Marsh Invertebrates

by Carl Strang

They are called “amphibian traps,” but the little cage-like contraptions I have been running at Mayslake Forest Preserve’s stream corridor marsh have revealed more about the wetland’s invertebrates than its amphibians, though last week’s tiger salamander was a major coup. The salamander was not alone in its trap, as there also was one of the predaceous diving beetles, Dytiscus verticalis, one of only two I’ve caught so far this year. That low number makes me wonder if predators such as the bufflehead pair and the hooded mergansers that frequented the marsh last year might have made inroads on the Dytiscus population. On the other hand, a few representatives of a new insect turned up in the traps last week. At first I thought they were water boatmen, as they preferred to sit on their bellies in the trap.

The pale backs made me suspicious, however, as this seemed likely to be the countershading one would expect in a backswimmer.

Sure enough, when placed in the water, the half-inch-long true bug flipped over.

The dark underside blends with the marsh bottom, while the pale back is better camo against the sky.

As best I can tell from the BugGuide website, these belong to the common species Notonecta undulata. I also found some amphipods.

Also known as scuds or sideswimmers, these tiny invertebrates are not identifiable to species from photos. There seem to be a lot of them, though, in the marsh.

A final, more familiar invertebrate in the traps has been the white river crayfish.

Two of the red-hued crustaceans turned up last Friday.

They are one of Mayslake’s two known crayfish species. I found a tragic example of the other while leaving the marsh on Tuesday.

It was a large, dead grassland crayfish.

Grassland crayfish live in tunnels they dig on land, away from open water, though the tunnels go deep enough to reach the water table. Sometimes these odd crustaceans climb up on land to wander on warm, humid nights. Now is the time of year when mothers leave their tunnels to carry their young to water and release them. The young grow to the point where they are big enough and strong enough to go out on land and dig their own tunnels. After letting the babies go the mothers remain in the water for some undetermined time, and then return to their homes. The dead crayfish was pointed toward the marsh. I turned her over.

Sure enough, the ground beneath her abdomen was littered with little dead crayfish. Others still were stuck to the mother.

She nearly had made it.

The dead crayfish is at the center of the bottom edge of this photo. She is only 15 feet from the water’s edge.

I’m inclined to attribute this tragedy to the unseasonably warm, dry weather we have enjoyed in recent days. In most years the mother crayfish can come out in March, day or night, with no danger of dehydration. Not so in this case.

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