I brought home a few blades of eelgrass, a sheet of sea lettuce, and a clump of brown, feathery stuff, plus some sand for rooting the miniature eelgrass, and added them to the aquarium. (To the great delight of the hermit crabs.)
Half an hour later, this critter was out exploring ...
It's a Spotted Aglaja*. It was probably buried in the sand, where these usually hang out, just under the surface.
... it preys on bubble snails along with other small mud dwellers including copepods. Lacking a radula or rasping tongue, it sucks in it's prey whole. (From Puget Sound Sea Life.)Here are a few more photos.
These animals are hermaphrodites; each one is both male and female. There's a good series of mating photos of a similar species on DORIS. (In French, but Google will translate, sort of.)
Mine (Spotty) is a young one; the average size is about 3/4 of an inch, although they grow up to about 2 inches.
After s/he did the rounds of the walls, s/he headed down towards the sand, and hasn't been seen since. There's plenty there for her to eat; copepods, amphipods, a few tiny clams, and loads of infant polychaete worms.
* Or possibly Diomede's Aglaja, but I think the head on DA is too rounded and narrow.