Family ECHINISCOIDIDAE Kristensen & Hallas, 1980

Echiniscoididae: Echiniscoides sp. female, habitus

© Copyright Dr S. Claxton, Camden, NSW

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Introduction

Echiniscoidids are halophilous rather than strictly marine animals. They are unplated Echiniscoidea with a ‘eutardigrade’ cuticle that often lacks pillars in the epicuticle. Secondary clavae (papilla cephalica) are dome-shaped or indistinct. Other cephalic appendages and leg appendages are small or reduced. Cirri A are similar to cirri E and the primary clavae are similar to the fourth leg appendages which are papillar (both may be absent). Two black eyes are present. The legs are stumpy and the segmentation of the legs is reduced to folds in the cuticle and to the tarsal segment with the claws which can be withdrawn into the leg. The two stylets are long, each with a large furca and a cuticular stylet sheath reaching half way down to the furca. Carbonate stylet supports are absent. The placoids may be calcium carbonate encrusted or consist only of cuticular bars, as in nearly all terrestrial Echiniscidae. The family consists of two genera, Echiniscoides and Anisonyches which may be paraphyletic.

Members of the genus Anisonyches have worm-like bodies, four claws on legs I-III and three claws on leg IV. Each claw has two asymmetrical basal spurs. The claws of Anisonyches have been compared those of eutardigrades of the order Apochela (Milnesium and Limmenius), which also have asymmetrical basal spurs. The type species A. diakidius was described from coralline sand from the Bahamas. Later the species was found in intertidal zones from the Galapagos Islands, Bermuda and the Mediterranean Sea. Recently, Grimaldi de Zio, D’Addabbo Gallo, Morone de Lucia & D’Addabbo (1987) described a second species, A. mauritianus from intertidal sand from the Indian Ocean. This species has some arthrotardigrade features, e.g. a short median cirrus and seminal receptacles with a long curled tube and a terminal vesicle.

The Australian species of Anisonyches was found at the high tidal zone on barnacles at North Stradbroke Island, Queensland. It closely resembles the type species but has a distinct median cirrus which should be missing in the type species. The seminal receptacles are similar to those of A. mauritianus, but these structures have not been described in the type species.

The genus Echiniscoides was monotypic until recently (Kristensen & Hallas 1980); the type species, E. sigismundi, was believed to be cosmopolitan and was found from 1000 m a.s.l. in the mountains of Zaire to the subtidal zone in the Dead Sea. Today, however, six species and six subspecies of E. sigismundi have been described and several new species are known from Australian waters. All species are unplated but several species have a distinct dorsal sculpture. The tidal nominate form of E. sigismundi has a totally smooth cuticle without pillars in the epicuticle. The subtidal E. higginsi is the only described species with pillars in the epicuticle as in all other heterotardigrades. E. higginsi from the USA is also the only Echiniscoides with relatively few claws on each leg (6-6-6-6) and is the only species other than the subtidal E. bruni (claw formula 10-10-10-10) from the Mediterranean Sea with the same number of claws on all legs. All intertidal species have one claw less on the hind legs. Echiniscoides hoepneri and E. pollocki have 8-8-8-7, while some subspecies of E. sigismundi may have 13-13-13-12, but the nominate form usually has 8-10 claws on the first three legs and 7-9 on the hind legs. Except for the number of claws and the fine structure of the cuticle, the different species look very similar. Subtidal species however, may have slightly longer sensory structures on the head, trunk and legs than species from the supralittoral and high tidal zones. The length of the stylets and the fine structure of the placoids have also been used as diagnostic characters to separate closely related species. Echiniscoides hoepneri is reported to be parasitic on barnacles but E. sigismundi is also associated with different species of barnacle. The Australian subspecies, E. s. polynesiensis, can be found in algae or on barnacles on rocky shores and has been recorded from Melbourne, Sydney, and along the eastern coast of Queensland. The nominate form from Europe has, however, been found on barnacles from Sydney. A new species has been found in supralittoral lichens from Macquarie Island and a subtidal species is common in polychaete tubes (Galeolaria) from Nielsen Park, Sydney.

General References

Møbjerg, N., Jørgensen, A. & Kristensen, R.M. 2019. Ongoing revision of Echiniscoididae (Heterotardigrada: Echiniscoidea), with the description of a new interstitial species and genus with unique anal structures. Zoological Journal of the Linnean Society 188(3): 663–680

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