatopora hystrix, which is the dominant coral of the 

 middle zone of the reef-flat, can not live within 500 feet of the shore on 

 account of its inability to withstand high temperature and muddy water; 

 nor can it live upon the outer 200 feet of the reef-flat, for here the waves 

 shatter its fragile stems; yet in regions between 1,000 and 13,000 feet from 

 shore it succeeds so well that all other species are either crowded out or 

 reduced in numbers, being unable to compete with it for a foothold. 



It is remarkable that the ability to resist high temperature is fairly well 

 correlated with a coral's ability to withstand the smothering effects of silt, 



'Cary, L. R., 1915, Froc. Nat. Acad. Sci., vol. I, pp. 285-289; also: Carnegie Inst. Wash., Year Book 

 No. 14, pp. 200201, 1915. Also the last paper of this volume. 



^here is very little silt upon this part of the reef-flat. In a muddy region silt would certainly be the 

 most important factor in its effect upon coral life. 



