A warning

A lionfish spreads out distinctively-patterned fins around its body and shows its zebra stripes. Such striking beauty hides a deadly secret: the lionfish has venomous spines hidden among the dorsal fins, and can deliver a very painful—though rarely fatal to humans—sting. The fish adopts a head-down posture and will swim aggressively towards intruders with spines exposed, to warn them away.
(Photo by Konrad Hughen, Woods Hole Oceanographic Institution)