Family ECHINISCIDAE Thulin, 1928

Echiniscidae: <i>Echiniscus</i> sp., female [scale bar =20 µ]

Echiniscidae: Echiniscus sp., female [scale bar =20 µ]

Echiniscidae: <i>Bryochoerus</i> sp., female [scale bar =20 µ]

Echiniscidae: Bryochoerus sp., female [scale bar =20 µ]

Echiniscidae: <i>Bryodelphax</i> sp., female [scale bar =20 µ]

Echiniscidae: Bryodelphax sp., female [scale bar =20 µ]

Echiniscidae: <i>Hypechiniscus</i> sp., female [scale bar = 20 µ]

Echiniscidae: Hypechiniscus sp., female [scale bar = 20 µ]

Echiniscidae: <i>Cornechiniscus</i> sp. [scale bar = 20 µ]

Echiniscidae: Cornechiniscus sp. [scale bar = 20 µ]

Echiniscidae: <i>Pseudechiniscus</i> sp., female [scale bar = 20 µ]

Echiniscidae: Pseudechiniscus sp., female [scale bar = 20 µ]

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Family ECHINISCIDAE Thulin, 1928

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

Echiniscids are terrestrial tardigrades characterised by the presence of thick dorsal, sclerotised cuticular plates. Adults have four claws on each leg and only the tarsus with claws can be retracted. The claw morphology is very conservative. Seminal receptacles are absent. Fourteen genera are known and they are diagnosed on the basis of plate metamery and buccal apparatus (Kristensen 1987). Representatives so far found in Australia belong to the genera Echiniscus, Bryochoerus, Bryodelphax, Hypechiniscus, Cornechiniscus, Mopsechiniscus, Pseudechiniscus and Antechiniscus. Echiniscidae are the most successful family of all tardigrades when judged by the number of species, but the two genera, Echiniscus and Pseudechiniscus, alone comprise about 85 per cent of the family (Kristensen 1987).

Echiniscidae have three or four fused head segments (segment A) and four trunk segments (segments B-E). Serially arranged cephalic appendages include paired internal and external buccal cirri (absent in Mopsechiniscus), paired primary and secondary clavae and a long flagellate cirrus, cirrus A, situated at the posterior margin of the first segment. The segmental plates of the trunk are designated by Roman numerals (I-IV) as are the legs. Leg IV always has a small chemoreceptive papilla. Leg I (and rarely legs II and III) may have a small spine or papilla. Median (intersegmental) plates are numbered from anterior to posterior (1-3) and may be divided by transverse lines into two or three parts. Lateral spines may occur at the post-lateral angles of the segmental plates and are designated by letters B-E. Dorsal spines are identified by the segment letter and superscript ‘d’. Cornechiniscus, Mopsechiniscus, Pseudechiniscus and Antechiniscus have additional segments (pseudosegmental plates) between segments III and IV. In most echiniscids, the mouth opening is terminal, except in Cornechiniscus and Mopsechiniscus in which it is located ventrally. In these two genera (and in Novechiniscus and Proechiniscus), the buccal tube has a flexible element in the posterior part of the buccal tube similar to that developed in several eutardigrade genera. A posterior flexible element is also present in a newly discovered species of Antechiniscus from Tasmania. Kristensen (1987) utilised the structure of the buccal apparatus to characterise genera of Echiniscidae for the first time and to reconstruct phylogenetic relationships in the Heterotardigrada.

Most echiniscids live in mosses, lichens and (rarely) soil. Specimens of E. vinculus were found in sand and leaf litter on Bribie Island, Queensland. Although species of the genus Echiniscus are known to favour dry environments, at least one species, E. tessellatus, has been collected only from rainforest sites in Australia. Echiniscus is the most successful terrestrial tardigrade genus (Kristensen 1987) and was considered to be totally parthenogenetic until 1987 when males were found in some species. A study by Claxton (1996) has shown that bisexual species are common in Australian Echiniscus from xeric environments, a situation not found elsewhere in the world. As in all heterotardigrades, gonopore structure differs in females and males. The male has an oval gonopore while in the female, the gonopore is surrounded by a six-leaved rosette of cells. In the bisexual Echiniscus species found in Australia, males are shorter and have longer clavae than females. Males of three species have significantly longer claws than females, and in two species, males have significantly longer lateral trunk appendages and the dorsal trunk appendage combinations differ from those of females. Common, relatively large males have also been found in three species of Echiniscus from the Australian Antarctic.

