Stream Sampling 1

by Carl Strang

Mayslake Forest Preserve has a small stream, much of it less than 3 feet wide, which is the outlet for a string of lakes including the preserve’s May’s Lake and Trinity Lake. Ultimately it winds its way to Salt Creek, thence to the Des Plaines, Illinois, and Mississippi Rivers. I have been curious about what forms of life call that little stream home, and last week I took part of a morning to begin a survey.

Here the stream is roughly halfway from May’s Lake to its exit from the preserve.

I used a fairly standard method, planting the flat side of a D-shaped net on the bottom of the stream and then shuffling my feet upstream from the net and allowing the current to carry any disturbed animals into the bag.

Here the net is placed in the stream, ready to receive any animals washed into it.

I lift up the net, then, to see what I have caught.

This is a fun moment, anticipating the delight of discovering what fell into the net.

I transfer any recognizable critters to a white tray with a little water in it.

The white background makes the little animals easier to see.

In the initial sampling effort I found too many different organisms to cover in just one post. Having described the methods, I’ll share a few of my finds today and then spread the rest over a couple more chapters. Let’s begin with some worms.

A free living flatworm, or planarian.

Planaria are tiny flatworms found in every one of our streams, as far as I know. With kids I have used the descriptive nickname “cross-eyed triangle-head.” They function as scavengers, and are renowned for their regenerative abilities. Also in the initial sample I saw a couple elongate forms that brought back childhood memories: leeches.

These are segmented worms, recognized by the suckers on each end.

Some but not all leeches are blood-sucking parasites. Many are predators of small invertebrates. Planaria and leeches were familiar, but I also encountered something new to my experience: aquatic earthworms.

This one looked just like the worms one finds in terrestrial soils.

My references indicate that many aquatic earthworms live much like their terrestrial counterparts, tunneling through the mud, swallowing soil and digesting microscopic life within it. Another such worm I caught was a little different.

Note the longer bristles, or setae, on the body of this worm.

I didn’t pay much attention to differences in substrate in this initial exploration. I wonder if the bristles help secure this species against current. Perhaps it dwells in riffles, or is a species that remains closer to the surface and acts more like a predator.

This is just the start. I found a dozen other kinds of animals that day, and will share them in future episodes.

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1 Comment

  1. jolene said,

    April 29, 2012 at 10:00 pm

    II found so many of those aquatic earthworms. today by accident i was digging in a fast moving stream with my kids for rocks and moving sediment.. well we kept getting these every few minutes or so. I freaked out thinking they were nematodes! Or at the very least … leeches… Good to know they are harmless earth worms!!


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