Invertebrates
Hydroid
Hydrozoa · also called hydroids
Growing across the underside of a dock piling, a colony of hydroids sways gently with the current, so convincingly plant-like that most people swimming right past would never guess they were looking at a colony of tiny stinging animals rather than a patch of ordinary seaweed.
Overview
The hydroid, class Hydrozoa, is a colonial cnidarian found attached to hard surfaces across nearly every marine habitat worldwide. Its plant-like branching growth form, close relationship to jellyfish, and inclusion of both the classroom-familiar Hydra and the remarkable immortal jellyfish make it one of the more quietly significant and widely overlooked animal groups in the ocean.
An animal disguised as a plant
Few animals manage to hide in plain sight as effectively as the hydroid, whose branching, stationary colonies fool most casual observers into walking, swimming, or diving right past an entire community of tiny stinging predators.
Status
Not Evaluated — common and widespread across suitable hard-surface marine habitat worldwide, with no documented widespread decline.
Hydroid: common questions
Is a hydroid a plant or an animal?
A hydroid is an animal, despite its plant-like branching growth form, made up of numerous tiny individual polyps living together as a colony, each equipped with stinging tentacles used to capture plankton and small invertebrates, a body plan that gives it a superficial resemblance to seaweed or moss rather than to a typical mobile animal.
How is a hydroid related to a jellyfish?
Hydroids belong to the same broad group of cnidarians as jellyfish and sea anemones, and many hydroid species share a similar life cycle, alternating between a fixed colonial polyp stage and a tiny free-swimming medusa stage that closely resembles a miniature jellyfish, making the two groups genuinely close relatives rather than just superficially similar.
Can a hydroid actually sting a person?
Yes, while many hydroid colonies go unnoticed entirely, some species carry stinging cells potent enough to cause real skin irritation in swimmers or divers who brush against them, a reaction sometimes mistaken for a rash from seaweed rather than recognised as a genuine sting from a colonial animal.
Is the freshwater Hydra used in classrooms actually a hydroid?
Yes, the well-known freshwater Hydra commonly used as a laboratory and classroom organism belongs to the same hydroid class, Hydrozoa, as many marine colonial species, sharing the same basic stinging-cell body plan despite Hydra itself living as a solitary individual rather than in a branching colony.