208
WILLIAM MACNAE
densest population of this crab that I have ever seen, six burrows were
counted in each of several squares of side 25 cm.
Sesarmas also chop up and devour fallen leaves, which may account,
to some extent, for the scarcity of these in a mangal. They nibble at
germinating seedlings of any of the mangrove trees and so may hinder
regeneration of a cut-over area (Watson, 1926). But as might be
expected they will also devour one another and any carrion available.
Their burrows always reach to the level of the water table and then are
somewhat enlarged providing a pool of water to which the crabs may
retreat in order to wet their gills. Other burrowing grapsids show a
similar dependence on the water table.
It is more difficult to know what determines the distribution of the
non-burrowing species. Metopograpsus thuicuhar in eastern Africa and
$1. frontalis (= M . gracilipes) in Australia and South East Asia may be
found within the bruguiera forests or rhizophora forests on firm mud,
seeking shelter in the crevices provided by knee roots, logs, etc. M .
latifrons is always associated with the prop roots of Rhizophora around
which it scuttles with ease, showing some reluctance to going into the
water or on to the surface of the mud.
A few species of the thalassinidean genera Thalassina and Upogebia
construct their burrows in mangrove mud. The burrows of most
thalassinideans which have been investigated are U-shaped. It appears
that the burrow is an important adjunct to the feeding techniques of the
animals. This use of the burrow has been described for Upogebia by
MacGinitie (1930, 1934) and MacGinitie and MacGinitie (1949). He
was describing the habits of U . pugettensis (Dana) and my observations
on U . africana (Ortm.) suggest that these descriptions may apply
throughout the genus.
Verwey (1930) stated that the burrows of Thalassina anomala
were single, with a long horizontal or near horizontal shaft and a pool
at the bottom. He described no ascending arm. Sankolli (1963) gives
a similar description of burrows some 200 km south of Bombay. Both
describe several side arms. Campbell (in Zitt. ) has told me that a burrow
he excavated in Queensland was found to be U-shaped with several
blindly ending side arms. One arm of the U and several side burrows
emerged on the hillock of mud, the other arm in a pool not far
away.
These accounts appear to be contradictory, but both may be correct
for the extensive digging at the end of a long horizontal arm will cause
a dropping in level on the ground above, so creating a conical pool
which may link up with the system of burrows beneath. This constant
activity of the mud lobsters is comparable with the activity of earth-
WILLIAM MACNAE
densest population of this crab that I have ever seen, six burrows were
counted in each of several squares of side 25 cm.
Sesarmas also chop up and devour fallen leaves, which may account,
to some extent, for the scarcity of these in a mangal. They nibble at
germinating seedlings of any of the mangrove trees and so may hinder
regeneration of a cut-over area (Watson, 1926). But as might be
expected they will also devour one another and any carrion available.
Their burrows always reach to the level of the water table and then are
somewhat enlarged providing a pool of water to which the crabs may
retreat in order to wet their gills. Other burrowing grapsids show a
similar dependence on the water table.
It is more difficult to know what determines the distribution of the
non-burrowing species. Metopograpsus thuicuhar in eastern Africa and
$1. frontalis (= M . gracilipes) in Australia and South East Asia may be
found within the bruguiera forests or rhizophora forests on firm mud,
seeking shelter in the crevices provided by knee roots, logs, etc. M .
latifrons is always associated with the prop roots of Rhizophora around
which it scuttles with ease, showing some reluctance to going into the
water or on to the surface of the mud.
A few species of the thalassinidean genera Thalassina and Upogebia
construct their burrows in mangrove mud. The burrows of most
thalassinideans which have been investigated are U-shaped. It appears
that the burrow is an important adjunct to the feeding techniques of the
animals. This use of the burrow has been described for Upogebia by
MacGinitie (1930, 1934) and MacGinitie and MacGinitie (1949). He
was describing the habits of U . pugettensis (Dana) and my observations
on U . africana (Ortm.) suggest that these descriptions may apply
throughout the genus.
Verwey (1930) stated that the burrows of Thalassina anomala
were single, with a long horizontal or near horizontal shaft and a pool
at the bottom. He described no ascending arm. Sankolli (1963) gives
a similar description of burrows some 200 km south of Bombay. Both
describe several side arms. Campbell (in Zitt. ) has told me that a burrow
he excavated in Queensland was found to be U-shaped with several
blindly ending side arms. One arm of the U and several side burrows
emerged on the hillock of mud, the other arm in a pool not far
away.
These accounts appear to be contradictory, but both may be correct
for the extensive digging at the end of a long horizontal arm will cause
a dropping in level on the ground above, so creating a conical pool
which may link up with the system of burrows beneath. This constant
activity of the mud lobsters is comparable with the activity of earth-