1.2 DISTRIBUTION1.2.1 Local distrib

1.2 DISTRIBUTION
1.2.1 Local distribution
Within local populations of mud crabs, their distribution is characterized by significant
ontogenic changes, with some studies reporting juveniles more common in seagrass
and algal beds associated with mangroves. In an Australian bay, S. serrata juveniles
of different sizes, subadults and adults were all found to favour different zones from
the upper intertidal through the mangrove forest, intertidal and subtidal. A sandstone
Figures 1.1–1.4 reproduced with permission from Keenan, Davie and Mann (1998).
Figure 1.1
Scylla serrata – dorsal view (top) and
claws (bottom)
Courtesy of Queensland Museu
m
Figure 1.4
Scylla tranquebarica – dorsal view (top) and
claws (bottom)
Figure 1.3
Scylla olivacea – dorsal view (top) and
claws (bottom)
Courtesy of Queensland Museu
m
Figure 1.2
Scylla paramamosain – dorsal view (top) and
claws (bottom)
Part 1 – Biology 3
shelf at the mouth of the Caboolture River, Queensland, Australia, associated with a
mangrove system was found to be a good location to collect juvenile S. serrata. The
juvenile crabs typically sheltered under loose slabs of sandstone and other rocks, or
within clumps of mangrove roots, shaded by mangrove trees (Avicennia marina and
Ceriops tagal) between mean high water and mean spring low water. In Micronesia, deep
soils alongside a river, branches, logs and hollow mangrove trunks (Sonneratia alba)
provided the best habitat for S. serrata as determined by burrow density. Significantly
larger S. serrata were found in fringe channels near the edge of the mangrove forest,
compared with the interior of the forest. Chemical tracers have been used to show
that while some adult populations of S. serrata feed predominantly within mangroves,
others forage more on reef flats and seagrass beds.
Examination of crab zonation patterns from mangrove forests in Australia,
Indonesia and Japan have shown that Scylla spp. dominate the zone below mean low
water of spring tides (LWS) in all three locations, with their mode of life of the genus
being classified as “decapods always living in a burrow”.
Apart from spawning migrations, mud crabs appear to move little within their
habitat, most remaining on site in distinct populations. However, longer-term tagging
has shown that crabs can move several kilometres from their home range over time.
Nightly movements of S. serrata fitted with transmitters averaged 461 m, with average
speeds in the range of 10–19 m/h.
Distinct differences have been reported for the habitat preferences of S. paramamosain
of different sizes. Small crablets (carapace width [CW] 0.5 cm) settle on the outer
edge of mangroves, gradually moving deeper into the forests living on the surface of
mangroves (CW 1.5 cm), while larger crabs dig burrows or live in the subtidal zone
migrating in to feed in the mangroves at high tide (CW 4.5 cm), with the main adult
crab population living subtidally, offshore (CW 12.5 cm). The boundary between the
mangroves and mud crab flats is identified as an area that can support higher densities
of crabs.
While several species of mud crab can be present in any one location, it appears
common that one species makes up a dominant percentage of the overall crab
population, for example in Aklan, the Philippines, S. olivacea comprised 95 percent of
the mud crab population, with 2 other species present in the same area.
As mud crabs appear to have an interdependent relationship with mangrove forests,
the loss of mangroves, for whatever reason, will typically be followed by lower crab
catches. However, mud crabs are found in estuaries without mangroves, so they are not
essential for their colonization or survival.