Ecology of Black Coral — Grigg 
255 
Fig. 9- Tied to a vertical line at 24 meters, this branch of A. grandis was quite healthy after 90 days. (Com- 
pare with Figure 8, a branch at 12 meters, also fixed to the line for the same 90-day period.) 
porian coral cemented massively by coralline 
algae, Bryazoa, and other calcium-depositing or- 
ganisms. In Figure 11 the population density is 
shown in five different areas having a calcium 
carbonate substrate. The population density off 
Molokini (a basaltic substrate) ranged between 
0.0 and 0.2 colonies/square meter, considerably 
less than that of a calcium carbonate substrate 
of equivalent depth and inclination. 
The topographical features of the bottom 
strikingly influence the distribution of A. gran- 
dis within the zone of observation (75 meters). 
A sloping substrate, the presence of overhangs, 
caves, ledges, and drop-offs, all significantly in- 
crease the population density. This tendency is 
illustrated in Figure 11, which shows a high pop- 
ulation density in such areas. Below 75 meters 
this tendency is not so apparent. As was pointed 
out earlier, the aggregation of A. grandis in 
dimly lit areas is probably a result of a negative 
phototaxis of the planulae. A vertical substrate, 
of course, is relatively free of silt and detrital 
material which might otherwise inhibit planulae 
fixation. 
In summary, then, the most favorable sub- 
strate consists of a rough-textured CaC0 3 ma- 
terial with many shaded areas resulting from an 
irregular bottom profile. 
Current 
Food and oxygen come to the sedentary corals 
by way of water eddies and currents. Such water 
movements also carry away metabolic wastes. 
Hickson (1932) found that a flow of water in 
one direction over a rocky bottom seems to favor 
the growth of gorgonian corals. Wells (1957) 
also pointed out that a current aids the growth 
of corals: "In still water, accumulation of sedi- 
