Goblet-shaped sea jelly relatives with miniature “arms.” A plump, legless creature resembling a sausage. Long, wormlike animals tipped with flat “holdfast” discs for anchoring to the seafloor.

Bees play by rolling wooden balls — apparently for fun. The cleaner wrasse fish appears to recognize its own visage in an underwater mirror. Octopuses seem to react to anesthetic drugs and will avoid ...