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PACIFIC SCIENCE, Vol. XIV, July I960 
Hg. 2. Metabetaeus minutus (Whitelegge). a, b, 
Anterior region, dorsai and lateral aspects; c, cheliped; 
d, chela, outer face. 
were definitely brackish, and the level of the 
water in the pool changed with the tides. 
M. minutus was collected in two pools on 
Jaluit. On the islet of Medyado it was found in 
a mangrove swamp; the base of this swamp was 
an old reef flat, cut off from both the ocean 
and the lagoon by boulder and sand ramparts, 
and through the coral platform flowed definitely 
brackish water, rising and lowering not only 
with the tide but also with rapid periodicity 
of the waves on the ocean reef. The water flowed 
in and out through a series of ragged holes, 
and it was in these holes that the shrimp hid, 
emerging to the surrounding pools when un- 
molested. On Jabor the shrimp were found in 
a bomb crater in the center of the islet; the 
crater was almost dry at low tide and the shrimp 
were withdrawn into the holes of the coral at 
the base; but at high tide, when it was almost 
waist deep with brackish water, the shrimp 
emerged in great numbers. 
However, in the ecology of the pools the 
two species appear to play entirely different 
roles. In the pool of Lohena there were two 
species of shrimp found; the smaller and much 
more abundant atyid was identified by Rathbun 
(1906: 919) as Caridina brevirostris Stimpson 
and the larger as Metabetaeus. The atyid, abun- 
dant in pools of subterranean origin about the 
Hawaiian Islands, is an herbivore evidently feed- 
ing upon the algae growing on the rocks of the 
pools and upon vegetable detritus falling into 
the pool. When undisturbed it settles in large 
numbers on rock surfaces, occasionally walking 
or swimming from rock to rock. The alpheid, 
about twice the size of the atyid, usually hides 
in the rubble at the bottom of the pool, or in 
cracks in its side, and makes sudden forays 
to capture the atyid in its long rapacious chelae. 
It carries the struggling atyid in its chela to- 
wards its mouth and disappears again into a 
hiding place, presumably to eat its prey. 
On the other hand, while in the pools of 
Jaluit there was also an atyid (not as yet 
identified ) ; the mature atyid and alpheid were 
of nearly the same size, and the alpheid was 
by far the more abundant and gave no evidence 
of predatory habits. Like the Caridina in Ha- 
waiian pools, AL minutus would emerge to rest 
on the rocks and vegetable debris in the pool; 
