Key to Australian Freshwater and Terrestrial Invertebrates
Phylum Cnidara
Class Hydrozoa
Families Hydridae & Oceaniidae (hydroid forms)
Common names: hydra, freshwater polyps, hyroids
Overview
Two families of hydroid Cnidaria occur in Australasian freshwaters: Hydridae, an exclusively freshwater group represented by solitary hydroids of the genus Hydra, and Oceaniidae, which has both marine and freshwater species. The body of hydras are a simple tube with a ring of tentacles around the mouth. The tentacles contain stinging cells, or �nematocysts�, which are used for defence and to capture food. Hydras can be green, brown, white, grey, tan or red in colour and may be up 3mm long when extended, but the tentacles and body are highly contractible. While feeding the body is elongate, however when disturbed, the body and tentacles can be contracted and appear small and blob-like. Hydras are generally sessile, but they can detach and move to another location by gliding slowly on their base. Sometimes they �somersault� from tentacles to base and back again or simply detach from the substrate and float away in the current. The other family, Oceaniidae, are colonial sessile hydrids, which look like a cluster of Hydra linked together by a framework of tubes. They have same basic body structure as species of Hydra, but instead of living as separate individuals (polyps), buds are interconnected by stalk-like structures (stolons), forming a continuous colony. Colonies measuring up to 100 mm in height have been reported, but are usually only 10-20 mm high.
Distribution and diversity
There are around 2,700 living species of hydrozoans. Two families of hydroid Cnidaria are known from Australian inland waters. Hydridae, represented by the solitary genus Hydra, are common in freshwater lakes, pools and creeks across temperate and tropical Australia and New Zealand. The other family, Oceaniidae, represented by the colonial genus Cordylophora, are found in fresh and brackish water and have been recorded from a number of shallow inland lakes across eastern and south-eastern Australia.
Life cycle
Hydroids most frequently reproduce asexually by budding where a bud or cyst forms in the body wall. The budding polyp develops tentacles, continues to grow and eventually separates from the parent polyp to continue a solitary existence. In addition, Cordylophora have two types of buds: polyps that usually have 10-20 tentacles and �gonophores�, which are reproductive and lack tentacles. Gonads develop and produce sex cells in response to environmental triggers for surviving periods of unfavourable conditions such as winter or at the onset of drought. Eggs form in the body wall that eventually splits to expose them. Sperm are subsequently released into the water to fertilise the exposed eggs which then form a chitinous shell before being released. The eggs are very hardy and remain viable even if frozen or dehydrated and will hatch into miniature adults when favourable conditions return.
Feeding
Hydrozoa are predaceous, feeding on tiny aquatic animals such as zooplankton, small crustaceans, insects and other invertebrates, even newly hatched fish. Prey are captured by tentacles armed with numerous stinging nematocysts which are sensitive to touch and automatically fire dart-like threads containing neurotoxins into whatever triggered their release. Once embedded in the flesh of its prey, poison is injected to paralyse it. The tentacles pass the prey to the mouth where it is consumed whole. Hydras can stretch their mouths and gut cavities to consume prey up to four times the diameter of their bodies. Indigestible remains are expelled through the mouth.
Ecology
Freshwater hyroids live attached to solid surfaces such as stones, aquatic vegetation and submerged plant detritus such as sticks or logs. Hydra occur in a wide range of freshwater habitats from the shallow shores to the depths of lakes to fast- or slow-flowing streams and pools where they are often seen in large congregations. Most Oceaniidae are marine, but Cordylophora occurs in fresh and brackish water and in Australia has been recorded from shallow inland lakes sometimes with a salinity up to two-thirds that of seawater. Hydroids do not appear to significantly affect human activities. However, Hydra are well-studied animals due to their tissue regeneration ability when injured or severed and the fact that they appear to senesce very slowly, if at all.
