Family HYPSIBIIDAE Pilato, 1969

Hypsibiidae, habitus, pharyngeal apparatus, leg

© Copyright Dr S. Claxton, Camden, NSW

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Introduction

Hypsibiids are distinguished by claws which are asymmetric with respect to the median plane of the leg and the two claws are always unequal in size and different in form and dimension.The family is characterised by great diversity of claw structure and buccal apparatus structure. Many species are small and are often rarer than macrobiotids, although members of the genus Diphascon appear to be more common in soil or soily habitats and also at higher altitudes. Six of the 17 genera have a flexible section in the buccal tube and of these, the genus most commonly occurring in the Australian fauna is Diphascon.

The family is divided into subfamilies on the basis of the buccal apparatus — the buccal tube is rigid (with or without a ventral strengthening bar) in the Hypsibiinae, and has a flexible section in the Itaquasconinae and the Diphasconinae. The aquisition of a flexible section has occurred several times in the evolution of the Hypsibiidae as it has in Echiniscidae.

The Hypsibiinae comprise 11 genera: Hypsibius, Isohypsibius, Doryphoribius, Microhypsibius, Ramazzottius, Ramajendas, Pseudobiotus, Halobiotus, Thulinia, Eremobiotus and Mixibius. Representatives of the first seven of these have been found in Australia. Species of the genera Hypsibius and Isohypsibius have a simple buccal tube which does not have a ventral support. In Hypsibius, the apophyses for muscle insertion are in the form of semilunar hooks while in Isohypsibius, they form a crest. Eggs are laid in the exuvium. Doryphoribius, however has the ventral support and claws which are of Isohypsibius type. A number of species of this genus occur in Australia including the New Zealand species Doryphoribius zyxiglobus. The species Microhypsibius japonicus has now been found in moss at New England National Park, New South Wales, and it has only otherwise been found in Japan. Members of this genus have a thin, rigid buccal tube which does not have a ventral support. The claws tend to be relatively small and rather similar in size on each leg and the two branches of each claw are always inserted onto a common basal part. The secondary branch of each external claw forms a continuous arc from the basal segment as in Hypsibius. Although it has not before been cited in the Australian fauna, the genus Ramazzottius, or rather its type species R. oberhaeuseri, has been found in almost every other country. This species with its characteristic free-laid, ornamented eggs has been found at many localities in Australia, often associated with Milnesium tardigradum in habitats subject to considerable drying as it is in other countries. Another species, not yet identified, has been found in lichens at Lake George, New South Wales. The adults closely resemble those of R. oberhaeuseri but the eggs have processes in the shape of fine spicules, not small hemispheres as in R. oberhaeuseri. Like Ramazzottius, Ramajendas has external claws with a long, thin main branch. The latter genus is rare, so far found only in Antarctica and Subantarctic islands and includes true marine and terrestrial forms. The freshwater species Pseudobiotus augusti, was found by Murray in 1909 in Sydney. The claws of this species and others in the genus are typically long and quite similar in size on each foot. The accessory claw is considerably reduced.

Of the four genera in the Itaquasconinae, Itaquascon, Mesocrista, Platicrista and Parascon, representatives of only the first have so far been found in Australia. The species Itaquascon umbellinae and I. trinacriae found in mosses and lichens in Australia are cosmopolitan. Only two specimens of I. pawlowski have been found, one in mosses from the Blue Mountains and another from New England National Park, New South Wales. This species has only otherwise been found in the highlands of Poland and on Vancouver Island, Canada. This species is considered to be rare but its small size and consequent decreased chance of being found may account for the widely disjunct distribution. This genus is notable for the lack of placoids in the pharynx.

The subfamily Diphasconinae includes Diphascon and Paradiphascon and possibly Hebesuncus (Dastych 1992). This latter taxon comprises two species with bipolar distribution. H. conjungens is known from the Arctic and some high mountains in the Northern and Southern Hemisphere. It has also been found in Australia at high elevations in New South Wales — Mt Kosciusko and New England National Park. This genus differs from the other two genera in this sub-family by having an asymmetrical anterior apophysis on the buccal tube and a very short, thin pharyngeal tube with relatively weak annulation. Both species in this genus lay deposit free, ornamented eggs, of the kind usually found only in the Macrobiotidae or Eohypsibiidae. Dastych (1992) suggested that Paradiphascon (found so far only in South Africa) could be most similar to an hypothetical common ancestor of Diphascon. Dastych (1992) suggested that the posterodorsal apodeme (the ‘drop-like’ structure found between the rigid and the flexible parts of the buccal tube) seems to be a unique synapomorphy of the genera Diphascon and Paradiphascon. In the subgenus Diphascon of the genus Diphascon the apodeme is round and smooth and is completely reduced (representing the most derived state), in the subgenus Adropion. Two Antarctic species (D. puniceum and D. sanae) exhibit an intermediate form of the apodeme and may merit a separate subgenus (Dastych 1992). Both species occur in Australia.

General References

Marley, N.J., McInnes, S.J. & Sands, C.J. 2011. Phylum Tardigrada: A re-evaluation of the Parachela. Zootaxa 2819: 51–64

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