FAUNA AND FLORA OF MANGROVE SWAMPS
201
as human food although Cardisoma and some of the larger species of
Sesarma are eaten locally. Each species tends to live in groups and
these groups are often exclusive and monospecific. A slight change in
the nature of the ground will often show a change in fauna.
a. Effects of nature of the substratum on crustaceans
As has been mentioned above, mangroves occur on beaches composed of several grades of particle size. Although the interior of the
mangal is almost always muddy there is usually some admixture of
sand. The seaward fringes are muddy on shores of accretion, but they
may be sandy on stable shores. The landward fringes are often sandy.
I n areas of sand with some silt but not an excess of it, particularly
along sunny glades and at the edges of the mangal, in both landward
and seaward fringes the dominant crab is one or other of the forms of
Uca lactea. At the lower levels of the seaward fringe on sandy beaches
there may be found crabs characteristic of the lower shore, and these
will tend to displace Uca lactea.
Species of Dotilla, Mictyris and Scopimera may all occur but never
together on sandy areas in or near a mangal. These are burrowers but
only Xcopimera appears to have its life centred round a permanent
burrow, and it will occur when there is little or no admixture of silt,
and normally at high levels. Dotilla and Mictyris, characteristic of
mid-tide levels, are similar in habit and replace one another geographically, they migrate up and down the beach remaining only in
areas where the level of the water table at low tide lies at or just below
the surface. Dried-out banks of sand show by their crumbly appearance
that they have been worked over as the tide was ebbing, and crabs will
return t o them to burrow in, as the tide flows. These crabs are not
normally part of the mangrove fauna but only exceptionally and
incidentally occur among the lowermost zone of pneumatophores of
Avicennia or of Xonneratia.
Mixed deposits of sand, silt and detritus are colonized by a wide
variety of crabs and representatives of all the genera listed above will
be found here. Some will be characteristic of well-compacted muds and
others will be found in ill-compacted muds.
The fauna of muds composed of fine silt is again a specialized one.
Such crabs are normally covered by a felt of fine hairs and when
washed they have a downy appearance. I have already pointed out
this feature of crabs found in semi-fluid mud (Macnae, 1957).
The work of Altevogt (1957a), Miller (1961), Ono (1965) and
Cameron (1966) makes it clear that all species of several genera of these
smaller ocypodid crabs show a depcndence on the size of particles in
201
as human food although Cardisoma and some of the larger species of
Sesarma are eaten locally. Each species tends to live in groups and
these groups are often exclusive and monospecific. A slight change in
the nature of the ground will often show a change in fauna.
a. Effects of nature of the substratum on crustaceans
As has been mentioned above, mangroves occur on beaches composed of several grades of particle size. Although the interior of the
mangal is almost always muddy there is usually some admixture of
sand. The seaward fringes are muddy on shores of accretion, but they
may be sandy on stable shores. The landward fringes are often sandy.
I n areas of sand with some silt but not an excess of it, particularly
along sunny glades and at the edges of the mangal, in both landward
and seaward fringes the dominant crab is one or other of the forms of
Uca lactea. At the lower levels of the seaward fringe on sandy beaches
there may be found crabs characteristic of the lower shore, and these
will tend to displace Uca lactea.
Species of Dotilla, Mictyris and Scopimera may all occur but never
together on sandy areas in or near a mangal. These are burrowers but
only Xcopimera appears to have its life centred round a permanent
burrow, and it will occur when there is little or no admixture of silt,
and normally at high levels. Dotilla and Mictyris, characteristic of
mid-tide levels, are similar in habit and replace one another geographically, they migrate up and down the beach remaining only in
areas where the level of the water table at low tide lies at or just below
the surface. Dried-out banks of sand show by their crumbly appearance
that they have been worked over as the tide was ebbing, and crabs will
return t o them to burrow in, as the tide flows. These crabs are not
normally part of the mangrove fauna but only exceptionally and
incidentally occur among the lowermost zone of pneumatophores of
Avicennia or of Xonneratia.
Mixed deposits of sand, silt and detritus are colonized by a wide
variety of crabs and representatives of all the genera listed above will
be found here. Some will be characteristic of well-compacted muds and
others will be found in ill-compacted muds.
The fauna of muds composed of fine silt is again a specialized one.
Such crabs are normally covered by a felt of fine hairs and when
washed they have a downy appearance. I have already pointed out
this feature of crabs found in semi-fluid mud (Macnae, 1957).
The work of Altevogt (1957a), Miller (1961), Ono (1965) and
Cameron (1966) makes it clear that all species of several genera of these
smaller ocypodid crabs show a depcndence on the size of particles in