Family HALECHINISCIDAE Thulin, 1928

Representative Halechiniscinidae: A. <i>Halechiniscus</i> sp. (ventral view of female); B. <i>Wingstrandarctus</i> sp. (dorsal view of male); C.<i>Florarctus</i> sp. (ventral view of male); and D. <i>Tanarctus</i> sp. (dorsal view of female)

Representative Halechiniscinidae: A. Halechiniscus sp. (ventral view of female); B. Wingstrandarctus sp. (dorsal view of male); C.Florarctus sp. (ventral view of male); and D. Tanarctus sp. (dorsal view of female)

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Family HALECHINISCIDAE Thulin, 1928

Compiler and date details

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

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

Introduction

Halechiniscids are arthrotardigrades without dorsal plates and with strongly segmental legs which can telescope at all joints. Larvae have two toes and adults have four toes with terminal claws. Seminal receptacles with ducts are always present. Secondary sexually dimorphism can be extensive; the smaller male usually has larger primary clavae than the female.

Halechiniscidae are the most diverse arthrotardigrade family (see Fig. 1A–D). In this checklist, the family is divided into six subfamilies following the cladogram of the Heterotardigrada (Kristensen & Higgins 1984b) and, in part, the cladogram of subfamilies of Halechiniscidae (Pollock 1995), but recognising Orzeliscidae and Archechiniscidae as separate families. Only the deep-sea subfamily, Euclavarctinae, is not recorded from Australian waters. The other five subfamilies, Halechiniscinae, Dipodarctinae, Florarctinae, Tanarctinae, Styraconyxinae, are common and are treated separately.

Halechiniscinids have relatively short club-shaped primary clavae. Buccal clavae (secondary clavae) may be club-shaped (Paradoxipus) or kidney-shaped (Halechiniscus) or reduced to an indistinct, flat organ surrounding the mouth cone (Halechiniscus, Chrysoarctus). Toes with simple claws are retractable into a membranous sheath. There are no toe or tarsal peduncles, but the tarsus may be triangular, followed by a lance-like tibia as in Tanarctinae. Paradoxipus has proximal suction pads and distal claws on all toes. The dorsal cuticle is thinner than the ventral cuticle; ventral plates may be present (Fig. A). Seminal receptacles have lateral vesicles and long convoluted genital ducts. Stylet supports may be lacking in Crysoarctus, but in other genera the stylets, stylet supports and the placoids are strongly calcified. Crysoarctus flabellatus and Paradoxipus orzeliscoides have transparent lateral and caudal alae.

Only the genus Halechiniscus has been recorded from Australia. Halechiniscus remanei has been recorded from intertidal coralline sand, Townsville, Queensland (Renaud-Mornant 1981), and H. perfectus and H. greveni are common in coralline sand or on dead corals overgrown with filamentous green algae on the reef crest at many Australian coral islands. Two new species of Halechiniscus with ventral plates (e.g. Fig. A) are known from the Coral Sea (Marion Reef and Chesterfield Reefs).

Dipodarctinines have, like floractinines and tanarctinines, primary clavae which are longer than the lateral cirri; secondary clavae are present as kidney-shaped sacs between internal and external cirri. The feet have strengthening bars (tarsal peduncles) inside the tarsus. The foot on leg IV of Tanarctus-type has short external toes and internal toes are long and flexible. On the first three pairs of legs the ventral three toes are all the same length and the dorsal one is longer. The stylets are relatively short and stylet supports are present.

Dipodarctininae are the most recently established halechiniscid subfamily (Pollock 1995), but the species D. subterraneus from Bahamas has been known since 1959 under the genus Halechiniscus. The type species, D. borrori, and also D. subterraneus are common in subtidal coralline sand at Heron Island.

