ZOOLOGY: A. a MAYER 
525 
PERCENTAGE OF CORAL HEADS 
NAME OF CORAL 
Murray Island, 
Australia 
Tutuila, Samoa 
per cent 
per cent 
38 
47 A 
18 
33.6 
10 
4.01 
Very rare 
10.0 
25 
0.0 
Totals 
91 
95.1 
It was found that when shallow reef-flats are impounded and cut off 
for about an hour from the open ocean at low tide, the water quickly 
becomes nearly twice as alkaline in places as in the open ocean, while 
in other places it becomes abnormally reduced in alkahnity. The in- 
crease in alkalinity is caused by the photo-synthetic action of plant 
cells within the corals and by sea weeds. 
Rail falling directly into the sea has far more effect in reducing the 
alkalinity of the surface water than has stream water pouring outward 
from the shore. Yet the torrential rain of 7.8 inches in about five hours 
on March 19 reduced the surface alkalinity only from 8.20 to 8.18 PH., 
and the salinity from 34.69 to 30.46 off the landing-stage of Blacklock's 
wharf near the inner end of Pago Pago Harbor, and the alkalinity of the 
harbor remained about 34.33 despite the average rainfall of more than 
one inch per day from January 1 to April 17. 
The scouring of sand from their floors by currents is a potent factor 
in deepening the shoreward parts of the reef -flats, and may result in 
changing a fringing reef into a barrier reef. The Hthothamnion ridge 
lying along the seaward edge withstands this process of disintegration for 
by growing, it resupplies such loss and maintains itself about 6 inches 
above low tide level. The corals growing over the shoreward parts of 
the reef-fiats also tend to replace the lost limestone and some reefs may 
thus maintain themselves as fringing reefs, as at Aua, Pago Pago Harbor, 
while others as at Black's Bay, Tutuila, become deepened near shore so 
as to change into barrier reefs. At Aua the current over the reef -flat 
ranges from 20 to 62.8 feet per minute, and is an effective transporter 
of coral sand; spilling it into deep water off the northern edge of the reef. 
This scouring process is doubtless facilitated by the holothurians which 
are well known to be sand swallowers. There is on the average one of 
these animals for every 8.6 square feet of reef flat off Aua, Pago Pago 
Harbor. 
