Crawdad or crayfish? Meet this resourceful freshwater crustacean
Earlier this year, a school class caught young crayfish during our pond study class. Crayfish are types of crustaceans that also include shrimp, crabs, and lobsters. So, let's check out those freshwater crayfish, or crawdads, if you like.
Crayfish have ten feet. The first two legs carry large pincers that the crawdads use to excavate their burrows. Burrows in water, you ask? Yes, and no.
Crawfish generally are found in lakes, streams, marshes and other wet areas. However, some of these wet refuges are temporary ponds and pools that dry out during warmer weather. Since crayfish filter oxygen through gills, the little crustaceans have to maintain some contact with water.
Here in northern Illinois, three species solve this problem by digging burrows. Given that they have to reach the water table, these burrows can be quite deep. Is it any wonder that crayfish pincers push up noticeable "chimneys" of excavated soil?
The crawdad's large claws are also used to catch their prey that includes a variety of plants and animals, dead or alive. Some crayfish are particularly fond of snails. Their idea of escargot includes eating the shells, which help strengthen their exoskeleton.
Interestingly, laboratory and field studies indicate that crayfish can significantly reduce populations of exotic zebra mussels living in streams.
Back to the pond study class mentioned earlier. When introducing the activity, I explained to the students that adult dragonflies eat adult mosquitoes in the air while dragonfly larvae, which use gills to breathe, eat mosquito larvae in the lake.
If given a chance, the omnivorous crayfish will snack on dragonfly larvae. Now, if you are researching the endangered Hine's emerald dragonfly here in northern Illinois, you might be unhappy to see crawfish chimneys turning up in your dragonfly study area. Think again.
Researchers from the Illinois Natural History Survey and the University of Illinois discovered that during dry periods, the larvae of the endangered dragonfly took refuge in the wet crayfish burrows.
As water levels dropped, the density of dragonfly larvae in crayfish burrows showed much lower fluctuations than those in a stream channel.
In addition, field experiments showed that if the crayfish were removed, the abundance of Hine's emerald dragonfly larvae was not significantly altered. There you have it. In order to help the prey, keep one of its predators around!
And where do populations of crayfish come from? Well, mature crayfish mate in winter. Once fertilized, the female applies a layer of glue to her underside and then curls her abdomen forward and deposits the eggs in the glue. The female carrying eggs is said to be "in berry," as her eggs resemble tiny, spherical black berries.
Within a month or so, the eggs hatch. The young, which remain attached to their mother, look like miniature adult crayfish.
You probably noticed that crayfish keep company with edible crustacea such as shrimp and lobsters. With that in mind, I'll leave you with a stanza from the traditional "crawdad hole" song.
They got two big claws and eight tiny feet, now honey
Two big claws and eight tiny feet, now babe,
Two big claws and eight tiny feet,
A tail full of meat that's good to eat,
Honey oh babe of mine.
• Mark Spreyer is executive director of the Stillman Nature Center in Barrington. Email him at firstname.lastname@example.org.