This “Christmas tree” attached to kelp that washed up on the beach is an animal colony known as a hydroid. Though they look like plants, hydroids are clusters of tiny stinging beasts. If you’re a planktonic creature adrift on ocean currents, this Christmas tree is your worst nightmare. If you’re a predatory sea slug, this hydroid is dinner.
When magnified, the tips of the hydroid’s “tree branches” show little polyps crowned with tentacles; each polyp is a miniature animal that resembles a sea anemone. Hydroids, jellyfish and anemones are close cousins in the same grouping of animals—cnidarian, meaning “stinging creature.” All species in this phylum fire poison darts to immobilize their prey.
Coral is also a cnidarian. Coral formations that look like pretty rocks to snorkelers gazing through masks look like collections of tiny, poisonous, predatory animals to scientists squinting into microscopes.