ZOOLOGICAL SOCIETY BULLETIN 



I picture on the right shons this c 



GROWTH-RATE OF ACROPORA PHARAONIS 



al as it was .in April in, 1917 and the large picture 



ale on July 11, 1918 



graph on the right represents this coral as it 

 was in April 1917 and the larger picture shows 

 it on the same scale as it was in July 19 IS. It 

 increased from two to seventy ounces in weight, 

 and developed more than 250 new branches, in- 

 creasing in dimensions from 3.3 x 2.3 x 2.6 

 inches to 16.5 x 15.2 x 9 inches high. 



On an average about five and one-quarter 

 ounces of limestone appear to be added each 

 year to each square foot of the upper surface of 

 the reef flats of Pago Pago Harbor, Samoa, due 

 to the growth of corals, and this would give an 

 average upward growth of 0.35 inches per 

 annum over the entire area of the upper surface 

 of the reef-flats. 



Now we have bored through the reef flat in 

 Pago Pago Harbor. Samoa, near its seaward 

 edge and find that 1 2 1 feet of coral reef over- 

 lies the volcanic rock. A submerged platform 

 from about one to three miles wide and about 100 

 to 210 feet deep extends all around the Island 

 of Tutuila. and the modern coral reefs are grow- 

 ing outward from the shores upon this platform, 

 and are now about 120 feet thick on their sea- 

 ward faces. 



We see then that if the reefs grow upward at 

 the rate of 0.35 of an inch a vear, a reef 121 



feet thick would be formed in 1120 years. It 

 is interesting to note that Professor G. Stanley 

 Gardiner calculated that in Fiji the corals might 

 grow so as to produce a reef seventeen feet 

 thick in 1000 years, and we see that at this 

 rate a reef 121 feet thick might be made In 

 7120 years. In a later publication, however, in 

 his Fauna and Geography of the Maldives, Pro- 

 fessor Gardiner expresses the opinion that a 

 reef might be built up to sea level from a depth 

 of 150 feet in 1000 years. Now, while these 

 various calculations are not very accordant they 

 all agree in ascribing a rapid growth rate to 

 reefs and force us to conclude that the reefs 

 which now surround the islands of the tropica] 

 world might easily have grown up since the 

 glacial period. 



My own calculation is doubtless based upon 

 too slow a growth-rate, for corals are more 

 densely clustered, and are less disturbed below 

 the level of wave action, than over shallow reef 

 flats where they are dashed by the heavy surges. 

 We must also bear in mind that holothurians. 

 boring plants, sponges, mollusks. worms and 

 many other animals are constantly destroying 

 the corals, while currents carry away small 

 loose particles, although these degrading in- 

 fluences are more or less offset by the growth of 



