FAUNA AND FLORA O F MANGROVE SWAMPS
203
and any edible detritus towards the mouth. The sand and other
mineral matter would sink t o the base of the buccal cavity and be
discarded when a pellet of suitable size had collected. (These pellets
hold together for some time, longer than similar pellets made artificially
with water, so one must presume that thcy contain mucoid material
from the saliva to give cohesion.) The water would return to the
branchial cavity through what has been called the Milne-Edwards
opening a t the base of the outer edge of the third niaxilliped. This
opening is protected by the epipodite of the third maxillipcd and the
armature of setae on the base of the epipodite is finer and more dense
in U . dussumiwi and U . urvillei than in the sand-feeding (J. lactea
(Fig. 71).
The provision of watcr for the flotation action is also important.
Some water runs into the gill cavity as has been described and is sucked
back during the upstroke of the scaphognathite. But there is an
inevitable wastage of water. If pools of water are available-even very
shallow pools-water can be taken up through an inhalant opening
between the third and fourth legs, by the action of the scaphognathite,
while the crab squats down into the watcr. But in some localities
water is not always available. I n such localities the crabs are dependent
on their burrows. rarely straying far from them and frequently returning
t o them. Since these burrows always penetrate t o the level of the water
table (Verwey, 1930; Macnae and Kalk, 1962), water is available here.
This would explain the apparent dependence of such species as U .
lactea, U . inversa and U . bellator on the closeness of the burrow. Miller
indicates that the Amerimn U . pugnax and U . pugilutor are dependent
on the burrow while U . minax which lives a t a lower level is not. From
his dercription U. minax is not very dissimilar in habit from the
Indo-Pacific U . vocans which only by an extension of definition may be
considered with the mangrove fauna.
Uca vocans (Latr.) (= U . marionis (Desm.)) makes certain, often
difficult to satisfy, demands on its environment. It requires the presence
of sand spits interspersed with muddy areas. Burrows are constructed
in the edges of the sand spits and the crabs feed on the mud flat. Since
these always have little pools of water scattered over the surface, a
supply of water for flotation of food is readily available and the crabs
are not tied to the vicinity of the burrows. Uca vocam thus tends t o
wander quite far from the home sand spit but always returns to it or
to a similar one as the tide flows.
The species which live in muddy areas discard most of the mud but
some may pass into the gut. Such particles are normally small enough
to pass through the filter in the distal chamber of the stomach. Sand
203
and any edible detritus towards the mouth. The sand and other
mineral matter would sink t o the base of the buccal cavity and be
discarded when a pellet of suitable size had collected. (These pellets
hold together for some time, longer than similar pellets made artificially
with water, so one must presume that thcy contain mucoid material
from the saliva to give cohesion.) The water would return to the
branchial cavity through what has been called the Milne-Edwards
opening a t the base of the outer edge of the third niaxilliped. This
opening is protected by the epipodite of the third maxillipcd and the
armature of setae on the base of the epipodite is finer and more dense
in U . dussumiwi and U . urvillei than in the sand-feeding (J. lactea
(Fig. 71).
The provision of watcr for the flotation action is also important.
Some water runs into the gill cavity as has been described and is sucked
back during the upstroke of the scaphognathite. But there is an
inevitable wastage of water. If pools of water are available-even very
shallow pools-water can be taken up through an inhalant opening
between the third and fourth legs, by the action of the scaphognathite,
while the crab squats down into the watcr. But in some localities
water is not always available. I n such localities the crabs are dependent
on their burrows. rarely straying far from them and frequently returning
t o them. Since these burrows always penetrate t o the level of the water
table (Verwey, 1930; Macnae and Kalk, 1962), water is available here.
This would explain the apparent dependence of such species as U .
lactea, U . inversa and U . bellator on the closeness of the burrow. Miller
indicates that the Amerimn U . pugnax and U . pugilutor are dependent
on the burrow while U . minax which lives a t a lower level is not. From
his dercription U. minax is not very dissimilar in habit from the
Indo-Pacific U . vocans which only by an extension of definition may be
considered with the mangrove fauna.
Uca vocans (Latr.) (= U . marionis (Desm.)) makes certain, often
difficult to satisfy, demands on its environment. It requires the presence
of sand spits interspersed with muddy areas. Burrows are constructed
in the edges of the sand spits and the crabs feed on the mud flat. Since
these always have little pools of water scattered over the surface, a
supply of water for flotation of food is readily available and the crabs
are not tied to the vicinity of the burrows. Uca vocam thus tends t o
wander quite far from the home sand spit but always returns to it or
to a similar one as the tide flows.
The species which live in muddy areas discard most of the mud but
some may pass into the gut. Such particles are normally small enough
to pass through the filter in the distal chamber of the stomach. Sand