Sony F-717 digital, tripod
How could I pass up a scene like this?
Porcelain crabs (either Petrolisthes armatus or Porcellana sayana) usually perch atop rocks in order to feed, giving them the best flow of water for filtering microorganisms. In a pinch, they'll use any handy surface that fits the bill, such as the shell of this large thinstripe hermit crab (Clibanarius vittatus). Thinstripe hermits get fairly sizable, this one inhabiting a shell just slightly smaller than my fist. I didn't retain it for very long in the tank, because there wouldn't be enough for it to eat.
Most crab species avoid direct, face-to-face contact, because they will indeed feed on one another, but they seem to know what's safe, too – the porcelain crab was completely blasé about the much larger species. This species in particular doesn't seem to care how close they are to one another, either, and have little competitive nature, unlike other crabs species I've observed. During feeding, I've seen them wave a pincer to get another encroaching porcelain to back off about a centimeter, but that's about it.
Also visible, just under the shell opening of the thinstripe, is a slipper snail, seen here, happy to ride along and snag its own food.