152
WILLIAM MACNAE
Just as there is a broad similarity between the facies shown by
mangrove trees all round the Indian Ocean and into the western
Pacific, so there is a broad similarity in the distribution of the marine
fauna of the mangrove areas. Genera and even species are common to
all mangals in the Indo-west-Pacific. Some genera and some species
are more restricted in distribution, and sibling species may replace one
another. Although no taxonomist has dared to suggest it, one has often
suspected that some of the sibling species will be found to be genetic
variants, differentiated from one another by a few genes only and may
well be stages of a cline. For example, Eggert (1935) has described a
series of often nearly sympatric species of Periophthalmus, species
which differ in small ways and several of which may in fact be, not
separate species, but rather local populations of variants of a much
smaller number of widespread polytypic species. Similarly several of
the smaller species of Sesarma and of certain ocypodid genera may, in
spite of their differences, be units of a cline.
While the plants in a mangal will always show a tendency towards,
and under optimal conditions always demonstrate a very distinct
and characteristic zonation, it is always more difficult to define a
zonation among animals. Most animals can readily move away from
places when conditions become unsatisfactory, and so their distribution
is governed by other factors in addition to those appertaining to the
soil, the weather and the intertidal level. A few more sedentary,
burrowing forms only, cannot easily move away. Animals are more
likely to be affected by conditions of shade and exposure to the sun
than by the nature of the soil, by salinity (except at extremes of high
and low salinity) or by distance above mean sea-level. Several species
of crab may be found wandering at any level in the mangal but will
hold territory at some particular level. Some burrowing crabs are
more widespread than non-burrowing forms, since to some extent they
create their environment.
The question is often asked as to whether there is a distinct mangrove
fauna or not. The answer is that there exists on all shores of mud, or
mixed mud and sand, a fauna which is not found elsewhere and of
which certain elements demand the shelter of vegetation. In the tropics
and subtropics these elements will always be associated with mangroves.
Beyond the region of occurrence of mangroves, representatives of such
a fauna may be found in the shelter of grasses such as species of
Xpartina.
A mangrove area, like any other intertidal area is an area of transition from the sea to the land and from the land t o the sea and hence
its fauna may be derived from either.
WILLIAM MACNAE
Just as there is a broad similarity between the facies shown by
mangrove trees all round the Indian Ocean and into the western
Pacific, so there is a broad similarity in the distribution of the marine
fauna of the mangrove areas. Genera and even species are common to
all mangals in the Indo-west-Pacific. Some genera and some species
are more restricted in distribution, and sibling species may replace one
another. Although no taxonomist has dared to suggest it, one has often
suspected that some of the sibling species will be found to be genetic
variants, differentiated from one another by a few genes only and may
well be stages of a cline. For example, Eggert (1935) has described a
series of often nearly sympatric species of Periophthalmus, species
which differ in small ways and several of which may in fact be, not
separate species, but rather local populations of variants of a much
smaller number of widespread polytypic species. Similarly several of
the smaller species of Sesarma and of certain ocypodid genera may, in
spite of their differences, be units of a cline.
While the plants in a mangal will always show a tendency towards,
and under optimal conditions always demonstrate a very distinct
and characteristic zonation, it is always more difficult to define a
zonation among animals. Most animals can readily move away from
places when conditions become unsatisfactory, and so their distribution
is governed by other factors in addition to those appertaining to the
soil, the weather and the intertidal level. A few more sedentary,
burrowing forms only, cannot easily move away. Animals are more
likely to be affected by conditions of shade and exposure to the sun
than by the nature of the soil, by salinity (except at extremes of high
and low salinity) or by distance above mean sea-level. Several species
of crab may be found wandering at any level in the mangal but will
hold territory at some particular level. Some burrowing crabs are
more widespread than non-burrowing forms, since to some extent they
create their environment.
The question is often asked as to whether there is a distinct mangrove
fauna or not. The answer is that there exists on all shores of mud, or
mixed mud and sand, a fauna which is not found elsewhere and of
which certain elements demand the shelter of vegetation. In the tropics
and subtropics these elements will always be associated with mangroves.
Beyond the region of occurrence of mangroves, representatives of such
a fauna may be found in the shelter of grasses such as species of
Xpartina.
A mangrove area, like any other intertidal area is an area of transition from the sea to the land and from the land t o the sea and hence
its fauna may be derived from either.