TARDIGRADE KEY

Author: Gary Grothman

Thulyphoribius

Genus description from Camarda et al. 2025: “Peribuccal lamellae or papulae absent, although in PCM, sometimes, internal septa in the buccal ring are visible giving the appearance of the presence of lamellae or papular lamellae. Buccal cone with 6 papular peribuccal lobes visible only under SEM and only when the buccal cone is fully extended. […]

Cucumibius

Cucumibius habitus

Genus description from Tumanov et al. 2024 (supplement): “Body surface with numerous tubercles formed by protrusions of procuticle and covered by epicuticle with intracuticular pillars. Peribuccal structures consisted of six peribuccal lobes. AISM in the shape of crests, asymmetrical with respect to the frontal plane. The dorsal apophysis consists of the following parts: (i) a relatively massive […]

Chilibius

Genus description (of Hypsibius (Isohypsibius) sculptus, with minor edits) from Ramazzotti 1962: “Lunghezza circa 200 µm. La cuticola è scolpita e presenta una granulazione estremamente fine, diffusa ovunque, anche sulla superficie ventrale. Il tubo boccale è stretto ed il suo diametro esterno è di circa 2.5 µm in un Tardigrado di 200 µm. Sempre per […]

Aerobius

Aerobius habitus

Genus description abridged from Mapalo et al. 2004 (based on only extant specimen): “Body length is ~100 μm. The cuticle appears to be smooth with no observable protuberances. Cuticular folds are observed on the dorsal side, mostly likely due to its preservation in a shriveled state. Eyespots were not observed. A faint oval-shaped outline can […]

Parahypsibius

Genus description from Gąsiorek et al. 2024: “Hypsibiids with Ramazzottius-like claws and rigid buccal tube bent posteriorly. Only posterior bars may be present. AISMs asymmetrical, dorsal AISM shorter and higher than ventral, with prominent apex and less evident caudal processes. Pharynx with two granular macroplacoids. Rudimentary elliptical sensory organs present on head. Cuticular sculpturing well […]

Dipodarctus

Genus description from Jørgensen et al. 2014: “Dipodarctinae with lanceolate tibia and triangular tarsus of the Tanarctus-type. Digits on the first 3 pairs of legs (I-III) are of different shape than those of fourth leg pair. Three ventral digits are of equal length, claws with calcar and one long dorsal digit with a simple claw. […]

Dipodarctinae

Dipodarctinae from Jørgensen et al. 2014: “Halechiniscids with primary clavae much longer than, and ventral to, the lateral cirri; indistinct secondary clavae present as kidney- or sausage-shaped elevations between internal and external cephalic cirri. Feet on legs I-III with at least three (digits I-III) of the four digits short and of equal length; digit IV […]

Hypsibius

Genus description from Gąsiorek et al. 2018: “Six weakly outlined peribuccal lobes present. Apophyses for the insertion of stylet muscles in the shape of symmetrical hooks; and with well-developed caudal processes pointing diagonally (backwards and sideways […]). Pharyngeal apophyses and placoids present. Stylet furcae with the triangular base, thin arms and enlarged apices (sensu Pilato […]

Claw Symmetry Relative To Median Plane Of Leg?

Image from Bingemer J, Hohberg K. 2017. An illustrated identification key to the eutardigrade species (Tardigrada, Eutardigrada) presently known from European soils. Soil Organisms. 89 (3): 127-149.

Stylet Support Insertion Point As Percentage Of Buccal Tube Length

Stylet support insertion point = ss divided by Buccal tube length, as %. Note anterior measurements begin at anterior margin of stylet sheaths, ss is centred where stylet supports reach buccal tube (this requires a good dorsal or lateral view for proper measurement)

Image from Tumanov DV. 2006. Five new species of the genus Milnesium (Tardigrada, Eutardigrada, Milnesiidae). Zootaxa. 1122: 1-23.

Buccal Tube Long, Mouth At Anterior Of A Protrusible Snout

Images from Pilato G, Binda MG. 2010. Definition of families, subfamilies, genera, and subgenera of the Eutardigrada, and keys to their identification. Zootaxa. 2404: 1-54.

Pharyngeal Tube With Spiral Reinforcement

Images from Pilato G, Binda MG. 2010. Definition of families, subfamilies, genera, and subgenera of the Eutardigrada, and keys to their identification. Zootaxa. 2404: 1-54.

