Midwinter is a very revealing time. With most trees and shrubs stripped of their leaves it is easier to see birds, the shape and architecture of the trees and various other things which are obscured at other times of the year.

Often prominent now are marble galls on oaks, especially young and scrub oaks.

As their name suggests marble galls are hard, round and about two centimetres in diameter.

They are brown and have a generally smooth surface. They are very easy to spot on small branches, sometimes singly but often in groups of three, four or more.

Life is not easy for white-clawed crayfish. Once found almost everywhere in suitable waters in England and Wales this miniature cousin to the lobster is declining everywhere.

Overall its numbers are down by between 50% and 80% compared to 20 or 30 years ago, and in some places it has become extinct or has been reduced to a few small and isolated populations.

The Midlands remain one of its strongholds but the species which, outside Britain and Ireland, is only found in north-west Europe (from the Balkans to Spain) is endangered everywhere.

Slow-moving and unobtrusive white-clawed crayfish are up to 12 cms long, olive-green or brown and look just like little lobsters complete with big claws.

They need clean calcareous water with places to hide such as overhanging banks, tree roots and rocks. They are mainly nocturnal and will eat plants, animals, or detritus, and can live for ten years or more.

Apart from the usual problems for freshwater wildlife, such as pollution, engineered changes to streams and rivers and the clearance of bank side plants, this, our only native crayfish, suffers from competition from an introduced species - the American signal crayfish - and a deadly fungal disease which the interloper carries.

Its situation almost exactly mirrors that of the red squirrel: in both case the native species is both smaller than its American relation, and is susceptible to a disease carried by, but not seriously affecting, the newcomer.

For this reason the Staffordshire Wildlife Trust is appealing for help following the discovery of the disease in the Sherbrook on Cannock Chase, where there is a good population of white-clawed crayfish.

The trust's senior wetlands ecologist, Nick Mott, said: "This is devastating news as Cannock Chase has always been a stronghold for our native crayfish. The disease can be transferred to them via a number of 'wet pathways' including direct contact, footwear, animal coats, fishing kit or pond nets."

To help prevent the disease spreading, people are being asked to avoid entering the Sherbrook for the next few months or until there is confirmation that the disease has died out.

Once confirmation has been received, stream users can help to stop it returning by removing all mud and plant matter from footwear, pond-dipping nets or anything that has come into contact with water, then washing in clean water at home and drying thoroughly or disinfecting it before using it again in another pond or stream.

Dog owners and horse riders can also help by keeping their animals out of the water.

These precautions could usefully be applied whenever and wherever people have been in and around rivers, streams and ponds. The more protection we can give our native crayfish the better.

If you want to know more a booklet about freshwater crayfish is available free from the Trust: call 01889 880100, or download a copy from http://www.staffs-wildlife.org.uk/page/white-clawed-crayfish.