CRAYFISH IN SUFFOLK RIVERS A. W.
DAVIES
(Essex River Board)
are two species of crayfish in British rivers : Astacus pallipes and A. fluviatilis (syn. A. astacus). The latter is the continental type which has been imported and has escaped to colonise some of the rivers of southern England. It is not yet known to occur in the River Stour, though it may well be present there. THERE
The differences between these two species are not at all obvious but Plate 1 shows the main respects in which they differ. These are :— A.
A. fluviatilis has two projections behind the eyes while A. pallipes has only a single projection, followed by a series of very small ones.
B.
A. pallipes has a very well-developed keel to the rostrum, not so A fluviatilis.
C.
The side of the basal part of the rostrum is straight in the case of A. fluviatilis while in A. pallipes it is somewhat acute, but this difference is not at all obvious.
D.
The posterior limit of the carapace is more concave in A. fluviatilis than in A. pallipes.
Other features used in differentiation of this sub-order include the basal segments of the first and second antennae, pincer morphology and shape of the telson. Crayfish show a world-wide distribution, being found in American, Australian, and New Zealand rivers. They live only in rivers and most of the time lie concealed and protected in a hole in the river bank with just their antennae protruding. As they breathe by gills, crayfish are very susceptible to lack of dissolved oxygen in the water and for this reason they are normally found in very shallow, fast flowing broken water which ensures good aeration. They are also very sensitive to pollution, especially from metals such as copper, zinc, chromium, etc., more so even than trout. They are more tolerant of organic pollution however, provided the dissolved oxygen is high, i.e., 8-10 parts per million or above. Crayfish are carnivorous and eat any live animal of suitable size, although I have also seen them eating dead fish which have not yet begun to decompose. In captivity they will readily eat earthworms. They normally catch prey in their pincers, using these to tear it into small pieces if too big to be introduced whole into the mouth.
12
Transactions of the Suffolk Naturalists',
Vol. 13, Part 1
It is quite common to find crayfish with one pair of pincers larger than the other. The reason is that the smaller one is being regrown after being lost or damaged. The regrown limb increases in size each time the crayfish moults. This power of regeneration is also possessed by other crustacea such as crabs and lobsters. Crayfish are common in the River Stour but because of their powers of concealment are not readily seen. General observations on the River Stour and tributaries indicate the presence of large numbers of crayfish at the following sampling points :— The uppermost reach of the Stour where crayfish have been found on the Stretch in the vicinity of Pentlow Bridge. Farther downstream they have been observed at Glemsford Station Bridge and between Henny Mill and Langham waterworks intake. The absence of crayfish above Pentlow Mill is probably due to pollution from sewage effluents and factory waste. Crayfish have also been observed on the River Glem between Scotchford Bridge and the confluence with the R. Stour, on the R. Brett between Higham and the confluence with the main river and on the River Box throughout most of its length. A biological river survey of the River Brett, tributary of the River Stour, over a total river bed area of one hundred Square yards, showed a density of four crayfish present per Square yard, which is very high compared with the fish carnivores, which were :— Density Square yard 0-05 1. Stickleback (Gasterosteus) 1-7 2. Bullhead (Cottus gobio) 0-17 3. Stone loach (Nemacheilus) T h e Author's thanks are due to the Chairman and members of the Essex River Board for permission to write this article.
C R A Y F I S H IN SUFFOLK
RIVERS PLATE
ctrapace
Dorsal View
Lateral View Position of Kve
(fluviatihs)
astacus
1
14 Transactions of the Suffolk Naturalists', Vol. 13