Peribuccal papillae?

Images from Pilato G, Binda MG. 2010. Definition of families, subfamilies, genera, and subgenera of the Eutardigrada, and keys to their identification. Zootaxa. 2404: 1-54.

(Lateral) cephalic papillae, peribuccal papillae, Milnesiidae - type claws, exceptionally wide (and usually short) buccal tube?

Cephalic image modified from Nelson DR, Guidetti R, Rebecchi L. 2009. Tardigrada. Ch. 14 in Ecology and Classification of North American Freshwater Invertebrates, Thorp JH, Covich AP (eds.), 3rd ed.

Claw image modified from Pilato G, Binda MG. 2010. Definition of families, subfamilies, genera, and subgenera of the Eutardigrada, and keys to their identification. Zootaxa. 2404: 1-54.

Pharyngeal tube flexible?

Note: in a tardigrade with snout extended, the flex of the pharyngeal tube may not be obvious.

Images from Pilato G, Binda MG. 2010. Definition of families, subfamilies, genera, and subgenera of the Eutardigrada, and keys to their identification. Zootaxa. 2404: 1-54.

Serrated / dentate cuff / collar on legs IV

Left image from Kristensen RM. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. pp. 261-335 in Bertolani R (ed). Biology of Tardigrades: Selected symposia and monographs.

Right Image from Richters F. 1926. Tardigrada. in Krumbach T. 1927. Handbuch der Zoologie, 3rd band, Walter de Gruyter & Co.

Leg sensory structures

Typically papilla on leg IV (se4 in image) and variable shape on leg I

Image from Kristensen RM. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. pp. 261-335 in Bertolani R (ed). Biology of Tardigrades: Selected symposia and monographs.

Trunk appendages

Appendages (after Cirrus A) labeled according to plate … B (Scapular plate), C (First segmental plate), D (Second segmental plate), E (terminal plate). “B” is lateral, “Bd” is dorsal

Be careful! There can be great variability within a population, with individuals (especially juveniles) lacking some appendages. When in doubt, focus on other characters first!

Left image from Ramazzotti G, Maucci W. 1983. Il phylum Tardigrada(III edizione riveduta e aggiornata). English translation by C. W. Beasley, 1995. Memorie dell’ Istituto Italiano di Idrobiologia 41: 1-1012.

Right image cropped from Kristensen RM. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. pp. 261-335 in Bertolani R (ed). Biology of Tardigrades: Selected symposia and monographs.

Cirrus A form

Images modified from Kristensen RM. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. pp. 261-335 in Bertolani R (ed). Biology of Tardigrades: Selected symposia and monographs.

Lower image (Mopsechiniscus) modified from du Bois-Reymond Marcus E. 1944. Sobre tardigrados brasileiros. Communicaciones Zoologicas del Museo de Historia Natural de Montevideo. 1(13): 1-19 plus plates.

Terminal / caudal plate with two notches?

Images modified from Kristensen RM. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. pp. 261-335 in Bertolani R (ed). Biology of Tardigrades: Selected symposia and monographs.

First image (Echiniscus) modified from Richters F. 1926. Tardigrada. in Krumbach T. 1927. Handbuch der Zoologie, 3rd band, Walter de Gruyter & Co.

Inner & outer buccal cirri present

Image modified from Nelson DR, Guidetti R, Rebecchi L. 2009. Tardigrada. Ch. 14 in Ecology and Classification of North American Freshwater Invertebrates, Thorp JH, Covich AP (eds.), 3rd ed.

Pseudosegmental plate between last medial plate & terminal plate?

Note on 2nd image, what you see before the terminal plate is either no plate, or a full-width medial plate; don’t confuse the latter with a pseudosegmental plate!
Also, sometimes the pseudosegmental plate is paired, like segmental plates II & III. It’s still pseudosegmental!

Images from Clifford HF. 1991. Aquatic invertebrates of Alberta. University of Alberta Press, Alberta, Canada.

Segmental plates larger than intersegmental?

Image modified from Lindahl & Balser (1999), http://www.iwu.edu/~tardisdp/Keypage35.html

Any Paired Plates?

Modified from Lindahl K, Balser S.  1999. Key to tardigrade genera [Internet]. Available from: https://sun.iwu.edu/~tardisdp/Keypage33.html