392 
ZOOLOGY: A. G. MAYOR 
It was found that on an average each barrel caught 0.7 of a pound of lime- 
stone sand per day; the sand being dehydrated in alcohol and dried in the sun 
before being weighed. 
Further tests showed that the barrels caught on an average only 12% of 
the sand which was carried in the current over their open tops from the edge 
of the reef-flat. 
The northern edge of the reef -flat is about 1200 feet wide, but sand spills 
over its edge along only about 800 feet of this length, the 400 outermost 
feet of reef edge being subjected to the constant inward wash of the waves. 
Using these data, it appears that about 100,000 pounds of sand are washed 
off from the reef-flat by currents each year. 
There are 290,000 holothurians on the Aua reef-flat between Breaker Point 
and the edge of the reef off Aus Village; and as the acidity of their gastric 
cavities appears to be the same as that of the holothurians of Florida, each 
one, according to experiments made at Tortugas in 1917, might be expected 
to dissolve 10 pounds of sand per annum; or 2,900,000 pounds of sand may 
be destroyed annually over this reef -flat by solution due to holothurians. 
Thus the corals appear to add annually about 840,000 pounds of limestone 
to the reef, but 3,000,000 pounds, or nearly 3| times this weight of limestone 
appears to be removed annually by holothurians and by currents, and other 
factors such as boring algae, mollusca and fishes; the efficacy of which we 
have not been able to calculate, add still more to the destruction of lime- 
stone although their influence is in some measure offset by the growth of 
nullipore algae, and lithothamnion. Alcyonaria although a decided factor in 
some reefs are so rare as to be negligible over the Aua reef-flat. 
In most respects this Aua reef is a typical average Pacific fringing reef and 
our data appear to explain the disappearance of the lithothamnion ridge over 
the shoreward parts of the reef -flat as the reef grows outward. Moreover, 
the reef-flat appears to be deepening at present although the average depth 
of water over it at low spring tide is less than one foot. 
This does not mean that such a fringing reef must necessarily change into a 
barrier reef through waste and solution of limestone from its floor, for as the 
reef-flat deepens the currents must lessen, and a sandy or muddy bottom, 
while unsuited to the growth of coral, is also not so favorable for holothurians 
as are the small patches of limestone sand surrounded by dead and living 
coral such as characterizes the Aua reef-flat today. 
Thus as the reef-flat deepens, the factors which degrade it probably di- 
minish and a balance may be attained between the accession of limestone 
due to growth of corals and other organisms and its loss due to mechanical 
and organic causes. 
It is interesting to see that although Porites heads are more numerous than 
Acropora, yet Acropora, due to its remarkable growth-rate, is the most im- 
portant coral genus in building up the Pacific reefs. 
