Current status of the tardigrada: evolution and ecology

The Tardigrada are bilaterally symmetrical micrometazoans withfour pairs of lobopod legs terminating in claws or sucking disks.They occupy a diversity of niches in marine, freshwater, andterrestrial environments throughout the world. Some have a cosmopolitandistribution, while others are endemic. About 900 species havebeen described thus far, but many more species are expectedas additional habitats are investigated. Most are less than1 mm in body length and are opaque or translucent, exhibitingcolors such as brown, green, orange, yellow, red, or pink inthe cuticle and/or gut. Marine species are more variable inbody shape and overall appearance and generally exhibit lowpopulation density with high species diversity. Reproductivemodes include sexual reproduction and parthenogenesis, but muchremains to be known about development. Tardigrades have a hemocoel-typeof fluid-filled body cavity, a complete digestive tract, anda lobed dorsal brain with a ventral nerve cord with fused ganglia.Recent molecular analyses and additional morphological studiesof the nervous system have confirmed the phylogenetic positionof tardigrades as a sister group of the arthropods. The abilityof tardigrades to undergo cryptobiosis has long intrigued scientists.Although tardigrades are active only when surrounded by a filmof water, they can enter latent states in response to desiccation(anhydrobiosis), temperature (cryobiosis), low oxygen (anoxybiosis),and salinity changes (osmobiosis). Cryptobiotic states aid indispersal.