Common clownfish in a sea anemone, Raja Ampat Islands, Indonesia (© Magnus Lundgren/Nature Picture Library)

Sea anemones may look like underwater flowers, but they're actually animals—close relatives of jellyfish and corals. Anchored to rocks or reefs, they use a sticky base and a crown of waving tentacles armed with tiny stingers to capture passing prey. Those stings can paralyse small fish, yet one famous neighbour is immune.