Code IR999999

Tardigrades are small animals, rarely and somewhat patchily collected from aquatic habitats but more usually found in the surface film of terrestrial bryophytes or in damp soil. The body is short and stout, usually with annulations, sometimes with more complex plates and spines. There is usually a single pair of simple eyes. There always are four pairs of stumpy, unjointed legs, terminating in claws, pegs or adhesive disks (sometimes on finger-like projections). Body length may reach 1.2mm but rarely exceeds 0.5mm. The mouth is terminal or sub-terminal and is flanked by paired stylets. The pharynx is of sucking type, as in some flatworms (Platyhelminthes). The anus is located posteriorly, usually with a single genital duct (females) or paired ducts (males) in a cloaca. Tardigrades move by sluggish crawling and are known as 'water-bears'.

The phylum is divided into three orders and eight families (Parker, 1982). Some families are wholly marine, others cosmopolitan in freshwater. Two orders and four families may occur in Australian inland waters:

Order Heterotardigrada:

Families Oreelidae and Echiniscidae

Order Eutardigrada:

Families Macrobiotidae and Milnesiidae.

Identification

Tardigrades, though small, are distinctive and not readily mistaken at the phylum level. Identification to Tardigrada can be based on size, body shape and the presence of four pairs of short, unjointed legs with one or more terminal claws or pegs or toe-like adhesive disks.

Heterotardigrada carry tentacles or antennae on the head, and Oreelidae have four smooth claws on each leg while Echiniscidae have heavily sculptured dorsal plates and either some claws with a spur or else some legs with other than four claws.

Eutardigrada do not carry tentacles or antennae. There are four claws on each leg, but in Milnesiidae each leg is divided at its base into two parts, while in Macrobiotidae the legs are not divided.

Reference:

Parker, S.P. (1982) Synopsis and Classification of Living Organisms. Volume 2. McGraw-Hill Book Company, New York.