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 Palcemonetes 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, Palcemonetes 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 batrachian 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 Palcemonetes 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. 
