81
ABUNDANCE OF THE SLIPPER LIMPET
(CREPIDULA) IN ESSEX WATERS.
(With Plate II.).
PROBABLY one of the most remarkable instances of
the increase of an alien species in England is afforded
by the American "Slipper Limpet" (Crepidula forni-
cata). This marine mollusc was first noticed in Essex by Mr.
Walter Crouch, who on the 6th of September 1891 found a
dead shell at Stone Point, St. Osyth, and exhibited it at a meeting
of the Essex Field Club the next day (see Essex Naturalist, v.,
260). In a consignment of shells from the Crouch River sent
to him on the 4th of March 1893, was a living example, the fish-
erman (John Bacon) calling it a "Crow Oyster." Bacon said
that he could remember them for 15 or 20 years, but they were
very scarce. Subsequently Mr. Crouch communicated further
notes (E.N., x., 253). He had received about 30 specimens
from the Oyster Layings in the River Crouch. Dr. Laver
presented to the Museum of the Club a very large specimen
attached to a stone, from the River Colne. Then observations
followed in rapid succession in the journals, and years later
Dr. Murie in the Zoologist (15th November 1911) summed up
the occurrences in the Kent and Essex beds, and stated that in
the Blackwater fisheries alone 35 tons of these shells had been
dredged up within four weeks.
Since then the increase has been most astonishing. We have
no precise information from Kent, but in the Colne Estuary Mr.
G. L. Trussell, the Manager to the Colne Fishery Board, informs
us that during the past three or four years "unabated efforts
have been made to collect these limpets from areas most affected,
and hundreds of tons have been taken by the dredge. In the
course of the last twelve months upwards of 1,000 tons, taken
chiefly from the estuaries of the Blackwater and the. Colne,
have been crushed and used for manure by the farmers of the
districts." By the courtesy of the Editor of the Essex Standard
we are enabled to print a picture (Plate II.), which gives a vivid
idea of the immense quantities of the shell in the River Black-
water. The crushing of the limpets is carried out in a large
barge fitted with powerful machinery driven by an oil engine.
In the picture some of the workmen are seen measuring out the
crushed product into a lighter for delivery to a farmer. Our
F