Animal profile

Obelia

Obelia geniculata

HydrozoaAttached to seaweed, pilings and rock in shallow coastal water

Field profile

Obelia is a colonial hydrozoan forming branching plant-like growths on kelp and hard surfaces. The colony produces tiny free-swimming medusae. It is a standard teaching specimen for the alternating hydrozoan life cycle.

Published field notes

2024-03-03

Obelia

Obelia lives as two entirely different animals

The fuzzy branching growth on a kelp blade and the millimetre-wide swimming medusa in the plankton are the same species in different phases. Neither looks like the other, and the colony's sole route to sexual reproduction runs through the swimmer.

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