Water Louse – Asellus

2008 May 3

The back part of our property drains into Cole’s Creek, which empties into Portage Lake about a mile to the west of us. It’s a pretty substantial stream, and one of the better places for catching smelt[1]. S_ has been going down there from time to time to check if the smelt run has started, so last weekend she suggested that we all go there to poke around in a stream bigger than the one beside the house and see what we could find. We probably didn’t find anything that isn’t in our little micro-creek[2], but we did find some photograph-worthy specimens. Like these guys:

Large water louse
Small water louse

These are “Water Lice”, genus Asellus. They are isopod crustaceans closely related to “wood lice” like Armadillidium and Porcellio, except that they are aquatic. They still live under rocks and branches a lot of the time, and eat stuff like fungus and decaying leaves. They just do it under water instead of on dry land.

I wanted to get a picture of the underside, and eventually I got the bigger one flipped over without having it flip right back again

This looked odd – there were too many legs. Something was on the underside. It took some time and a lot of pictures, but eventually I got a good clear shot of what was going on under there:


There was another, smaller water louse being held underneath! The big one is evidently a male, and he’s guarding a female so that he can mate with her on her next molt. It looks like he’s devoting one pair of legs to holding her, and then just proceeding with life as if nothing is going on. I expect that she picks up a lot of food from the debris that he stirs up as he eats.

There is apparently a tradeoff going on here regarding the size of the females: larger females can lay more eggs and would therefore seem to be more desirable mates, but the smaller females are easier to guard, and are less likely to be “poached” by other males. This one, for example, is so much smaller than the male that it’s hard to tell she is even there, so he has her pretty much all to himself. So, next time she molts, he’ll mate with her, and then she will carry the eggs in an internal pouch until they hatch.

Water lice are reported to be easy to raise in aquariums, or even in a shallow dish. I bet they are at least as much fun as “sea monkeys”, and a lot cheaper to come by.

[1] For anyone who doesn’t live where there are smelt, they are little fish, about 3-6 inches long, that swim up little creeks to spawn in the early spring. During a good smelt run, you can scoop them out of the stream with a net by dozens. They are very tasty little fish, they don’t have noticeable scales, and the bones are soft enough that you don’t even notice them when you eat them. All you need to do is cut off the heads, remove the entrails, and fry the fish up. Mmm, calcium. Well, technically, you don’t *have* to cut off the heads and remove the entrails, but most people are a bit squeamish about that.

[2] Cole’s Creek is actually kind of barren compared to our little stream. It has a mostly sandy bottom and a pretty fast current, and turning over any given rock in it exposes maybe two or three crawling things, instead of dozens or hundreds like what we see in our stream. I think that the sandy bottom doesn’t give a lot of shelter against the current, so the little arthropods get swept out into the lake instead of hanging around in the stream. Plus, Cole’s Creek obviously has fish, which snap up anything they find exposed. The bugs are there, they just aren’t present in any serious quantities.

8 Responses
  1. May 7, 2008

    Another triumph for the Backyard Arthropod Project!

    You’re my hero, dude.

  2. June 2, 2008

    Wow! Sex slaves of the animal kingdom on the next Maury Povich!

  3. June 3, 2008

    Isn’t it amazing how things that are morally reprehensible when people do them, turn out to be standard procedure when you start looking at certain bugs?

  4. June 3, 2008

    I wonder if the bugs get a thrill out of watching other bugs do this. God, I hope not. It’s bad enough when we do it.

  5. Leanne permalink
    July 10, 2008

    That Is Bloody Disgusting

  6. February 8, 2011

    I Was Nearly Sick When I First Saw This!!

  7. February 9, 2011

    Sian: They aren’t so bad, at least as long as you don’t find them in a cup that you have been drinking out of.

  8. Olivia permalink
    June 21, 2013

    Thanks for the very interesting pictures and post… I would want to read again with my children when they get bigger..

    By the way for this insect particularly, Could it be the big one was female and the small one that it carried underneath was male? I am a layman but what I know from general reading is that female is by far larger than male most of the time.

    🙂 please let me know if you have a way to identify the sex of the insect. Thanks.

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