Jellyfish - hydroid forms

Code IB999999

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Two families of hydroid Cnidaria are known from Australian inland waters. One, family Hydridae , is represented by solitary hydrids of the genus Hydra . These are common in ponds and streams and often are found on aquatic vegetation. The other family, Clavidae , comprises colonial, sessile hydrids which look like a cluster of Hydra linked together by a framework of tubes. Clavidae occur in fresh and brackish water.

Hydridae (Code IB019999)

The body is a simple tube with tentacles around the mouth. Size is variable up to about 3mm when extended, but all species are highly contractile. Colours vary from whitish to green. Hydra are slow-moving. They creep on the ab-oral foot or progress by a cartwheel or somersault motion. Reproduction is mostly by budding, but egg and sperm cells are produced seasonally or in response to environmental triggers. Fertilised eggs hatch directly into small hydras without going through a medusoid (jellyfish) stage.

Small, tentacled flatworms of the family Temnocephalide might sometimes be mistaken for Hydra . Temnocephalids live attached to crayfish or other aquatic animals. They hold on by glands at the rear and waving tentacles in the water. To distinguish them: the hydroid gut is a simple blind sack but temnocephalids possess a muscular pharynx, and Hydridae possess microscopic stinging cells while temnocephalids do not.

Clavidae (Code IB029999)

The colony resembles a cluster of Hydra linked by a framework of tubes. The genus Cordylophora is recorded from a number of shallow lakes across eastern Australia (Williams, 1980). Clavidae might be mistaken for branching colonial Bryozoa but can be distinguished on the basis of zooid form and the presence of tentacles with stinging cells.

Reference:

Williams, W.D. (1980) Australian Freshwater Life: The Invertebrates of Australian Inland Waters. The Macmillan Company of Australia, Melbourne.