Phylum TARDIGRADA

Semidiagrammatic drawing of the hypothetical ancestral tardigrade (female) showing general features

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Phylum TARDIGRADA

Water Bears

Compiler and date details

September 2013 - Introductions, Dr S. Claxton, Camden, NSW & Dr Reinhardt Kristensen, University of Copenhagen, Denmark

February 2011 - checklist compiled by Jo Wood, South Australian Museum, Adelaide

Introduction

DEFINITION AND GENERAL DESCRIPTION

Tardigrades, or water bears, are aquatic animals, being active only when surrounded by water. These microscopic metazoans are found in a diversity of extreme habitats, ranging from the deep sea, at depths to 6000 m, to the highest elevations in the Himalayas, and from hot springs to the ice cathedrals inside the ice cap of Greenland. Tardigrades are amongst the smallest Metazoa with most of the 718 described species ranging in length from 0.1 to 0.5 mm. The adults of the smallest marine species are only 0.85 mm long but some terrestrial forms may reach 1.2 mm.

The body is bilaterally symmetrical and the generally cylindrical shape may be extremely dorsolaterally flattened in some species. The majority of tardigrade species are white to translucent but some terrestrial species are strongly coloured — yellow, orange, green, red to olivine-black. There are five distinct body segments including a cephalic segment and four trunk segments each bearing a pair of segmented legs. Marine forms may have telescopic legs with up to 13 claws or four to six toes with complex claws or distal adhesive discs. In terrestrial and limnic forms, the segmentation of the stumpy legs is reduced and they bear two or four claws which may be missing in some soil inhabiting forms. Although tardigrades have no circulatory or respiratory structures, other systems, such as the digestive system, are quite complex.

The phylum comprises three classes: Heterotardigrada, Eutardigrada and Mesotardigrada. Heterotardigrada, consisting of two orders, Arthrotardigrada and Echiniscoidea, have cephalic appendages. The arthrotardigrades are marine forms which usually have a median cirrus and telescopic legs with or without digits, while the echiniscoids are terrestrial armoured forms and marine unarmoured forms. The echiniscoids have no median cirrus and the legs are non-digitate. In Eutardigrada, which are primarily terrestrial and comprise the two orders Parachela and Apochela, cephalic appendages (which are not homologous to those of the heterotardigrades) are absent in the former but present in the latter. The cuticle of eutardigrades is not armoured and the legs are non-digitate. The class Mesotardigrada was established on the basis of the description of a single species found in a hot spring in Japan (Rahm 1937). The type material does not exist and the species description has been criticised.

Of the nine marine families, seven are found in Australia, with 13 genera and 31 species having been collected. Most of the collections have been made on the east coast of Australia and in the Coral Sea. There are no collections from the Australian deep sea.

Fourteen families with 43 genera and133 species have so far been recorded in Australia. However, further genera and species have been collected. Collections have been made from Lord Howe, Macquarie and Heard Islands. Tardigrades from the Australian Antarctic Territories have also been studied.

HISTORY OF DISCOVERY

The Tardigrada have been known and studied since 1773 when Goeze first described ‘kleinen Wasserbaren’ (Goeze 1773, cited in Ramazzotti & Maucci 1983). In 1776, Spallanzani named the group ‘Il Tardigrado’, which means ‘the slow walkers’ after the way most terrestrial species moved. Spallanzani was also the first to discover that terrestrial tardigrades have cryptobiotic stages. Cryptobiosis (latent life) is a stage in which all metabolic activity is suspended for a period of time (Wright, Westh & Ramløv 1992).

