Fossil Record Life & Ecology Systematics More on Morphology

Hydrozoa: Life History and Ecology

With about 2700 species, the Hydrozoa are a fairly diverse group, but they usually receive little notice, because many species form small branched colonies that can be mistaken for seaweed. Such colonial hydrozoans may be found attached to rocks or other hard substrates. Some colonies encrust hard substrates, while others form little feather-shaped or bushlike erect colonies. The members of one hydrozoan genus in particular, Hydractinia, form dense crusts on shells occupied by hermit crabs, and apparently defend the crab from predators. Other colonial hydrozoans, such as the chondrophorines and siphonophorans, are pelagic; many of these have developed internal gas-filled floats as an aid to buoyancy. Some siphonophores may reach lengths of over a meter and migrate up to 300 meters vertically in the water column in an hour.

Most hydrozoans are marine, but hydrozoans also include the few cnidarians that have adapted to fresh water: these are the polyps in the genus Hydra and a few species of small freshwater jellyfish.


  • Ruppert, E.E., and R.D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders College Publishing, Fort Worth.

Authors Copyright