Well, what are you ...

[ysengrin]

Spotted in the reef tank because it was moving fast ... didn't get a good photo, but did snap two rough images. Size is just under 1/4" not counting the "antenna" (which I think are actually on the rear and not antenna). Pretty sure it isn't something we introduced to the tank, unless it hitched a ride inside a hermit crab shell. Pretty sure it's a hexapod (or isopod) of some sort, and pretty sure I've seen something like it in tidepools, though I don't remember what it was IDed as.

EDIT: Just saw a bristleworm in the tank, too. Got pix, post 'em later.

EDIT THE SECOND: seems to be a munnid isopod.