Knowledge of these minute arthropod-like animals has advanced rapidly in recent years, largely because of the availability of improved microscopical and extraction techniques. The use of fresh water shocking of large samples of marine sediment has resulted in the discovery of totally new forms of Arthrotardigrada (Kristensen & Higgins 1984a, b). During the last 20 years the number of genera has more than doubled (from 37 to 94) and many of the new genera and three new families are marine. The fact that 42 genera of marine tardigrades include only 101 species, demonstrates the wide morphological diversity of the group and indicates its very ancient origin. Recently, a more than half billion year old stemgroup tardigrade was found in a Siberian Cambrian limestone (Muller, Walossek & Zakharov 1995). The Siberian fossil has some clearly arthrotardigrade characters as well as some unique characters not seen in recent tardigrades.

Australian tardigrades are poorly known with only 133 species recorded in the literature as of 2013. There are no data at all on the freshwater tardigrades and the first papers on marine tardigrades from the east coast have only recently been published. Richters (1908) was the first to record an Australian tardigrade, as Macrobiotus hufelandi which was thought, until recently, to be a cosmopolitan species. Murray (1910) reported 31 species of terrestrial tardigrades collected during the British Antarctic (Shackelton) Expedition in 1907-1909, from three sites on the east coast. Murray’s material has not survived but his descriptions of six new species and a new genus, Oreella, remain. The species of Oreella, O. mollis, which Murray described from material collected at Leura in the Blue Mountains west of Sydney is the only one of his new species so far not recollected. A species of Echiniscus described but not named by Murray has been recollected at Mt Kosciuszko and given the name Echiniscus jamesi (Claxton 1996).

In the sixty-six years following Murray’s paper, only a single reference was made to tardigrades found in Australia — Colledge (1921) reported finding Milnesium tardigradum in a tuft of grass. Pilato & D’Urso (1976) recorded ten species, seven of which had not previously been recorded from Australia and two of which were new to science. The type material for these two species, Macrobiotus santoroi and Macrobiotus australis, is deposited in the Istituto Policattedra di Biologia Animale dell’Universita, Catania, as is that for Doryphoribius macrodon described by Binda, Pilato & Dastych (1980) and Macrobiotus joannae described by Pilato & Binda (1983). Echiniscoides sigismundi polynesiensis, a marine species, was recorded from Victoria by Kristensen & Hallas (1980), and Renaud-Mornant (1981) mentioned a beach tardigrade from Queensland. Kristensen (1984) described a new genus (Wingstrandarctus) of marine tardigrade from specimens found off the east coast of Australia and also from Florida Keys, USA. The type material for the new species Wingstrandarctus corallinus is preserved in the Zoological Museum, Copenhagen. Kristensen & Higgins (1984a) revised the marine genus Styraconyx using 12 Australian records of two species from coralline sand and intertidal barnacles. Apodibius serventyi, an interesting clawless species, was described by Morgan & Nicholls (1986) and the types are deposited in the Western Australian Museum. New species described by Pilato & Claxton (1988) (Macrobiotus hieronimi and Minibiotus maculartus), Pilato, Claxton & Binda (1989a,b) (Minibiotus fallax, Echiniscus marcusi and Macrobiotus peteri) and Pilato, Claxton & Horning (1991) (Diphascon (Adropion) gordonense) are preserved in the Macleay Museum, University of Sydney, and the new species Echiniscus curiosus, E. jamesi and E. rodnae (Claxton 1996) are preserved in the Australian Museum, Sydney. The type material for Mopsechiniscus tasmanicus is lodged overseas in the Zoologisches Museum, University of Hamburg (Dastych & Moscal 1992).

Recently, studies on the tardigrades of the Australian Antarctic Territories have appeared (Miller & Heatwole 1995; Miller, Heatwole, Pidgeon & Gardiner 1994; and Miller, Miller & Heatwole 1994). Miller, Horning & Dastych (1995) reported two new species, Echiniscus darienae and Ramajendas heatwolei, from Macquarie Island, the types of which are deposited at The Australian National Insect Collection, CSIRO Division of Entomology, Canberra.

CLASSIFICATION

The classification adopted here follows Marley at al. (2011), Guidetti & Bertolani (2011) and the online “Actual checklist of Tardigrade species, Ver. 23: 15-07-2013” of Degma et al. (2013).