38 
PACIFIC SCIENCE, Vol. XXII, January 1968 
GENERAL DISTRIBUTION OF BIOMASS 
The epifaunal biomass is clearly concentrated 
on the upper levels of Reef 500. From the totals 
shown in Table 1, it is apparent that the mean 
biomass of the Reef-top Zone is about twice 
that of the Mid-reef Zone, seven times that of 
the Reef-base Zone, and 15 times that of the 
Mixed-bottom Zone. Analysis of variance of the 
differences in biomass among the zones of the 
reef -complex are highly significant at the 99% 
level (Tabular F (.01) = 5.70). Even though 
the biomass of the mixed-bottom region is a 
small fraction of that of the reef-top, it is 
large compared with that of many level-bottom 
communities. This is attributed to the quantities 
of debris and detritus swept from the reef 
by the strong water movements. 
The top-to-bottom decline in biomass is con- 
sonant with the general decline of species and 
individuals, but these two entities need not be 
causally related. The fact that they are so related 
indicates that environmental conditions exist at 
the top of the reef and not at the base that favor 
the development of species whose biological 
characteristics include production of large stand- 
ing crops. All of the largest producers of bio- 
mass attain maximum population densities on 
the reef’s upper levels, and most of them are 
sessile or sedentary suspension-feeders. Among 
these, in descending order of biomass per m 2 , 
are the sessile pelecypod Chama pellucida , a 
sponge mat composed of such amorphous 
sponges as Lissodendoryx noxiosa, the seden- 
tary, plankton-feeding sea cucumber Cucumaria 
lubrica, and the burrowing date mussel Li- 
thophaga plumula. The conditions that favor 
maximum development of their populations, 
such as ample supplies of suspended material 
and suitable water movements, are present only 
on the reef’s upper levels. 
BIOMASS AND FEEDING TYPES 
On a weight basis, suspension-feeders pre- 
dominate in all zones of the reef-complex 
(Table 1). They are followed in order by 
carnivores, scavengers, herbivores, and deposit- 
feeders. The principal carnivores are, in descend- 
ing order of standing crop, the starfish Pis aster 
giganteus, various nemerteans, eunicid poly- 
chaetes, and gastropod mollusks. Among the 
principal scavengers are such crabs as Paraxan- 
thias taylori and Lophopanopeus leucomanus, 
the ophiuroid Ophioderma panamensis , and the 
hermit crab Paguristhes ulreyi. The chief 
herbivores are the algaphagous sea urchins, the 
limpet Megathura crenulata, and such chitons as 
Callistochiton crassicostatus. The deposit-feeders 
are represented by the sea cucumbers Parasti- 
c hop us parvimensis and Leptosynapta albicans, 
and the terebellid polychaete Amp hi trite 
robusta. 
Table 1 reveals that on the reef -proper the 
mean weight of suspension-feeders decreases 
sharply from reef -top to base, as do the weights 
of carnivores and scavengers, though their 
decline is more gradual. Herbivores and deposit- 
TABLE 1 
Distribution of Dry-Weight Biomass on Reef 500 by Zones and by Principal Feeding Types* 
FEEDING types 
REEF-TOP 
MID-REEF 
REEF-BASE 
MIXED-BOTTOM 
MEAN 
WT. 
g/m 2 
% wt. 
g/m 2 
% wt. 
g/m 2 
% wt. 
g./m 2 
% wt. 
Suspension-feeders 
2238.4 
86.4 
1120.4 
79.6 
202.5 
53.9 
55.6 
31.9 
904.2 
Carnivores 
219.8 
8.5 
128.0 
9.1 
53.9 
14.3 
25.0 
14.3 
106.7 
Scavengers 
122.8 
4.8 
117.1 
8.3 
45.1 
12.0 
63.3 
36.3 
87.1 
Herbivores 
4.1 
0.2 
37.6 
2.7 
63.3 
16.9 
6.6 
3.8 
27.9 
Deposit-feeders 
0.1 
0.1 
4.5 
0.3 
10.9 
2.9 
24.0 
13.7 
9.9 
Totals 
2585.2 
1407.6 
375.7 
174.5 
* The marked drop in total biomass between the mid-reef and reef-base zones is accounted for in part by 
lack of water movement and by feeding activities of sea urchins that destroy settling larvae. 
