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 Major Groups | Cnidaria (hydra, jelly fish) | Hydrozoa (hydra, jellyfish) | Hydridae


Major Group: Cnidaria
Minor Group: Hydrozoa (Pennak 1989, ABRS 2004)
Order: Hydrozoa (Williams 1980) or Hydroida (Pennak 1989, ABRS 2004)
There is some discrepancy in the literature about the taxonomic level of Hydrozoa. This does not affect the identification of Hydridae.

Descriptive Features:

  • sessile animals
  • colour variable, depending on age, type and amount of food ingested
  • radially symmetrical, main body, “column”, is an elongated cylinder
  • pedal disc, for substrate attachment, consists of special secretory cells
  • distal end of column has a circlet of 3 -12 tentacles, typically 5 or 6, length varies from ˝ - 5 X length of column
  • raised, dome-like hypostome present at end of column in centre of the circlet of tentacles, bears a single opening that is the combined mouth and anus
  • single, continuous, internal body cavity is a gastrovascular cavity; it continues out into hollow tentacles
  • nematocysts, minute stinging capsules, imbedded in certain epidermal cells
  • Total length: 1 - 25 mm

      Hydra sp.


      Hydra sp.

      Taxonomic Checklist: Species (Williams 1980)
      Hydra oligoactis Pallas
      Hydra hexactinella
      Chlorohydra viridissima Pallas

      Distribution: all states

      Sensitivity Rating: SIGNAL grade 2

      Functional Feeding Group: predators


      Flying Fox Creek, Darwin NT

      Ecology: Instream habitat: Hydra are common in freshwater ponds, mountain streams and small lakes. They are found attached to stones, twigs and other submerged plant detritus, often in large aggregations.
      Feeding ecology: All cnidarians are predators feeding on minute animals such as cladocerans and copepods. They catch their prey with tentacles armed with stinging nematocysts.
      Life history: In Hydra, the medusa stage is usually absent. The polyp reproduces both sexually and asexually by budding.


      Information Sources: Williams 1980, Pennak 1989, ABRS 2004, Gooderham & Tsyrlin 2002
      Key to Genera: Pennak 1989 (USA)
      Key to Species: none