DEPARTMENT OF MARINE BIOLOGY. 



229 



As has been stated, the primary object of this investigation was to get an 

 approximate measure of the rate at which corals might build reefs. In order 

 to make this estimate the true reef corals must be considered separately from 

 those which live in other habitats. The reef species par excellence in the 

 Recent and Pleistocene reefs of Florida and the West Indies is Orhicella annu- 

 laris; after it in importance are Mcea7idra strigosa, M. lahyrinthiformis, 

 and Siderastrea siderea. Other corals, the most important of which is Pontes 

 astreoides, with Agaricia and Favia fragum of secondary importance, occur in 

 the areas intermediate between the prominent heads. In some areas Acropora 

 palmata is the dominant species. The massive heads form the strong frame- 

 work of the reef, with infilling by other corals and other organisms. Therefore 

 the upward growth rate of Orhicella annularis on the reef is critical. The data 

 on it will be repeated: 



Upward growth-rale of Orhicella annularis. 



Location. 



Fort Jefferson, tiles, outside moat wall . . . 



Loggerhead Reef, tiles 



Loggerhead Reef, naturally attached 



Golding Cay, tiles 



Golding Cay, naturally attached 



Annual 

 average. 



mm. 

 6.57 



5.28 

 6.80 

 5.67 

 5.00 



The highest figure is for naturally attached specimens on Loggerhead Reef, 

 but the average is based on only 5 measurements, which are not so accurate 

 as those of specimens on tiles. The specimens attached to tiles all thrived and 

 gave an annual average of 5.28 mm. for 14 measurements; while the Golding 

 Cay specimens, which also thrived, gave an annual average of 5.67 mm. for 

 6 specimens, 2 years' growth each. An estimate of 6 mm. for upward growth 

 per year is probably somewhat liberal. This would indicate for an upward 



24 5 X 12 

 growth of a foot, — '—^ = 50.8 years. Should 7 mm. be taken as the aver- 

 age the rate would be 1 foot in 43.54 years. Using these figures as the basis of 

 at further computation, a reef by the continuous upward growth of corals 

 might attain at a rate of 6 mm. per year a thickness of 25 fathoms = 150 feet 

 in 7,620 years; and at a rate of 7 mm. per year it might attain the same thick- 

 ness in 6,531 years. 



Should the growth-rate of Acropora palmata be taken as a measure, the 

 time to accumulate such a thickness would be considerably less. This species 

 forms spreading, palmate fronds, rising from stout bases. As age advances the 

 fronds thicken and can Avithstand the pounding of surf and breakers. The 

 average upward growth is between 25 and 40 mm. per year, but as the inter- 

 spaces between the fronds are considerable in volume, comparisons with 

 Orhicella annularis must be based upon relative increases in weight for a known 

 period. The tables on page 234 will supply such a basis i'^ 



According to weight, the specimens of Acropora palmata have grown 3.91 

 times as fast as those of Orhicella annularis; this may be stated in round num- 

 bers as 4 times as rapidly, while the increase in height is 4.28 times as rapid. 

 Therefore a reef composed of Acropora palmata might grow upward at the 

 rate of about an inch per year, a growth which would produce a thickness of 

 150 feet in 150X12 = 1,800 years, but it is not probable that conditions so 



^The weights are of the wet, living corals. 



