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We encountered these guys in a small inland (freshwater) creek in southern WA. There were hundreds, covering low flat rocks in the water; every one carrying a shell made out of either tiny rocks, or a piece of a twig. They're probably 1/8" in diameter, maybe 3/4" long with the shell. Some were moving at a slow crawl along the rocks (below and above the surface), others were burrowing in and out of the dirt on the creek bottom.

Anyone know what these might've been? Are they native to the area or invasive?

Crab? Crab?

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    This might be better at biology.stackexchange.com
    – James Jenkins
    Commented Jun 10, 2016 at 17:13
  • Thanks @JamesJenkins, didn't realize that existed -- if no one here knows, I'll ask for it to be migrated.
    – Jeremy Nikolai
    Commented Jun 10, 2016 at 18:53
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    @Sue: Just looking for an identification; we saw them while camping and were curious about them. The creek was freshwater; near a campground and downriver from a small waterfall. I'll add in one more picture and some more details (they were hard to get close-ups of given their size).
    – Jeremy Nikolai
    Commented Jun 10, 2016 at 18:53

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Most likely a Caddisfly. They make homes of twigs and stones.

CADDISFLY: The caddisfly lives only a short time as an adult but may spend several years as a larva. Many larvae can do something few aquatic insects can – they build their own shelter. Different kinds of caddisflies build different types of homes. Some species build homes of leaves or twigs; others use tiny stones, while others are freeliving. A few types of caddisfies build a pebble house attached to a larger rock. Some caddisflies like to live in temporary stagnant ponds while other types will only live in swift streams. All caddisflies eat plants but some catch tiny bits of plants by building a net to trap food as it drifts past. The caddisflies are very important food for fish, especially trout.

enter image description here

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    I think that's it! or, more specifically, caddisfly larvae. Also, that led me to the discovery of this artist that makes jewelry by giving them new materials to work with.
    – Jeremy Nikolai
    Commented Jun 11, 2016 at 1:07
  • Re the jewelery -- Art or abuse? Depends on what he does with them. If they are kept in an appropriate habitat, it could be art. Otherwise, abuse.
    – ab2
    Commented Jul 28, 2019 at 14:37

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