A6 stream bridge

This Signal Crayfish was scampering about on the bottom of the shallow stream at this site midday yesterday. Notice the white area on the claw. The Swedish biologist, Gunnar Svärdson, who gave this species its colloquial name, observed individuals waving their claws at one another, and the white patches reminded him of train dispatchers signalling to the engineer!

Signal crayfish, originally hailing from the United States, have been spreading throughout the UK ever since escaping from ponds where they were farmed for food in the 1970s. They readily walk overland. The bad news is that they reproduce rapidly and numerously and, even more ominously, carry a plague that can wipe out our native White-clawed Crayfish.

The small Bedfordshire population of White-clawed Crayfish collapsed a few years ago, though we don’t know whether the Signal Crayfish invasion was to blame. Other factors that are worrying conservationists are the amount of native species that the omnivorous Signal Crayfish consume – upsetting delicate ecosystems; and the way in which they burrow into the banks of rivers, streams and ditches – thus destabilising them.

Here's some other interesting Signal Crayfish facts that I came across:

• The female breeds from the age of about two when it is 40mm long.
• She breeds once a year and averages 275 eggs.
• The eggs are fertilised by the male in October/November.
• They are carried by the female folded within her tail until May when the young are released - if they can escape her jaws.
• The Signal is bigger and more aggressive than native crayfish.
• They are less fussy in what they eat and more successful and rapidly colonise new areas.
• They can live up to 12 years.