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THE ESSEX NATURALIST
The Non-marine Mollusca of Mersea Island
By D. S. Davis*
Mersea Island lies at the confluence of the Colne and Black-
water Estuaries, on the Essex coast, about eight miles south of
Colchester. As is typical for the Essex coast, the island is quite
flat, not rising to more than 65 feet above sea level, and is fringed
with saltings and drained marshes. The land is largely used for
agriculture but urbanisation is increasing particularly at West
Mersea. The saltings and marshes are most extensive along the
north shore; much of the original area of saltings having been
enclosed by sea walls and drained. The south shore is more or
less open coast being subjected to erosion, and has a steep shell
and shingle beach and extensive tidal mud and sand flats. There
are also some small areas of salting along the south shore which
have mostly been enclosed by the development of the shingle
beach. As the island is cut off from the mainland by high tide
water covering the causeway at The Strood, it forms a well defined
geographic unit.
The geology of Mersea Island is shown in Fig. 1. The island
consists basically of London Clay partly overlain by alluvium and
fluvio-glacial sands. The alluvium is mostly saltings and drained
marsh around the edges of the island and the sands and gravels
form most of the highest points, reaching the coast only at Cud-
more Grove and West Mersea. The sand and gravel gives better
drained soils than the London Clay, but this does not really affect
the distribution of molluscs as the area is mostly rather dry above
the marsh level.
The most important factors influencing the distribution of
molluscs are results of land use. The agricultural land which lies
above sea level restricts molluscs to hedgerows and ditches or else
to road-sides and gardens. On higher ground freshwater species
are only found in a few small ponds which generally dry out in
the late summer. On the drained marshes snails and slugs can be
found abundantly along the edges of the drainage ditches, which
also contain some freshwater species. The ditches may become
brackish towards the sea wall. At some places along the south
shore, for example near the County Youth Camp site, there are
small areas of sand blown inland from the beach and colonised
by brambles, grasses and other plants. These sandy areas have
proved to be good collecting grounds for snails.
Although the saltings are covered by the spring tides and the
species of mollusc that they contain are usually associated with
marine or brackish waters, certain pulmonate and prosobranch
snails are generally included as non-marine species. The five
*Present address: Department of Biology, Dalhousie University. Halifax.
N.S., Canada.