176 



DISCOVERY 



Mayor has made exact determinations of the abund- 

 ance and distribution of the various species of corals 

 b}' tracing a line across the reef from shore to edge 

 and surveying squares of 50 feet along it. In each 

 square the number of heads of coral was counted and 

 classified according to species. A survey of this kind 

 made at Murray Island on the Great Barrier Reef 

 brought out many interesting points. It showed, for 

 instance, that the greatest number of coral heads 

 (belonging to eighteen species) was found about 200 feet 

 behind the Lithothamnion ridge and about 1,400 feet 

 from the shore, while the greatest number of species 

 occurred on the ridge itself, though the number of 

 colonies is smaller, many being broken off in time of 

 storm. In the middle region of the reef, where growth 

 is easy in the calm water and nutritive conditions 

 good, there is a struggle between the various species 

 for mastery, and one, Seriatopora hystrix, emerges as 

 an undoubted victor, crowding out a number of others. 

 Its zone of dominance is restricted, however, and else- 



FIG. 2.—POCILLOPORA GROWTH VARIETIES. 



where it never occurs. Toward the shore colonies 

 become fewer and fewer with many gaps between, 

 and it is found by experiment that they belong to the 

 hardy species, which will stand exposure to over- 

 heating of the shallow water by the sun and suspended 

 mud or sand from the adjacent shore. These, like 

 some species of Poriks, are very different, in this respect, 

 from the more delicate corals of the middle region and 

 reef edge. Some genera are distributed widely over 

 the breadth of the reef, and in this case there are 

 several species replacing each other, or, as in the genus 

 Pocillopora, one very variable species. The photo- 

 graph here given (Fig. 2) illustrates a colony with 

 massive branches from the reef edge, and a looser 

 delicately branching one from the quieter waters 

 toward the shore, but both belonging to this species. 

 Even more interesting than the correspondence to 

 position is the effect produced in Pocillopora by a small 

 crab (Hapalocarcinus) which associates itself with the 

 coral particularly. The female, which attains the size 

 of a pea, settles down between two growing buds and 

 controls their growth by the current of water she sends 

 out from her gill chambers. The two buds broaden 



out to short palmate branches, quite unlike the ordin- 

 ary slender forms, and curve over and unite to form 

 a closed chamber about the size of a hazelnut, with 

 small perforations serving for the passage of water 

 and food, and kept from closing only by the prisoner's 

 respiratory activity. The female crab is fertilised by 

 the much smaller male before the "gall " closes, and 

 sends out her numerous offspring through the pores. 

 But, though the form of these curious structures 

 reminds one strongly of vegetable "galls," yet there 

 is no exact parallel between the two cases, for the 

 crab works by directing the growth of the coral rather 

 than stimulating the tissues to abnormal activity. 



Experimental work on the growth of coral colonies 

 has been conducted by Mayor for some years. The 

 simplest of these are estimations of rates of growth. 

 These are made firstly by taking a colony, photograph- 

 ing and weighing it, putting it on a cement base and 

 attaching a numbered label. The colony is then re- 

 placed on the reef and, after a year or eighteen months, 

 if a hurricane has not meanwhile removed all traces 

 of the experiment, it is retrieved, photographed, and 

 weighed again. Such experiments show a consider- 

 able rate of growth and confirm the statement made 

 by Stanley Gardiner that in the Indo-Pacific region 

 a reef 150 feet thick might be built in 1,000 years. 

 But the growth rates obtained in the ^^'est Indies and 

 the Pacific by the Carnegie Institution differ greatly. 

 It may be said, in fact, that growth is roughly twice 

 as rapid in the latter as in the former, but an explana- 

 tion of this is yet to be made. 



Another line of experiment is the transplantation 

 of coral colonies from one kind of position to another, 

 say from the reef edge to the quiet waters of a channel 

 20 feet below the surface. Some species, as one would 

 expect, respond to such a change. After a few months 

 it will be seen that their stumpy branches have put 

 out slender shoots, and a remarkable difference may 

 be noticed between the earlier and later parts of the 

 colony, as if a different species had been grafted on 

 the original stock. But in other species the response 

 is limited or entirely absent. 



Many observers have recorded the influence of the 

 heavy rainstorms of the tropics on coral reefs. Such 

 a one in May 1920 in Samoa, when 28 inches fell in 

 thirty-six hours, washed down such loads of soil from 

 the island hills into the harbour of Pago Pago that the 

 water changed from dark blue to chocolate, and when 

 it began to clear a thin film of mud coated over most 

 of the coral heads and caused their death. Mud or 

 silt has a selective efi'ect. The brain corals {Pontes, 

 etc.) are as a rule able to resist its suft'ocating effects 

 where Acropora and Pocillopora will perish. But in 

 the case mentioned, huge brain corals, probably fifty 

 years old, were killed and whole stretches of reef 



