Tardigrada
Heterotardigrada
Mesotardigrada
Eutardigrada
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Tardigrades (
Tardigrada), or "water bears", are a
phylum of small, segmented
animals, similar and related to the
Arthropods. Tardigrades were first described by J.A.E. Goeze in 1773 ('kleiner Wasserbär'=little waterbear). The name Tardigrada means "slow walker" and had been given by Spallanzani in 1776.
Tardigrades are small animals. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Fresh hatched larvae may be smaller than 0.05 mm.
There are about 750 species described. Tardigrades occur over the whole world, from the high
Himalaya (above 6,000 m) to the deep sea (below 4,000 m) and from the polar regions to the
equator. Some species are able to dry out (cryptobiosis) and resist extreme conditions (
dryness,
temperatures above 350 K and below 30 K,
radiation). All
species need to be in liquid water for active life. Some species are marine, other live in
freshwater environments.
The most convenient place to find tardigrades is roof
mosses, where they live in the water films around lichens and mosses. Other environments are dunes, beaches, soil and marine or freshwater sediments, where they may occur quite frequent (up to 25,000 animals/l).
Scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice and in ocean sediments. Many species can be found in a milder environment like lakes, ponds and meadows, while others can be found in stone walls and roofs.
It is possible to spend some time in mushroom-smelling forests, scrutinizing mosses on rotten trunks in a search for water bears until coming to the conclusion that they can be found virtually everywhere.
Most tardigrades are phytophage or bacteriophage, but some are predators (e.g. Milnesium tardigradum).
Fossil records are sparse. The species
Beorn leggi has been recorded from Canadian
Cretaceous amber.
Aysheaia from the middle Cambrian
Burgess shale might be related to tardigrades.
Recent
DNA and
RNA sequencing data indicate that Tardigrades are the closest sister group to the
Arthropods, even closer than
Onychophora. These groups have been traditionally thought of as close relatives of the
Annelids, but newer schemes consider them
Ecdysozoa, together with the
roundworms and several smaller phyla. The ecdysozoa-concept resolves the problem of the nematode-like pharynx as well as some data from 18S-
rRNA and
HOX (
homeobox) gene data, which indicate a relation to roundworms.
Tardigrade characteristics
- body with four segments and head
- ventral nervous system with one ganglion per segment
- multilobed brain
- triradiate muscular sucking pharynx
- stylets
- cuticle with chitin, molting
- males and females present (though some species are parthenogenetic)
- four pair of legs (in genus Hexapodibius reduced to three) without joints
- feet with claws or toes
- oviparous
See also
References
TSN: 155166
