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FEEDING RATES OF MACROPHALMUS CONVEXUS
Introduction
Sentinel
crabs have been little studied and the available literature has been printed
mostly in Japan and is not available for investigation. The initial purpose on
arrival was to establish the number and type of species on the beach. The first
is Macrophalmus convexus this
has a long rectangular carapace with small sized chelae and is located mainly on
the upper shore. The second is Macrophalmus definitus this is
primarily located on the lower part of the beach. These have much larger chelae
and square carapace. The third species has not been looked at in any form and
will not be mentioned here.
The
beach in question adjoins the pier in Kaladupa for the town of Ambawa. It
extends at full low tide approximately 100m from the high tide mark and is
backed by mangrove plants. There is a considerable amount of plant growth from
60m towards the sea; these are small well-formed plants that produce a thick
covering across the entire lower part of the beach. The middle shore consists of
a mixture of fine filamentous algae and sand. There is considerably more algae
coverage as distance is covered away from the pier. The upper shore consists
mainly of sand.
The
only species under investigation is Macrophalmus
convexus.
After
establishing the distribution of the crabs (through observation) the beach was
divided into three sections extending away from the pier. A mark was then
established at the first point in which M.Convexus occurs. This was named
Horizon 1. Four males and four females where recorded within each section along
horizon 1. Each crab was observed for one minuet in which the number of times a
crab raised its chelae to its mouth was counted. This was repeated four times.
Each of the crabs was then colleted and measured. Measurements where taken using
graduated calipers across the width and length of the carapace, the lower chelae
and the upper chelae. The crabs where then released.
The
Horizon the moved ten meters further down the beach and the process repeated. In
all there are a total of six horizons. After this point very few convexus are
found.
Initial
results seem promising. Although the data as yet has not been analyzed there
appears to be a visual difference in the feeding rates as distance from the pier
and distance down the shore increase. Animals found on the more sandy parts of
the beach feed more rapidly than those in areas with more algae. Differences
between sexes have been more difficult to establish and must await further
investigation.
The
difference in the rate of feeding may well be due to the smaller amount of
organic material available within sandy areas. This may induce a rapid rate of
feeding before available nutrients are leached. However as it is unclear what it
is that sentinels eat this will remain a subject for investigation.
On
interesting observation concerns the appearance of convexus. It was noted
that there is a difference in chelae size between males. This was graphically
shown when one small male with large chelae was placed next to a much larger
male in whom the chelae was half the size. What this could signify is unclear
although initial observations could suggest a new species to the beach.
A
final year dissertation report entitled Feeding Rates of Macrophalmus covexus
will be produced by Karen Prytherch, University of Plymouth by May 2003.