FAUNA AND FLORA OF MANGROVE SWAMPS
171
these animals, for the Thalassina are active in their excavation only
at night and on days when it is dark and rainy and so tend to escape
notice.
The level to which crabs of the genus Sesarma ascend on the beach
seems to depend on the local climate. I n the ever-wet of Malaya and
adjacent territories S. fasciata Lanchester and S. plicata (Latr.) occur
on the artificial banks of prawn and fish ponds and of roadside ditches
where they reach well above high water mark. These two species often
extend on to the swards of halophytic grasses alongside small streams
at high levels but yet occasionally intertidal.
S. ortmanni Crosnier and X. eulimene de Man appear to occupy a
similar niche in the western Indian Ocean. No exact equivalent was
seen in Australia. In Pondoland, Natal, Mopambique and northwards
along the coasts of East Africa as well as in Madagascar S. ortmanni
and S. eulimene occur in the belt of Juncus kraussii and J . maritimus
which delimits the upper intertidal region. S. eulimene extends farther
upstream into the fresh water, while S. ortmanni is more strictly
marine. There is a very small overlap in occurrence. These two crabs
are frequently very abundant (up to eight have been counted for each
in a square metre at Inhaca) and until recently have not been distinguished from one another.
At a level only fractionally lower the larger species of Sesarma come
to dominate the fauna of the mangal. None of the taxonomic accounts
gives even the slightest hint as to the level at which occur any of the
large number of species of Sesarma which have been recorded from the
Indo-west-Pacific.
I n Singapore, Malaya and adjacent areas seven species of large
sesarmas occur : these are S. indica A.M.-Edw., S. mederi H.M.-Edw.
(= S. taeniolatu auct.), S. palawanensis Rathbun, S. singaporensis
Tweedie, X. tetragona Fabr., 8. versicolor Tweedie and the recently
described S. chentongensis SerAne & Soh. All of these occur in the
landward fringes. The slightly smaller species 8. moeschii de Man,
S. crassimana de Man and S. sediliensis Tweedie together with
Sarmatium inerme (de Man) are common in nypa plantations and in
ditches in the landward fringe of the mangal (Tweedie, 1940).
I n eastern Africa and in Queensland Sesarma meinerti de Man and
S. smithii H.M.-Edw. (Fig. 53) live in this zone-a much less complex
picture than that found in Malaysia. I n areas where Lumnitxera
racemosa and other bushes of the landward mangrove fringe give some
shelter the burrows of these two species may be very close togetherless than 15 em apart-i.e. as close as would seem feasible. At Morrumbene (lat. 22's.) in Mspambique such burrows occur in a strip only
171
these animals, for the Thalassina are active in their excavation only
at night and on days when it is dark and rainy and so tend to escape
notice.
The level to which crabs of the genus Sesarma ascend on the beach
seems to depend on the local climate. I n the ever-wet of Malaya and
adjacent territories S. fasciata Lanchester and S. plicata (Latr.) occur
on the artificial banks of prawn and fish ponds and of roadside ditches
where they reach well above high water mark. These two species often
extend on to the swards of halophytic grasses alongside small streams
at high levels but yet occasionally intertidal.
S. ortmanni Crosnier and X. eulimene de Man appear to occupy a
similar niche in the western Indian Ocean. No exact equivalent was
seen in Australia. In Pondoland, Natal, Mopambique and northwards
along the coasts of East Africa as well as in Madagascar S. ortmanni
and S. eulimene occur in the belt of Juncus kraussii and J . maritimus
which delimits the upper intertidal region. S. eulimene extends farther
upstream into the fresh water, while S. ortmanni is more strictly
marine. There is a very small overlap in occurrence. These two crabs
are frequently very abundant (up to eight have been counted for each
in a square metre at Inhaca) and until recently have not been distinguished from one another.
At a level only fractionally lower the larger species of Sesarma come
to dominate the fauna of the mangal. None of the taxonomic accounts
gives even the slightest hint as to the level at which occur any of the
large number of species of Sesarma which have been recorded from the
Indo-west-Pacific.
I n Singapore, Malaya and adjacent areas seven species of large
sesarmas occur : these are S. indica A.M.-Edw., S. mederi H.M.-Edw.
(= S. taeniolatu auct.), S. palawanensis Rathbun, S. singaporensis
Tweedie, X. tetragona Fabr., 8. versicolor Tweedie and the recently
described S. chentongensis SerAne & Soh. All of these occur in the
landward fringes. The slightly smaller species 8. moeschii de Man,
S. crassimana de Man and S. sediliensis Tweedie together with
Sarmatium inerme (de Man) are common in nypa plantations and in
ditches in the landward fringe of the mangal (Tweedie, 1940).
I n eastern Africa and in Queensland Sesarma meinerti de Man and
S. smithii H.M.-Edw. (Fig. 53) live in this zone-a much less complex
picture than that found in Malaysia. I n areas where Lumnitxera
racemosa and other bushes of the landward mangrove fringe give some
shelter the burrows of these two species may be very close togetherless than 15 em apart-i.e. as close as would seem feasible. At Morrumbene (lat. 22's.) in Mspambique such burrows occur in a strip only