Also known as water-bears, these extremely tiny animals belong to a group sharing features of both arthropods and worms. Their bizarre adaptations to survive adverse environmental conditions make them longevity record-holders among invertebrates, with some species reaching 100 years of age. Te Papa's collection of New Zealand water-bears is the largest and most comprehensive in the world.
There are more than 750 described species of tardigrades, ranging in length from 100 to 500 millionths of a metre, and a few may reach 1200 millionths of a metre long. Their bear-like appearance - legs with claws, and slow side-to-side lumbering gait - give them the common name of water-bears.
After these aquatic invertebrates were discovered in a drop of freshwater more than 220 years ago, they have been found in a vast array of habitats: in the sea, in freshwater, and on the land. However, regardless of habitat, all water-bears are aquatic, because all require a film of water surrounding the body to be active.
Occurring from the Arctic to the Antarctic, these invertebrates live in habitats such as the deep ocean, ocean beach sands, soil, leaf litter, mosses, lichens, liverworts, algae, and vascular plants.
Water-bears share similarities to both arthropods and worms, and their scientific classification into a separate phylum (that is, a taxonomic category above class and below kingdom) remains debatable. At different times, they have been placed within the Infusoria, the Annelida, and the Arthropoda (Nelson, 1991).
Water-bears have a two-sided symmetrical body and four pairs of legs ending in claws or paddles. Generally convex on their upper side and flattened on their under side, the body is indistinctly divided into a head segment, three trunk segments - each bearing a pair of legs - and a hind segment with the fourth pair of legs projecting backward.
Their feeding apparatus consists of a mouth tube, a muscular pharynx, and a pair of calcareous mouthparts that are well adapted for puncturing the walls of plant cells, or the bodies of other small animals, and sucking out the contents. Many water-bears’ eggs have characteristic surface pores and conical projections.
Many species have the ability to withstand environmental extremes. When conditions are not favourable water-bears dehydrate and form a cyst or tun (shaped like a wine cask). This process is known as anhydrobiosis and enables water-bears to survive the periodic drying that occurs naturally in their environment.
These tuns are also a means for their dispersal, as they form part of the atmospheric ‘dust’ that blows with the wind. Thus, through an alternation of active and anhydrobiotic periods, a tardigrade may have a lifespan from less that a year to more than 100 years.
The collection of New Zealand water-bears held in Te Papa is the largest and most comprehensive in the world. It comprises about 2500 specimens, including fifteen primary types - the original specimens on which published descriptions are based - all mounted on micro-slides. These specimens represent most of the seventy-eight species known from New Zealand (Horning et al, 1978).
References
Horning, D S, Schuster, R O, and Grigarick, A A. 1978. Tardigrada of New Zealand. New Zealand Journal of Zoology 5: pp185-280.
Nelson, D R. 1991. 'Tardigrada.' in Thorp, J H, and Covich, A P, eds. Ecology and classification of North American freshwater invertebrates. New York: Academic Press. pp501-521.