DISTRIBUTION AND ABUNDANCE OF GRAPSID CRABS (GRAPSIDAE) IN A MANGROVEESTUARY - EFFECTS OF SEDIMENT CHARACTERISTICS, SALINITY TOLERANCES, AND OSMOREGULATORY ABILITY
Sd. Frusher et al., DISTRIBUTION AND ABUNDANCE OF GRAPSID CRABS (GRAPSIDAE) IN A MANGROVEESTUARY - EFFECTS OF SEDIMENT CHARACTERISTICS, SALINITY TOLERANCES, AND OSMOREGULATORY ABILITY, Estuaries, 17(3), 1994, pp. 647-654
Crabs (Grapsidae, Sesarma) are the dominant macrofaunal group of mangr
ove forest soils in northern Australia. Little is known about the ecol
ogy of these crabs or the factors that influence their distribution in
mangrove forests. Pitfall traps were used to sample grapsid crabs in
the Murray River estuary in north Queensland. Sampling was conducted a
t five sites along a salinity gradient from <1 parts-per-thousand at u
pstream sites to >35 parts-per-thousand at the river mouth. At each si
te, trapping was done in both low and high intertidal forests. We char
acterized the sediments at each site by measuring percent sand, silt,
clay and organic matter, Eh, pH, and soil pore-water salinity. Four sp
ecies of grapsids dominated the crab fauna along the Murray River (Ses
arma semperi-longicristatum, S. messa, S. brevicristatum, and S. brevi
pes). Distinct zonation patterns were found along the salinty gradient
and between high and low intertidal forests. S. messa was dominant in
high intertidal, downstream forests, high and low intertidal forests
in the middle to downstream portion of the river, and in low intertida
l forests in the central reach of the river. S. brevipes was dominant
in both low and high intertidal zone forests at low salinity upstream
sites. S. brevicristatum was most abundant in the central reaches of t
he river and only in the high intertidal zone. S. semperi-longicristat
um was found only in the low intertidal zone, downstream forest. Subse
quently, tests of salinity tolerances of these crabs were carried out
in the laboratory. These indicated very wide tolerances over salinitie
s from completely fresh to hypersaline (60 parts-per-thousand). The os
moregulatory abilities of the crabs were also found to vary. However,
neither their salinity tolerance nor osmoregulatory ability adequately
explain the zonation patterns we measured in the field. For example,
S. brevicristatum had the most restricted distribution, but it had the
second broadest salinity tolerance and osmoregulatory ability. Sedime
nt characteristics explained a significant amount of the variation in
abundance for two of the crab species. Pore-water salinity provided no
explanatory power for any of the species. Individual species abundanc
es are probably influenced by additional factors such as interspecific
competition and predation.