56 
PACIFIC SCIENCE, Vol. XXI, January 1967 
feathered bristles (Fig. IF) which extend 
under each dactylus and proceed an equal dis- 
tance up the antero ventral surface of each dac- 
tylus. The combs are well developed and are 
presumably used in concentrating mucus from 
other legs, and in transferring mucus to other 
legs or to the mouthparts as described above. 
The endopodites of maxillipeds 3 are usually 
well bristled, and those of maxillipeds 2 have 
a fan of spines and dense tufts of bristles (Fig. 
1 G) for combing food into the mouth (these 
are to be known as maxilliped food brushes). 
Furthermore, the longitudinally bifurcated basal 
exopodite segment of maxillipeds 1 of Trapezia 
may have a valve function to prevent the loss 
of food during food-transfer operations. 
The dactyli of Tetralia heterodactyla (Fig. 1 
H—K ) have relatively small food brushes which 
are almost equally developed on all legs. The 
ventral concavity is very shallow and contains 
two rows of food combs consisting of flat, 
blunt, unfeathered bristles. These combs are 
almost restricted to the ventral surface of the 
dactylus. A conspicuous groove is also present, 
proximal to the end of each dactylus. These 
grooves are better developed on WL-1 and 4. 
The maxilliped food brushes of Tetralia are 
very well developed only on maxillipeds 2 
(Fig. 1 L), and consist of dense masses of 
bristles which are feathered (Fig. 1 M) dis- 
tally. The basal segment of the exopodite of 
maxillipeds 1 are also bifurcated as described 
for Trapezia. 
Live corals, in seawater, were examined under 
a dissecting microscope (Experiment 15), then 
"scratched” as described for Trapezia, with a 
dull probe. The probe was repeatedly coated 
with mucus and debris from the coral animal. 
When examined at 430 diameters, this material 
proved to be identical with that material on the 
crabs’ food brushes (Experiment 14) and with 
material in the stomachs of newly-killed and 
also field-preserved specimens. 
These experiments demonstrate that Trapezia 
f. ferruginea is actually a parasite with a strong 
host specificity, at least on the coral family 
level. The presence of food brushes on other 
Eniwetok Trapezia species and on Tetralia 
species warrants recognizing them as parasites. 
The use of the food brushes in feeding would 
also explain why Artemia eggs were found in 
the stomachs of crabs offered Artemia nauplii 
for filter feeding. Presumably the eggs were 
transferred from the aquarium floor to the crabs’ 
mouthparts after they had been picked up on 
the food brushes. 
In a search of the literature for similar coral 
parasites, Gerlach’s paper (1961:3) describes 
"an as yet unidentified aberrant copepod with 
a worm-like body which apparently lives on 
coral, mainly Pocilloporidae .... These ani- 
mals could be observed as they crawled about 
on the surface of the coral and slashed at the 
tissues of the coral polyps with the sharp claws 
of the first pair of legs. Here the point to 
be considered is that this is a form which has 
become particularly adapted to a mode of life 
parasitic on coral.” This behavior parallels the 
behavior of Trapeziinae described herein. 
As to the degree of parasitism, that is, the 
effect of the crabs’ parasitism upon pocilloporid 
corals, no data are available. The parasites are 
probably quite efficient, that is, they do not 
quickly kill or greatly harm their host. Other- 
wise, every case where numerous crabs are 
found occupying a coral head would result in 
the rapid destruction of the crabs’ microhabitat. 
The amount of food produced for crab con- 
sumption (or the number of polyps per head) 
probably serves as a basis for territoriality ob- 
served by Garth (1964:142). 
The coral-host preference of the two genera 
of crabs may well be correlated with the rela- 
tive difference in size, and thus efficiency, of 
the food brushes and combs. This conclusion 
is based on the fact that when live acroporid 
corals are removed from seawater and placed 
in the shade they secrete vast quantities of 
mucus, while pocilloporid corals secrete little 
mucus under the same condition. Tetralia, with 
the smaller and less efficient brushes and combs, 
thus takes advantage of a coral family which 
presumably is capable of secreting the greatest 
amount of mucus. Trapezia, on the other hand, 
has larger brushes and combs but lives with a 
less "productive” coral family. More research 
is being done on this aspect. 
The exact basis of the host specificity dis- 
played by these genera of crabs may well be 
related to the crab-size coral-space premise 
suggested by Garth, or to the distinct difference 
in the chemical odor and probable chemical 
