Crying blue murder

This is amazing, this river crayfish should be shabby brown but is kingfisher blue. It is beautiful. I never thought I'd say that about Austropotamobius pallipes, the country's only native crayfish species, the White-claw. Blue crayfish are a rare genetic phenomenon, akin to an albino bird or animal.

Blue boy, known as Reggie, is in a tank with its drab brown female mate (Heidi) at Malham Tarn Field Centre, once the remote moortop home of Walter Morrison, a wealthy Victorian who was MP for Skipton and ate crayfish regularly. It is leased from the National Trust by the Field Studies Council and since 1947 it has held outdoor courses for all age groups. It has some 4,000 students a year.

Reggie and Heidi are having babies. The parents arrived last May in a trial from a major education project in Ribblesdale. The female was carrying eggs and these hatched with a 50 per cent survival rate – better than achieved in open streams. The 20 or so youngsters, the size of an earwig, are in a separate tank, safe from the two adults, which may eat them. Now Heidi has conceived with Reggie in the small tank at Malham. This is exciting news for crayfish watchers. It's a bit like trying to breed giant pandas in captivity.

The crayfish are fed with net-loads of stream bottom dredgings, rich in their natural food. This is how the brown trout arrived as a tiny thing. Now it is the size of a decent goldfish and shares the tank with the two adult crayfish.

Robin Sutton, assistant head of the centre is delighted with the response of the students. The younger ones from cities, many of whom have never seen a stream, let alone a crayfish, are immensely interested, says Robin. "They may have watched them on the telly but it's an abstract idea until they see them. Then they almost want to get their heads under water with the crayfish."

This high moors idyll is a spin-off from the country's major crayfish revival programme, which was started in 2000 by a farmer and Environment Agency fisheries officer called Neil Handy. He became worried about the devastation being caused by a water-borne disease, aka plague, and the predation of the American Signal crayfish.

This aggressor is larger, stronger, hungrier and immune to the plague which it spreads. Its activities weaken river banks. It eats anything it can tackle, including small game fish, the eggs of salmon and trout.

It is also the one you'll eat in your deli sandwich, bred for the table in fish farms. It has proliferated in the wild since escaping sometime in the 1970s.

It is as hard to cull as the grey squirrel, yet almost invisible to the public. Neil, who lives near Settle, on the River Ribble, sounded a warning many years ago. If left unmolested, the Signal crayfish could not only wipe out the native White-claw but wreak havoc with the valuable salmon rivers in Scotland. He says this could happen in 10 or 15 years.

In 2003, the Yorkshire Post reported on his plan to breed the crustacean in tanks. It was early days but the tentative signs were promising. We went back in 2005 and he was a little further on. There were young.