Posted: 7/21/04

Crayfish

When I was a child, the crayfish confused me. I could not understand why it should be called a fish. It had nothing in common with any fish I had seen, other than the small matter of living in the water. I was startled by its movement, which was quicker than expected, and backwards. I was in awe of the big claws, imagining how it would feel to have such things clamped on my foot.

The northern crayfish is the native species; the rusty crayfish is an invader. The rusty is bigger, and takes over shallow habitat where both try to live. The rusty claims the cover of rock ledges, leaving its competitor exposed to predation. The rusty eats enough vegetation to change the conditions of the weed beds.
How can such a small, water-bound organism "invade" a vast region of rivers and lakes?

A slow invasion has been taking place since the last ice age. Even the northern crayfish is in this sense an invader. But the influx of the rusty has been sudden, thanks to bait buckets of fishermen from Ohio, Tennessee, and Indiana.

A crayfish can walk short distances on land, but its unaided passage across a continental divide is doubtful. Once introduced, movement can be quick, and reproduction can be prodigious. A study in Wisconsin found that a crayfish travels as much as a hundred meters a dayñin a lake with no current to help. Since the female can store sperm for fertilization, and can produce several hundred eggs at a time, there is no need for a coordinated release of male and female pairs. The population can explode quickly.

I like to see the crayfish, but I like to think it is a population like othersñin check. With the rusty species still in a takeover mode, and with scientists concerned about the harm it may cause, I would prefer not to see a lot of them.

Top of Page