Florarctinines have lateral or caudal cuticular expansions, called alae (Fig. C). The alae are formed by the epicuticule including the pillars. In the genus Florarctus the epicuticular alae can be supported by procuticular structures called caesti which are lacking in two other florarctine genera, Wingstrandarctus (Fig. B) and Ligiarctus. Both Wingstrandarctus and Florarctus can have symbiotic bacteria in 2-3 cephalic vesicles. All florarctines have two hook-shaped cuticular structures in the foot, called tarsal peduncles. Cephalic cirri are 3-segmented and very long. The primary clavae are longer than the lateral cirri, and the male may have extremely long primary clavae. The flat secondary clavae are located ventrally and are fused to form an H around the mouth in Florarctus. The fourth leg papilla is s-shaped. The seminal receptacles each have a very large vesicle located laterally and the duct is s-shaped, opening close to the female gonopore.

The Australian florarctine fauna is very rich, with 11 species of Florarctus and three species of Wingstrandarctus found in Queensland and the Coral Sea. The genus Wingstrandarctus was described from One Tree Island in fine coralline sand, but the type species W. corallinus, as well as W. crypticus and a new congeneric species have been found at Heron Island. The most common tardigrades in coralline sands of the Great Barrier Reef are all species of Florarctus; species already recorded from New Caledonia such as F. antiliensis, F. cinctus, F. salvati, F. heimi and F. cervinus have now been found at the Great Barrier Reef. Florarctus heimi and F. cervinus are the only Florarctus species with the caudal ala carrying four long filaments. Florarctus cervinus is only half the size of F. heimi, but has longer primary clavae. The caudal ala is also different and therefore they were described as two species. At Heron island these two types of florarctines were found together. Only F. cervinus had testis and only F. heimi had an ovary and seminal receptacles. Furthermore, the male of F.cervinus had the same type of caudal filaments as juveniles. One juvenile female was found in moult and the old juvenile filaments were of F. cervinus type, but the new cuticle carried the F. heimi type of caudal filaments. In fact, F. cervinus is the juvenile or the male of F. heimi. This is first time that such strong secondary sexual dimorphism has been demonstrated in tardigrades. In the male of a new species of Florarctus from the Coral Sea, the primary clavae are twice as long as the body, and in the female the clavae are the usual Florarctus size.

Tanarctinines are common in the deep-sea in the South Pacific, but there is only a single record from intertidal coralline sand from Marion Reef (Coral Sea) where a new species of Tanarctus with extreme long smooth primary clavae and leg IV sense organs has been found (Fig. D). The species has the same club-shaped secondary clavae as the type species T. tauricus from Bahamas.

Styraconyxinines are halechiniscids with two or four peduncles on four toes. Heart-shaped proximal pads may be present at the base of internal toes, if peduncles are lacking. Three to four hooks are present on each crescent-shaped claw, sometimes secondarily reduced to only one or two hooks. The complete set of cephalic cirri and primary clavae are present, but the secondary clavae are usually reduced or lacking. Cirrus A and Cirrus E are similar in shape. The cuticle is variable — it may have long lateral pillars (Raiarctus) or a double loose cuticle (Tetrakentron, Tholoarctus). Seminal receptacles are very complex and vary a great deal within a genus. Real placoids are lacking in Tetrakentron, Tholoarctus and Lepoarctus.

In Australia, members of the type genus Styraconyx are very common in both tidal and subtidal zones. Kristensen & Higgins (1984a) provided a map of over 10 Australian records of two species, S. craticulus and S. kristenseni. Styraconyx craticulus can be found in tidal algae, and the species can be dispersed easily. It may be found on a new buoy after only few weeks. Styraconyx kristenseni comprises two subspecies S. k. kristenseni and S. k. neocaledoniensis. The nominate form is common in coral sand along the whole eastern coast of Australia. Several undescribed species of Styraconyx are found in shell gravel (North Stradbroke Island) and Amphioxus sand (Heron Island). Raiarctus is only found subtidally in Australia. Even though this genus contains some of the smallest tardigrades (85–137 mm), its members are very easily recognised by the long lateral pillars. Raiarctus aureolatus is a rare species in coralline sand at Heron Island.

Tholoarctus natans is a very aberrant semi-benthic tardigrade with a bell-shaped cuticle, especially well developed in the male. The Australian form is very similar to the type material from Roscoff, France.