52 
they could not otherwise obtain, but are not 
harmful to their coral hosts. 
Crane (1947:83) and Garth (1964:142) 
note that Trapezia and/or Tetralia are limited 
in number to a single mated pair (with their 
young, Garth adds) on smaller coral heads, 
establishing a territory which they somehow 
protect (Garth). It must be noted, however, 
that the crabs have normal pelagic larvae, the 
megalops of which must seek out the proper 
species of host coral before settling and under- 
going metamorphosis. Thus, hundreds of corals 
must be rejected by each megalops until the 
proper species is found. Gurney (1938:76-77) 
described the zoeal stages of Trapezia cymodoce 
and Tetralia glaberrima which he reared from 
adults in captivity, but made no note of the 
larvae being photonegative. Thus, one may 
assume that normal photopositive larvae would 
be attracted away from the coral host of the 
adult crab, to take up a normal pelagic existence. 
Furthermore, of the 306 clutches of eggs that 
we hatched out at the Eniwetok laboratory, 22 
belonged to species of Trapezia and Tetralia. 
All of their larvae proved to be photopositive, 
thus indicating that they are pelagic in their 
larval development. 
To test the supposition that size-space, ocean- 
ographic, or food relationships may serve as a 
key factor in commensalism, Experiment No. 1 
was conducted in a deep pool on the ocean side 
of the north end of Eniwetok Island. Pocillo- 
porid and acroporid corals attached to large but 
movable rocks were collected and -killed by air 
drying. Some of these corals were dried for 
three days while others were cleaned by rotting 
the tissue, washing, soaking out, and then air 
drying. Next, these corals were tagged, restocked 
with newly collected crabs ( Trapezia on pocil- 
loporid coral and Tetralia on acroporid coral, 
the normal hosts), and placed in the bottom of 
the pool. Controls consisted of tagged live 
corals which were stocked with newly collected 
crabs of the proper species. 
After three days it was observed that all of 
the experimental crabs were missing from the 
dead corals (though some galatheid crabs had 
taken up residence). All crabs on the control 
corals were present (even when rechecked sev- 
eral weeks later). This suggested that either 
live coral or coral with the natural color (as 
PACIFIC SCIENCE, Vol. XXI, January 1967 
opposed to the white color of dead coral) is 
required for protection. 
To test the premise that coral color may be 
an important factor (Experiment No. 2), new 
corals were killed, cleaned, washed, dyed with 
vital stains of an appropriate color value so as 
to match the live corals, stocked with newly 
collected crabs of the proper genera, and placed 
back in the Eniwetok Island pool. Controls 
were used as in Experiment 1. After three days 
the stained acroporid heads lacked the Tetralia 
crabs and the pocilloporid heads lacked the 
Trapezia specimens, though some galatheid 
crabs and a few shrimps had taken up residence. 
All but one of the control crabs were found on 
their respective corals. 
The results of Experiments 1 and 2 suggest 
that size-space relationship (which is the same 
in dead and live coral) is not an exclusive factor. 
Also, the oceanographic conditions of currents, 
wave action, water temperature, and dissolved 
oxygen were constant in the environment of the 
live and dead corals, and thus oceanographic 
factors and the availability of water-borne food 
were not exclusively responsible for the com- 
mensal association. Any of these, or other, fac- 
tors could be critical in the distribution of these 
crabs, but the requisite of live coral appeared 
to be important. 
To check this premise, selective collections 
of corals (Experiments 3-5) were made on the 
outer reef at the north end of Eniwetok Island. 
Pocillopora danae heads were obtained just in- 
side the algal ridge where this coral abounds. 
Six buckets of completely dead coral heads of 
this species (Experiment 3, station 58) were 
collected from situations that were totally iso- 
lated from living corals. These corals yielded 
16 species of crabs (to be described in another 
paper), but contained no specimens of Trapezia 
or Tetralia. 
Next, live heads of Pocillopora danae which 
were partially dead and overgrown with algae, 
were processed as follows: Each head was 
snapped loose from the reef flat with a geol- 
ogist’s hammer and instantly lifted from the 
water and cleaved. The live portion with small 
areas of dead coral (Experiment 4, station 59 
from which 14 species of crabs were obtained) 
was placed in one bucket, the totally dead por- 
tion (Experiment 5, station 60 with 18 species 
