199
On the Occurrence of the Amphipod
Crustacean, Eucrangonyx gracilis
(S. I. Smith), in Epping Forest
BY RAYMOND W. INGLE
IN august, 1958, Mr. R. H. Harris, of Loughton, asked me to identify
some specimens of a small amphipod crustacean that he had collected
from the surface weed in Fairmead Bottom Pond, Epping Forest. He
suspected that these were juvenile specimens of Gammarus pulex
(Linnaeus), the common freshwater shrimp and recorded by Scourfield
(1926, p.98) as the only freshwater amphipod in the Epping Forest
district. However, when the specimens were carefully examined, they
proved to be Eucrangonyx gracilis (S. I. Smith), a lesser known species
of freshwater gammarid amphipod, of North American origin but now
established in several counties of Britain, although previously unrecorded
from the Epping Forest district. Later, in March, 1959, Mr. A. C.
Wheeler, of Theydon Bois, collected a large number of specimens from
various Forest ponds including Baldwins Hill, Blackweir, Goldings
Hill, Earls Path, Strawberry Hill and Wake Valley Pond, thus con-
firming that this species is well established in many of the smaller
Forest ponds.
Eucrangonyx gracilis is a small species of gammarid. Adult females
may reach 9mm. in total length. When alive this species crawls in an
upright position unlike Gammarus pulex which crawls on its side and
is a much larger species, as adult females may grow to a total length
of 12mm. More positive characters which separate the two are given
by Reid (1944, pp.8-10). Briefly, the shallow median notch of the telson
and the reduced inner branch of the third uropod distinguish E. gracilis
from any of the known species of Gammarus.
E. gracilis was first described by S. I. Smith (1871) from Lake
Superior in North America. It is now known to occur in various parts
of N. America (Shoemaker, 1933, p.17; Hubricht and Makin, 1940,
p.200). but it is not known how it came to Britain. It was first noticed
in this country about 1934, when a specimen was found in a glass
of water, drawn from the domestic water supply of a house in Hackney.
The specimen was sent to Dr. W. M. Tattersall who recognised the
species but refrained from establishing a new record on this single
specimen (Tattersall, 1937, p.593). Later, Crawford (1937, p.327), was
able to show that it occurred in the culverts below the filter beds at
the Lea Bridge Waterworks (surprisingly enough only twelve miles or
so from the Forest ponds in which it is now quite plentiful). According
to Spooner (1951, p.530) E. gracilis was well established in the Mid-
lands in 1938, as he collected specimens in Northamptonshire, Glouces-
tershire, and Warwickshire. The species has since been found in many