Definition of hydroid in English:



  • A coelenterate of an order which includes the hydras. They are distinguished by the dominance of the polyp phase.

    Order Hydroida, class Hydrozoa

    • ‘Aside from its temporary and permanently resident fish, the underside is decorated with sponges, hydroids, tunicates, tube worms, cup corals and anemones.’
    • ‘There were dense walls of soft corals, sponges, anemones and hydroids, nudibranchs, creepy-crawlies and lots of fish.’
    • ‘On the walls are a riot of soft corals, sponges, hydroids and anemones in all shapes and sizes.’
    • ‘The most numerous inhabitants are hydroids and copepods.’
    • ‘Colonies may be mistaken for hydroids, corals, or even seaweeds.’


  • 1Relating to coelenterates of the hydroid group.

    • ‘He had shown that hydroid jellyfish known as naked-eyed medusae reproduce not only by spewing eggs, but also by asexual budding, which he found marvelous to behold.’
    • ‘The hydroid tissues, in particular, are the general run of moss water storage cells and are probably only remotely related to the xylem of tracheophytes.’
    • ‘Before long, similar green proteins were detected in many bioluminescent coelenterates including various medusae, apparently all luminescent hydroid polyps, and a few others.’
    • ‘Along the tops of the walls and gullies are many small hydroid clumps which, in turn, attract a range of delicately coloured nudibranchs that feed on them.’
    1. 1.1
      another term for polypoid (sense 1)


Mid 19th century from hydra+ -oid.