DISCOVERY 



175 



environments are met with. At the edge of a reef 

 which fronts the ocean the best growth conditions 

 are attained. It may be supposed that the fresh ocean 

 water brings rich supplies of animal plankton, which 

 the coral polyps here are the first to sample and, as 

 the water sweeps shoreward, it is robbed of a good 

 share of the food it carries. A quantitative estimate 

 of plankton over the various parts of the reef has not, 

 however, been made. 



Whatever the effect of the agitation of the water 



live on the summit amongst the breaking waves, the 

 hardy plant thrives. The edge of Indo-Pacific reefs 

 commonly rises i8 inches or so above the rest of the 

 re^f on account of the vigorous growth, and is dry 

 at spring tides. It is called the Lithothamnion ridge 

 and is absent from the West Indian reefs. 



On the seaward edge below the rim the mechanical 

 effect of the waves is felt less and less, and 6 feet or so 

 below, growth is luxuriant and unrestrained, the Acro- 

 poras, of a different species, putting out such slender 



Piotograplt by A. G. Mayor, 



-EDOE; of CORAT. reef IX PAGO PAGO harbour, SAMOA. 



on the rate of growth, it certainly results in character- 

 istic modifications of coral form. In the picture 

 already alluded to the colonies of Acropora (= Madrc- 

 pora) are shown with their spreading mushroom-shaped 

 base and closely set stumpy branches, hugging the reef 

 surface tightly and offering as little chance as possible 

 to the disruptive force of the Pacific rollers. Many 

 colonies die, but their form is preserved by an enamel- 

 like coat of encrusting Lithothamnion — a calcareous 

 plant. (Such colonies appear in the photographs as 

 white patches.) This will grow, however exposed the 

 position ; and on coral pinnacles, where no corals 



and graceful, or spreading branches, as are generally 

 associated with their growth. This, too, is the effect 

 always to be traced passing shoreward over the reef 

 flat. The Lithothamnion ridge acts as a breakwater 

 a^ad protects the inshore corals from the full brunt 

 of the waves. It also ponds back the seawater at 

 low tide, so that on a fringing reef a gigantic shore 

 pool extends between the reef edge and the sandy 

 beach, of uniform depth, in which the numberless 

 coral colonies flourish— resembling a series of low 

 bushes witli their tops reaching a uniform level of about 

 i8 inches. 



