328 THE ESSEX NATURALIST.
Freshwater Prawns at Benfleet.—Students of the Crustacea are
well aware that both in Europe and in North America individual species
of the genus Palaemonetes occur occasionally in freshwater as well as in
brackish water and in the sea. A visit, in August, 1921, to a pond on
Kersey Marsh, near South Benfleet, showed that numbers of a common
prawn, Palaemonetes varians Leach, were present in perfectly fresh water,
at which horses were seen to drink and of which the remaining flora and
fauna were distinctly of fresh-water facies. Potamogeton pectinatus in
fruit, and a submerged batrachiau Ranunculus, were the only phanero-
gams observable in the pond, the water of which was tasteless, clear and
deep, with a muddy bottom. Pond-skaters, a water-vole, and a common
frog, gave additional testimony to its fresh-water character, and subsequent
examination under the microscope of an evaporated drop of the water
showed the merest trace of salt-crystals.
The pond had evidently been cut off from any connection with tidal
waters for a long time ; enquiry elicited that the last time the marsh was
flooded was in or about 1907, during an exceptionally high tide, upon
which occasion doubtless the prawns were introduced from the sea, and
since when the water in the pond has been gradually becoming less saline,
until now it is quite fresh. The same species of Palaemonetes occurred
numerously in brackish ditches at Leigh and in the open sea at the same
place.
Specimens of the prawns, which are not at all dwarfed by their resi-
dence in fresh water, have been preserved in the Club's Museum.—Percy
Thompson.
End of Vol. XIX.