True Crabs

Nine species, each listed quarantined and non-quarantined, doing very different jobs. Emerald and porcelain crabs are the reef-safe end: the emerald grazes bubble algae, the porcelain fans food out of the current and bothers nothing. Arrow, sally lightfoot, decorator and sponge crabs are opportunists, better suited to a fish-only tank.

The horseshoe crab needs the hardest thought. It plows a deep sand bed for a living, wants far more floor space than most displays have, and starves on a bare bottom. Arrow crabs clear bristleworms, then start on small sleeping fish. If you want cleanup with no ambush risk, buy the emerald and stop. Quarantined stock ships with a 48-hour DOA window.