DESCRIPTION OF THE PHYLUM TARDIGRADA (SPALLANZANI 1777)

EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> PROTOSTOMATA> ECDYSOZOA> PANARTHROPODA> TARDIGRADA

Tardigrada (tar-di-GRA-da) is formed from two Latin roots that mean “slow step” (slow – tardus; and step -gradus). The reference is to the very slow and deliberate movements of the eight stubby legs. Lazaro Spallanzani (1777) coined the name for the “slow walkers” when he was making observations on the abilities of tardigrades and rotifers to dehydrate and recover.

INTRODUCTION TO THE TARDIGRADA The tardigrades are distinctive microscopic animals that range in size from 50 to 1200 µm long (Nelson 2002). They are longer than wide and have an obvious head followed by 4 serial pairs of stubby lobopod-like legs, each one of which terminates in claws or a sucker disk. The body is covered by a cuticle, which is organized into scaly plates that hint at some degree of segmentation (Figure 1). When mature, tardigrades have a highly reduced coelom with the organs being bathed by fluids in the haemocoel. Waterbears have a complete digestive tract with a terminal anus. The terrestrial and freshwater taxa feed on moss and algae by means of a pair of stylets which pull cell sap into the gut with a pharyngial pump (Figure 2). However, tardigrades may be more carnivorous than generally thought. Esser (1990) reports observing Hypsibius feeding on the soil nematode, Enchodelus. Romano (2003) also lists “protozoa, rotifers, larvae, and other small invertebrates” as potential prey, which would be the only possibilities for soil and deep ocean taxa. Tardigrades are nearly ubiquitous being found in marine, freshwater and terrestrial environments (Nelson 2002 and Romano 2003). However active animals must be immersed in water; so, even terrestrial tardigrades must be in environments that hold a thin film of water. Thus, the “terrestrial” forms tend to be found associated with mosses, liverworts, and leaf litter. To accommodate fluctuations in water availability, most “terrestrial” forms can enter into a kind of suspended animation in which they dehydrate and remain viable [a situation called anhydrobiosis, Figure 3 from Schill et al. (2004)]. Though they are not able to survive cryptobiosis for centuries, Bertolani et al. (2004) maintained adult tardigrades in a cryptobiotic state for 4 years with standard atmosphere (oxygen exposure), and they had a high recovery rate. Tardigrades are even able to survive exposure to the vacuum of space in low earth orbit while in a cryptobiotic state (Jönsson et al. 2008). Their abilities to survive cryptobiotically have allowed them to exploit all manner of extreme environments from abyssal ocean regions to mountain tops, the poles and hot springs (Kinchin 1994, Nelson 2003, and Romano 2002). Though there is strong support for the Panarthropoda as a clade within the Ecdysozoa (e.g. Patel et al. 1989, Garey 2001, and Gabriel and Goldstein 2007), the position of the tardigrades remains questionable. Lartillot and Phillippe (2008) and Dunn et al. (2008) show that tardigrades may have greater affinities with the nematodes than they do with the arthropods. Budd (2001) supposes that the tardigrades are sisters to the Arthropods (a zygotaxon that he calls Tactopoda) and interprets Cambrian arthropod fossils according to similarities with tardigrades. Conclusions for the tardigrades, if they are sisters to the arthropods, are: small size is derived, and the anomalocarids (Figure 4), a top predator of the Burgess Shale community and animals generally assumed to be stem arthropods, were in the line leading to tardigrades and arthropods. Within the tardigrades, the classical classification systems have two major groups, which are regarded as classes: Eutardigrada and Heterotardigrada. The Eutardigrada (true tardigrades) mainly are terrestrial and freshwater taxa. Heterotardigrada (the other tardigrades) have lateral spines or bristles (Figure 5). Although the work of Jorgensen and Kristensen (2004) is equivocal, Blaxter et al. (2004) find the monophyly of the heterotardigrada is questionable and likely paraphyletic. Nelson (2002) adds a third class, the Mesotardigrada, which includes a genus from a hot spring. See Figure 6 for the relative positions of the classes of Tardigrada and the other members of the Panarthropoda.

FIGURE 1. SEM micrographs of the ventral views of Hypsibius. Cephalization and four pairs of stubby lobopods that terminate in claws are obvious features. Image from http://serc.carleton.edu/images/microbelife/topics/special_collections/tardigrade.jpg	FIGURE 2. A DIC micrograph of the head of Hypsibius showing the retraced stylets and the muscular buccal pump. Image from http://www.baertierchen.de/mai2005/p3243885.jpg	FIGURE 3. SEM micrographs of Milnesium. A. the active animal. B. The animal in a cryptobiotic state. Images from Schill et al. (2004)	FIGURE 4. An illustration of Anamalocaris, one of the top predators of the Burgess Shale era Cambrian seas. Image by Nobu Tamura, Wikimedia Commons	FIGURE 5. A micrograph of Echiniscus, a member of the Heterotardigrada, showing the characteristic lateral spines or bristles. Image from http://www.baertierchen.de/jan2002/etotal3b.jpg

FIGURE 6. The three classes of tardigrades (taxa in shaded box) and their relationships with phyla in the Panarthropoda (clade PA). P = Protostomata E = Ecdysozoa S = Scalidophora N = Nematoida PA = Panarthropoda

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By Jack R. Holt and Carlos A. Iudica. Last revised: 02/03/2014