200 THE ESSEX NATURALIST
counties including Surrey (Ellis, 1948, p.60), Middlesex (Reid, 1944,
p. 10), Norfolk and the River Dee (Hynes, 1955, p.624), Oxfordshire
(Mackay, 1951, p.14), the Rivers Avon and Sowe (Bassindale, 1940,
p.485) and Yorkshire (Fryer, 1952, p.65). In addition to these localities
there are specimens in the collection of the British Museum (Natural
History) from: Masebury, Shropshire; River Derwent, Derbyshire;
pond in Regent's Park, London.
It seems probably that E. gracilis has spread along the canals and
rivers of England and has colonised nearby suitable habitats (such as
small ponds), as Hynes (1955, p.624) has already suggested.
The present discovery of Eucrangonyx gracilis in the Forest ponds
is, therefore, in no way remarkable as these ponds are only twelve miles
or so from the locality where the species was first discovered in this
Country. There is no evidence to indicate when it was introduced into
the Forest ponds, but it is doubtful if E. gracilis occurred there before
1926 as it was not known to the late D. J. Scourfield when he wrote on
the freshwater crustacea of Essex (1926, pp.98-103), nor is there any
mention of E. gracilis in his manuscript notes which were deposited
in the British Museum (Natural History) after his death.
A predominant feature of E. gracilis is its high potential reproduc-
tive rate. Hynes (1955, p.382) has calculated that one female will
produce an average of 33 offspring in one brood. This is high when
compared with Gammarus pulex, which produces an average of 16 per
brood. The latter species is, at present, the dominant freshwater Gam-
marid in England and is widely distributed throughout the Country.
In some localities both species are found together (Hynes, 1955, p.625).
E. gracilis seems to prefer weedy places and water containing deposits
of vegetable debris, whereas generally, G. pulex is found in cleaner
habitats and in the larger ponds or lakes and in clear streams. In Epping
Forest G. pulex has been recorded from lakes, large ponds and running
water (Scourfield, 1926, p.98). The distribution of both species in the
Forest would seem worth investigating; it may reveal that in some areas
E. gracilis may be competing with G. pulex or even replacing it. E.
gracilis seems to be the dominant species in all the ponds investigated
so far.
REFERENCES
Bassindale, R. (1946). New Records of Eucrangonyx gracilis, S. I.
Smith and Gammarus tigrinus Sexton. Nature, Lond., 157,
p.485.
Crawford, G. I. (1937). An Amphipod, Eucrangonyx gracilis, S. I. Smith,
new to Britain. Nature, Lond., 139, p. 327.
Ellis, A. E. (1948). The Survey of Bookham Common. Amphipoda.
London Naturalist, 27, p.60.
Fryer, G. (1952). The Amphipod Eucrangonyx gracilis (S. I. Smith)
and its occurrence in Yorkshire. Naturalist, April-June, p.65-
66, 2 figs.