Ecology of Black Coral — Grigg 
249 
above the 25 % 4 5 level. This value would approxi- 
mate 35 meters of depth, in the clearest water 
off the Hawaiian Islands (Fig. 7). Thus, if strong 
light intensity is a limiting factor, it must be 
operational only on the larval stage. The larvae, 
of course, could be limited to deeper water be- 
cause of other ecological factors. 
Nevertheless, in corals that have no photo- 
philous algal symbiants light is not essential; 
and in fact where these corals live within the 
photic zone they shun direct illumination, living 
in caves, or on undersurfaces of projecting rocks 
(Wells, 1957). 
Microscopic examination of the tissues of A. 
grandis has not revealed the presence of any 
zooxanthellae. The planulae, however, have not 
been observed, and therefore it cannot be said 
for certain that the larval stage is also free of 
dinoflagellate symbiants. 
Dendrophyllia, 5 a deeper water coral that has 
extended its vertical range into shallow water, 
has no zooxanthellae and shows no positive re- 
action to light. Moreover, the planulae settle in 
darkness, while those of reef builders do not 
(Edmondson, 1929). Edmondson found that 
Dendrophyllia, like other planulae observed, ex- 
hibits a positive phototaxis when first expelled. 
This response, however, is only temporary, and 
within a few days the organism moves toward 
the darker portion of the bottom and either 
settles or dies. The author suspects that in the 
case of A. grandis larvae a similar mechanism 
takes place, thereby limiting these animals to 
the deeper and darker recesses of the reef. This 
hypothesis is supported by observations in sites 
shallower than 40 meters, where colonies are 
found only when the basal attachment is situated 
in a dimly lit area, such as a cave or an over- 
hang. Beyond 40 meters the population density 
slowly increases with depth, and at 75 meters 
the colonies no longer aggregate in shaded areas. 
Below 75 meters very little observation has been 
done; only several glimpses by the author have 
been made in drop-off areas where the popula- 
tion appears to become increasingly dense. 
4 Approximately 200 gm cal cm" 2 24 hr" 1 . 
5 Dendrophyllia is said to be incorrectly identified 
in Hawaii and should be placed within the genus 
Tubastrea (Wain wright, personal communication). 
Surface 
< — floats 
Fig. 4. A line was anchored to the bottom at 43 
meters, while the other end was buoyed up by three 
sub-surface floats. Branches of A. grandis were tied at 
the indicated levels along this line. 
Transplants and Surge 
Colonies of A. grandis require a firm substrate 
so that water movements, current and surge, do 
not dislodge them. Once dislodged, the colonies 
are eroded along the bottom and eventually die 
because of tissue abrasion. Hence, for these 
studies, it was necessary to improvise anchoring 
fixtures such that branches could be transplanted 
from their normal habitat into shallow water 
(Fig. 3). 
Extreme magnitudes of surge were measured 
at various depths on days when excessively large 
waves were present. On February 22, 1963, for 
example, the sea surface was very calm but at 
the same time a 15 -foot swell predominated 
from the north. In an area with a bottom depth 
of 40 meters, at slack current, the horizontal 
movement due to surge at the bottom was 1.5 
feet, 3 feet at 20 meters, 6 feet at 10 meters, and 
8 feet at the surface. On this day, it was ob- 
served that a current of 0.25 knots was super- 
imposed upon the surge at 40 meters, causing 
water movement to be uni-directional, speeding 
up and slowing down depending upon whether 
the current and surge were in or out of phase. 
A control was set up by cutting a branch, 
