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Hydroids (Hydra and Chlorohydra)

Taxonomic group

Hydridae: Hydra and Chlorohydra

Diagnostic features

Diagnostic features – Hydrozoans (Phylum Cnidaria) are small, predatory animals, most of which are marine, but a few live in fresh water. Two common genera, Hydra and the green Chlorohydra resemble small sea anemones (which are also members of the Phylum Cnidaria). They have very stretchy bodies, but when preserved they tend to contract into a short stout body form.

Typical habitats

Hydra and Chlorohydra are most common in weedy, slow-flowing streams and ponds.


They use stinging tentacles to capture small invertebrate prey, but Chlorohydra also uses symbiotic green algae to manufacture food within the body.

Indicator value

  • Hard bottom: Hydra 3
  • Soft bottom: Hydra 1.6
  • Hard bottom: Chlorohydra unassigned
  • Soft bottom: Chlorohydra unassigned

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

Both genera are common in open farmland streams with some nutrient enrichment. Hydra has low tolerance values of 3 (hard bottom sites) and 1.6 (soft bottom sites). Chlorohydra does not have separate tolerance values (the two genera are typically grouped together as “hydroids”).