Macrothricidae: Ilyocryptus sordidus

by Howard Webb (St. Louis, MO, USA)


The common image of a water-flea (Cladocera) is of a small, transparent zooplankton, swimming in a short jerky motion. While this is my prototype, it does not fit I. sordidus; a bright red creature that does not swim.

I discovered I. sordidus this spring while sampling in the Mississippi River. I was canoeing off of the main channel in a small slough.  Being out of the channel, there is no current, so suspended matter settles out.  My plankton net came up full of mud, and rather than following my first impulse of dumping it all out, I sifted the mud from the net.

The Daphnia

My regular sampling produced the usual suspects (Daphnia pulex, Daphnia ?, Bosmina, rotifers and ciliates) (ref. 1), but the mud contained a number of what at first appeared to be mites. Even after looking at them under the scope, it was hard to believe they were cladocera.

I. sordidus have been previously reported in the Missouri (ref. 2). What little environmental characteristics I could find matched my findings (ref. 3); living in muddy sediment and not free swimming. There is some dispute about the exact taxonomy of this species (ref. 4).

Attempts to culture them failed, and I was not satisfied with the few slides and photos I had. I went back to the same location in July. The water was several feet lower, and I was unable to find any daphnia at all (though enjoyed a large number of blue herons, egrets and a flock of pelicans). In late August I went back to another location (upstream in the same pool), and deliberately scooped the mud looking for (and finding) I. sordidus.


Macrothricidae: Ilyocryptus sordidus

The composition of these two is not what I would like, but this nicely shows the fringe on the carapace, and the long 'tail hairs' coming off of the post-abdominal process. These were one of the first images (live).

Macrothricidae: Ilyocryptus sordidus
Another live image, from the August collecting. Notice the relatively large claw.

Macrothricidae: Ilyocryptus sordidus

Mounted slide from August collection.  While there is more overall focus, the specimen suffers from being crushed.

Macrothricidae: Ilyocryptus sordidus

Another mounted specimen from the August collection.  Definitely not the normal delicate antenna.

Side Note

Macrothricidae are difficult to photograph, they have a very round body which exceeds my depth of focus (at 40X). The roundness also keeps them from showing a good profile.   Another problem is that these are mud dwellers, they always seem to have some small particles clinging to them, no matter how much I run a stream of water over them.

For the first live image, I used the 'burst' mode to take a series of shots, capturing fleeting exposures of various anatomy.  The other images were captured with the camera operated through the computer.

Technical Details

Environmental Conditions:
Water temperature: Spring 16 C, Fall 27 C
Depth: 2 meters
Location: Spring: Brickhouse Slough (behind Dresser Island) Pool 26 (Alton, Il/Portage des Sioux, MO)
UTM 15 740343E 4309378N
Fall: Pool 26 (Alton, Il/Portage des Sioux, MO) mile 214 (about 4 miles upstream of Brickhouse Slough)

Microscope: Bausch & Lomb monocular, 10x ocular, 4x, 10x and 40x objectives.

Mountant: PVA-G (Polyvinyl Alcohol with glycerol).  Thanks to Walter Dioni for introducing this to me.

Camera: Canon A70

Software: Photoshop Elements


1. Interesting by their absence were D. lumholtzi, which I found upstream in the same pool last September. It may be too early in the season (too cold) for this species to be present.

2. Zooplankton Project (Aquatic biology at South West Missouri State University)

3. Dodson, S.I., and D.G. Frey. 1991. Pp. 723-786 in Thorp, J.H., and A.P. Covich (eds). 1991. Ecology and classification of North American freshwater invertebrates. Academic Press. (Up to date keys to genera; particularly useful for chydorids.)

4. Alexey A. Kotov, Manuel Elías-Gutiérrez, Judith L. Williams, A preliminary revision of sordidus-like species of Ilyocryptus Sars, 1862 (Anomopoda, Branchiopoda) in North America, with description of I. bernerae n. sp.,   Hydrobiologia, Volume 472, Issue 1-3, March  2002, Pages 141 - 176.

Comments to the author Howard Webb are welcomed.

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