The genera Bryochoerus and Bryodelphax are extremely similar and should be synonymised. They differ only in the state of the median plate 3 which is divided in the former and undivided in the latter. The appearance of this plate, however, may be altered according to fixation technique or to the degree of compression in the mounted specimen. While there is some doubt as to the validity of the genus Bryochoerus, specimens obtained from the type locality (Eumundi, Queensland) have, along with a divided median plate 3, small, lateral intersegmental plates which are absent in the species of Bryodelphax found in Australia. Both genera have rigid, calcium carbonate encrusted stylet supports. The species of Bryodelphax found in Australia has ventral plates like those of B. sinensis.

Hypechiniscus has indistinct dorsal cuticular plates and the large head plate has a suture which nearly divides the plate into five pieces. Two cosmopolitan species, H. gladiator and H. exarmatus, have been found in Australia. Hypechiniscus gladiator has a single trunk appendage located middorsally between median plate 2 and segmental plate III. The claws are only slightly curved and the secondary spurs on the internal claws are large and bent from the main branch. The pharyngeal apparatus is aberrant (and similar to that of Tetrakenton) with a very small pharyngeal bulb and reduced, calcium carbonate encrusted placoids. There are no stylet supports but the buccal tube is thickened just outside the bulb.

Zoogeographically, the genus Cornechiniscus is considered to be a Palaearctic element (Kristensen 1987). However, a single specimen (a cuticle containing three eggs) has been collected from moss on rock at Jenolan Caves, New South Wales. The specimen has a distinctive horn-shaped cirrus A which differentiates this genus from other genera of the family. Cirrus A has a distinct cirrophore with a ball and socket joining the rigid flagellum which has a double cuticular wall (Kristensen 1987).

Mopsechiniscus is associated with Nothofagus in South America and Tasmania (Dastych & Moscal 1992) and has also been found in the South Georgia Islands. Mopsechiniscus has apomorphic features such as reduced head appendages (except cephalic papillae), and is close to the Pseudechiniscus line.

Pseudechiniscus is, after Echiniscus, the most successful genus in the family Echiniscidae in Australia as elsewhere in the world. The cosmopolitan species, P. suillus has been found in mosses and lichens from both xeric and rainforest biotopes. However, there is considerable variation among descriptions of this species which suggests that they may, in fact, represent a group of closely related species (Dastych 1984). Pseudechiniscus novaezeelandiae is less widely distributed and appears restricted to higher altitudes in Australia. In both of these species, males and females of about the same size have been found.

Although males of Antechiniscus lateromamillatus from South America are about half the size of females, males of the three species found in Australia are only about 15 percent shorter than females. Sexual dimorphism is limited; the shape and length of clavae are different in the sexes of all three species (longer in males). In specimens of the New Zealand species, Antechiniscus parvisentus, found in the New England National Park, New South Wales, males have a long filament at D where females have a short spine. Males were not noted in the original description of this species. Antechiniscids have two unpaired pseudosegmental plates behind each of the paired segmental plates and also, anterior to the caudal plate, a small bar-shaped pseudosegmental plate which is slightly overlapped by a large undivided median plate. There is also a series of small plates on the ventral surface of Antechiniscus pulcher, and Antechiniscus sp. (undescribed) from Tasmania which have not been seen in any other species of this Gondwanan genus, but which are present on some species within four other echiniscid genera and in the arthrotardigrade genus Renaudarctus. The genus is apparently common in Nothofagus rain forest in South America and New Zealand. In Australia, it is associated with Nothofagus in the New England National Park and Tasmania but not on Mount Kosciuszko.