Rare snail found in Chichester Harbour
ASussex scientist has located the only known UK site for a rare snail which has thrilled the world of conchology and conservation in general. The snail was thought to be extinct in Britain where it had never previously been found alive. The man is Dr Martin Willing of Midhurst, honorary president of the Conchological Society, and the site is in Chichester Harbour on Thorney Island in the Little Deep. The snail has an elegant spiral shell in shape reminiscent of those large conch shells from Pacific islands. But there the likeness ends for it is minute, only 5mm in length. Dr Willing has sent me a copy of his paper published in the latest Journal of Conchology (2020 Vol. 43: 621 – 635) in order that I may share this extraordinary find with readers of our Sussex newspapers.
You may think the presence or absence of such a tiny organism on the planet is hardly worth a candle. You would be wrong. Size does not matter, presence is all. Every living being is an indicator of the health of our habitat on which we all depend for survival.
The Lagoon Spiral Snail (Semisalsa stagnorum) was formerly only known from dead shells found in two places in England: an archeological complex at Bath in Somerset and a brackish lagoon on Farlington Marshes north of Portsmouth where the snail died out between 1960 – 70.
Then in 2005 Dr Willing, whilst surveying for the Chichester Harbour Conservancy, trawled Little Deep and found freshly dead shells. A year later he returned and found the tiny snail alive. A thriving population has continued there ever since and its distribution mapped and its ecology studied.
Little Deep is close to Great Deep and the two were once connected as part of the tidal creek joined to both Nutbourne Marshes and Emsworth Channel. Thorney Island was once an island that could only be reached at low tide. In 1870 a sea-wall was constructed and a road built which today serves the Army camp. This road isolated the Little Deep from its former confluence with Great Deep. So Little Deep was landlocked and became a non-tidal lagoon. This created exactly the right conditions of low salinity that our snail wanted, about per cent (parts per thousand or PPT).
The Environment Agency (Chichester) provided salinity testing equipment that pinpointed the snail’s exact needs with Chichester Harbour Conservancy providing help and support throughout. The rare snail was not found in the Great Deep where tidal flushing through sluices leaves a high salinity of about 30 per cent and so similar to sea water.
Lagoon Spire Snails are now found in fewer and fewer world sites since the 1960s. In England it is assessed in the UK Status Review of Non-marine Mollusca as Critically Endangered. In the Netherlands the snail was lost from seven of the eleven colonies in just 40 years. They do still live in some Mediterranean lagoons.
I have known the importance of Little Deep for 55 years though not as a site for the rare snail. In the 1960s it was proposed as a landfill in the same way that parts of Pagham and Langstone Harbours were infilled; thankfully I was able to prevent that through my work in what was then the Nature Conservancy. Surrounded by beds of common reed, it is a sheltered ecosystem that is unique within The Solent Maritime Special Area of Protection. At present its lagoon is becoming smaller as the reeds invade and that may need management. It has been a haven for many species of birds such as shoveller, tufted, and teal duck; egret, herons, water rails, sedge and reed warblers, and the rare bearded tit or reedling.
You can see the Little Deep from the seawall footpath when Covid conditions allow again as part of this week’s walk. Meanwhile we may rejoice that a tiny fragment of our planet remains as a small jewel, supporting a wide range of rare and fascinating animals.