Freshwater Mussels at Campsea Ashe

FRESHWATER MUSSELS AT CAMPSEA ASHE T H E E A R L OF C R A N B R O O K

THE Canal at High House, Campsea Ashe, is an artificial Stretch of water some 300 yards long and 14 yards wide. As the name suggests it looks like a length of canal with brick walls at sides and ends. These walls are 4 feet high with a hard gravel shelf at the base shelving gradually towards the centre. In its present (August, 1973) dry condition there is about a foot of black organic mud and below that a mixture of yellowish clay with pebbles, chalk stones, etc., presumably the original impervious lining dug from a pit in the nearest bit of heavy land. At the lowest point there was still (August, 1973) a large patch of watery mud and when the solid material was scraped out to a depth of a foot the hole was filled with water which quickly flowed in from the surrounding semi-liquid mud. At this point, presumably typical of the whole, a hard bottom was reached by probing 3 feet 6 inches below the surface of the mud. The Canal has certainly been in existence since 1839 that and other smaller artificial pieces of water within the garden of High House being shown on the tithe map of that year—the late Lord Ullswater (The Gardens at Campsea Ashe. J. Roy. Hort. Soc. LI 11.11. pp 1-7,1928) thought that they were probably originally intended for fish ponds or stews and made about 1680. Of late years the Canal's most attractive feature has been the half dozen or so species of water-lilies which grow in it. There were also some carp and many freshwater mussels (Anodonta cygnaea). The latter are said to have been introduced by Lord Ullswater "to keep the water clear" and therefore after 1883 when he purchased the estate. If he did so it seems probable that he got his mussels from the decoy at Loudham where A. cygnaea is abundant.

Whether due to the mussels or for other reasons the water in the Canal was clear and Mrs. Schreiber, the wife of the present owner, teils me that she has often sat watching the mussels moving "with slow intermittent jerks" along the bottom. Anodonta are "filter feeders", lying front end downwards in the mud or sand in which they live, the hinder portion showing above it, and constantly inhaling and exhaling a stream of water the oxygen of which is absorbed by the gills, and small particles of animal and vegetable matter in suspension passing into the mouth. They move by extending the foot which then becomes turgid and anchored to the substrate when the muscles contract, pulling forward the shell with the animal inside it—the "slow intermittent jerks" described above.

In June, 1973, the Canal started to dry up and one of the more curious effects on the inhabitants of the Canal occurred when the water become just too shallow for the half-bred mallard which were

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kept there to swim freely. These could not walk onto the bank being prevented by the brick walls at the sides and ends and when the water had sunk so low that the duck touched the mud with their feet as they swam, a number were found to be unable to lift themselves into the air being weighed down by large Anodonta which had closed on their feet.

When the Canal was visited in August, 1973, no water was to be seen. Most of the surface had dried out, with wide cracks up to 6 inches wide running through the dry mud. The shells of dead Anodonta showing mainly in the cracks but also protruding through the dried patches of mud between them. Mrs. Schreiber told me that as the water evaporated what were eventually to develop into cracks remained in semi-liquid mud, the intervening patches solidifying earlier. It would seem that while some of the mussels were trapped in these early drying patches, most were able to work their way into the intervening semi-liquid mud only to die there in the end. When seen in August the remaining shells were mostly half buried in the mud or down the cracks: I was told that when the water had first evaporated a number had been left on top of the mud.

Even in August when many had been destroyed by predators or taken away as curiosities by visitors many hundreds were left, in some places in concentrations as high as twenty-five to thirty visible on the surface to the Square yard. Digging down below the surface or probing with a garden fork to the depth of a foot over four of such concentrations produced three, three, four, four, additional individuals and digging or probing to the same depth in places where there were fewer on the surface produced no more.

It would seem therefore that the mussels came up towards the surface as the water evaporated.

In the patch of liquid mud which remained the presence of live mussels was shown by occasional small streams of water Coming out on the surface or the round holes left when exhalations ceased: when so revealed the animals could be forked out and a large number were being removed to other ponds.

Nearly all those visible on the surface of the dried out mud or taken alive when revealed by the water from the exhalent syphon were of a considerable size, 150-200 mm., and amongst many hundreds visible in August I only saw four less than 100 mm.

Mrs. Schreiber teils me that that had been the case from the time when the Canal first started to dry up and dead or dying mussels were seen on the surface. The patch of liquid mud was permeable by a man's feet or a stick to a depth of 3 feet 6 inches but contained too much undecayed vegetable matter in the shape of leaves, etc., to allow it to be easily worked through with a fisherman's landing net of half inch mesh. A number of netfulls were

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collected by forcing the frame down into the mud and then drawing it forward until the net was fĂźll: no mussei less than 100 mm. in length was recovered.

Most of the Anodonta recovered alive were moved to the other ponds in the garden, a few to ponds at Debach and Gt. Glemham; the carp were all rescued in time and put into the ponds in the garden. Like other British river mussels Anodonta depends upon fish for one stage in its reproductive process. The eggs develop in the gills of the parent mussei into a larval stage, the Glochidium, which, when they are expelled, attach themselves to a passing fish. Encysted in the gill membrane or skin of the fish the Glochidia develop into young mussels and then escape from the cyst to lead a separate existence. The carp may well carry Glochidia to develop into mussels, the transported mussels will be unable to reproduce themselves unless they find fish in their new home.

The Earl of Cranbrook, C.B.E., Red House Farm, Gt. Glemham, Suffolk.