FAUNA AND FLORA O F MANGROVE SWAMPS
211
c. Effects of saltness of the water on crustaceans
Within the mangal there may be a considerable variation in the
mineral content of the water, both above and within the substratum.
This variation is due to several factors. In long-shore mangals (and this
includes embayments where the salinity of the water is close to normal)
variation is due to two factors only. The water may be diluted by
rainfall, and under cyclonic storm conditions this may be so considerable as to be lethal for many organisms, or as the result of high insolation in regions of low rainfall the salinity of the soil water may be
increased due to evaporation and transpiration. Under these conditions
very high salinities may occur in the soil watcr of the bare areas
colonized in eastern Africa, Pakistan and India by Uca inversa or in
Australia by U . bellator. These may well exceed 10% in the dry season.
But the balance of ions does not appear to be greatly modified until the
magnesium salts are deposited a t considerably higher concentrations.
In estuarine regions variations may be caused both by rainfall and
by run-off from the land. Under these conditions the ionic balance may
be so altered that the Knudsen formula does not apply (Flemister and
Flemister, 1951).
Snelling (1959) and Ono (1965) have studied the distribution of
crabs along an estuary, and the distributions they describe must in the
main be due to salinity changes as one proceeds upstream. Snelling
lists two species of CEeistostoma : C. wardi, restricted to the vicinity of
the mouth, and C. mxcneilli which replaces it upstream. One may
presume that the latter is the more euryhaline of the two. Again, of the
species of Uca, U . longidigitum goes farther upstream and so presumably
is the most euryhaline of the three species occurring. U . vocans (= U .
marionis) and U . coarctata (H.M.-Edw.) do not go far from the mouth
(the last is not recorded by Snelling, but is present a t Bulimba
close to the mouth (personal observation)). Heloecius cordiformi8
H.M.Edw. is restricted by firm mud rather than by salinity. Of
sesarmas, Snelliiig found that S. erythrodactyla Hess was replaced by an
unnamed species some 28 miles (44 km) from the mouth. I n a similar
way, S. eulimene replaces S . ortmanni as one goes upstream on the
Maputo river, entering the Bay of Lourengo Marques (personal observation). Ono (1965) found that the limits of upstream penetration by the
species of crabs he studied were regulated by the following chlorinities:
Scopimera globosa (de Haan), 3-5%,; Macrophthalmus japonicus de
Haan and Uca lactea, 2-3%,; Uca arcuata (de Haan), Cleistostoma
dilatatum (de Haan) and Paracleistostoma cristatum de Man, 0.5-0 .lo%,,
and Ilyoplax pusilla de Haan, 0.03-0.5%,, Such figures suggest that
these crabs are all very tolerant of variable salinities.
211
c. Effects of saltness of the water on crustaceans
Within the mangal there may be a considerable variation in the
mineral content of the water, both above and within the substratum.
This variation is due to several factors. In long-shore mangals (and this
includes embayments where the salinity of the water is close to normal)
variation is due to two factors only. The water may be diluted by
rainfall, and under cyclonic storm conditions this may be so considerable as to be lethal for many organisms, or as the result of high insolation in regions of low rainfall the salinity of the soil water may be
increased due to evaporation and transpiration. Under these conditions
very high salinities may occur in the soil watcr of the bare areas
colonized in eastern Africa, Pakistan and India by Uca inversa or in
Australia by U . bellator. These may well exceed 10% in the dry season.
But the balance of ions does not appear to be greatly modified until the
magnesium salts are deposited a t considerably higher concentrations.
In estuarine regions variations may be caused both by rainfall and
by run-off from the land. Under these conditions the ionic balance may
be so altered that the Knudsen formula does not apply (Flemister and
Flemister, 1951).
Snelling (1959) and Ono (1965) have studied the distribution of
crabs along an estuary, and the distributions they describe must in the
main be due to salinity changes as one proceeds upstream. Snelling
lists two species of CEeistostoma : C. wardi, restricted to the vicinity of
the mouth, and C. mxcneilli which replaces it upstream. One may
presume that the latter is the more euryhaline of the two. Again, of the
species of Uca, U . longidigitum goes farther upstream and so presumably
is the most euryhaline of the three species occurring. U . vocans (= U .
marionis) and U . coarctata (H.M.-Edw.) do not go far from the mouth
(the last is not recorded by Snelling, but is present a t Bulimba
close to the mouth (personal observation)). Heloecius cordiformi8
H.M.Edw. is restricted by firm mud rather than by salinity. Of
sesarmas, Snelliiig found that S. erythrodactyla Hess was replaced by an
unnamed species some 28 miles (44 km) from the mouth. I n a similar
way, S. eulimene replaces S . ortmanni as one goes upstream on the
Maputo river, entering the Bay of Lourengo Marques (personal observation). Ono (1965) found that the limits of upstream penetration by the
species of crabs he studied were regulated by the following chlorinities:
Scopimera globosa (de Haan), 3-5%,; Macrophthalmus japonicus de
Haan and Uca lactea, 2-3%,; Uca arcuata (de Haan), Cleistostoma
dilatatum (de Haan) and Paracleistostoma cristatum de Man, 0.5-0 .lo%,,
and Ilyoplax pusilla de Haan, 0.03-0.5%,, Such figures suggest that
these crabs are all very tolerant of variable salinities.