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Common names: water bears, tardigrades
Probability of encounter: high in appropriate products (e.g. mosses, wet soil) if appropriate collection techniques are used. Since tardigrades require a film of water for activity and they enter inactive stages when dry, only wet collection techniques (funnels, flotation) are likely to recover them.
Quarantine importance: no known importance. The species that have been studied graze on algae, fungi, lichens and similar microbial material or are predatory on small invertebrates, including other tardigrades.
Similarity to mites: very small size, 4 pairs of 'legs' (lobopods), and rarely with long seta-like structures, but without jointed limbs, chelicerae, or gnathosoma. Tardigrades could easily be confused with mites, especially the armored species, but all mites will have typical segmented arthropod legs.
Morphology
Normal adult length:
0.25 - 0.5 mm (range 0.05-1.7 mm)
Body tagmata: trunk; true head absent, although cephalic segment
present
Eyes: a pair of lateral eyespots or blind
Antennae: absent
Mouthparts: internal sucking pharynx, stylets and supports
Legs: 4 pairs of lobopods ending in 2-6 claws (rarely more) or
4-6 digits ending in suckers.
Respiration: cuticular
Gonopore: obscure, subterminal
Distinguishing features: internal basket-like pharynx, placoids, buccal
support, stylets, 4 pairs of lobopods
with terminal claws (or toe-like suckers), 5 indistinct segments (cephalic, 3 trunk segments, terminal
segment), soft-bodied or with series of armor plates.
Comments:
Tardigrades are commonly known as 'water bears', presumably because of their
large claws and ursine build, or perhaps because of their teddy bear-like
aspects. Tardi-grade means 'slow walk', an apt description of their movement.
Most tardigrades have short, cylindrical
bodies with either a smooth integument that lacks armor (Eutardigrada) or a
series of armored plates (Heterotardigrada). Some, particularly marine species,
have large spines or body lobes.
Depending on species and
diet, live tardigrades can be white, green, brown or red. They have four pairs
of stubby, unjointed lobopod legs; the fourth pair may be reduced. Each leg ends in
2-4 or more claws that
may be attached to toe-like processes. The head is not clearly set off from the body,
but may bear a pair of eye spots. One or more seta- or horn-like cirri may
be present on the head. Mouthparts are internal stylets that are protruded through
the pore-like mouth when feeding. Anal and genital openings are terminal or
between the bases of the fourth pair of legs. Tardigrades range in length from
0.1 to 1.7 mm long, with most terrestrial taxa being 0.5 mm or smaller.
Diversity: ca. 900 described species in 3 classes, 5 orders; many families and genera
References
Tree of Life - http://tolweb.org/tree?group=Tardigrada&contgroup=Bilateria
Nelson DR. 2002. Current status of the Tardigrada: Evolution and ecology. Integ. and Comp. Biol. 42: 652-659.
