174 CARNEGIE INSTITUTION OF WASHINGTON. 



richest of the south Pacific reefs, they represent a condition from which gen- 

 eral conclusions may be more safely drawn than from those prevailing upon 

 the much richer reefs farther to the westward. 



On account of the limited time at our disposal, practically all of our observa- 

 tions were confined to the reefs in Pago Pago Harbor. In this harbor, which 

 occupies an old crater bottom open on one side, one finds a series of environ- 

 mental conditions embracing all those occurring on any of the reefs about the 

 island. The reefs at either side of the harbor entrance are fully exposed to the 

 waves, which, during the trade-wind season, beat upon them incessantly, 

 while at the inner end of the harbor, which runs nearly at right angles to its 

 outer portion, the water is calm, except in the most severe storms, and there is 

 no evidence of any considerable damage, even under such extreme conditions. 

 The numerous streams which enter the harbor cause considerable local differ- 

 ences in the salinity of the water and in the amount of sediment that it carries 

 in suspension. The configuration of the shore-line is such that several shel- 

 tered coves are found near the harbor entrance, while off the headlands 

 between these coves the reefs are exposed to strong currents and heavy 

 breakers. 



Only 6 species of Alcyonaria were collected at Tutuila, and of these 2 oc- 

 curred so rarely that they constitute a negligible factor in the reef fauna. 

 Of the 4 remaining species, Alcyonium flexile and A. glaucum can contribute 

 to reef formation only after the death of the colony, as the spicules are borne 

 free in the tissues. The colonies of A. rigidum and A. confertum form at 

 their bases dense masses of coral rock composed of fused spicules. As the 

 colony expands the tissues die about its base, and often in the center, so that 

 the spicule rock is left exposed. In this manner masses of limestone 4 feet 

 high and more than 20 feet in circumference are formed. Sometimes, due 

 apparently to a stimulus to growth caused by the presence of boring moUusks 

 or annelids in the dead skeletal portion of a colony of Alcyonium, columns are 

 formed with an internal axis of spicule rock from 6 inches to 9 inches in diam- 

 eter and extending 10 feet above the general level of the reef. 



These 4 species of Alcyonaria differ very much as regards their distribution 

 on the reefs. A. rigidum occurs only in deep water or in shallow places where 

 strong currents provide a comparatively low temperature and an abundant 

 supply of oxygen. A. glaucum has nearly as weak powers of resisting adverse 

 conditions as the former species, but can live in somewhat more quiet water 

 and is consequently found nearer shore and on less exposed portions of the 

 reef. A. confertum is by far the most resistant species. It consequently 

 occurs on all parts of the reef from near the shore at low-water mark to the 

 deepest visible part of the vertical outer face of the reef. A. flexile, while 

 able to withstand the conditions in shallow water, is never found in exposed 

 places and is consequently the most restricted in its distribution. 



The resistance of these species to high temperature corresponds closely to 

 their resistance to unfavorable conditions in their natural environment. 

 A. rigidum succumbs much sooner than either of the others, while A. confertum 

 is by far the most resistant. The other two species stand near one another 

 in an intermediate position as regards resistance to heat. 



Although the least resistant of the 4 important species, A. rigidum is found 

 to cover much greater areas than any of the others and contributes by far the 

 greater amount to the upbuilding of these particular reefs. It is the only 

 alcyonarian found abundantly on the most exposed reefs, where, in many 

 instances, it carpets large areas of the vertical outer face of the reef. Since it 

 grows so much more rapidly than any of the massive stony corals, the latter 

 are covered up and killed by the alcyonarian, so that on many of the reefs 



