Family ARCHECHINISCIDAE Binda, 1978

Archechiniscus sp. male, habitus

© Copyright Dr S. Claxton, Camden, NSW

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Introduction

Archechiniscids are medium-sized unplated arthrotardigrades (133–198 mm) with or without a vestigial median cirrus; all other cephalic cirri are reduced or without cirrophore. Primary clavae are ovoid and the secondary clavae are lens-shaped with a van der Land's body in the middle. The leg spines are absent; only a small papilla on leg IV may be present. Two black eyes are present. The stumpy legs can only telescope slightly. The two internal toes have claws inserted via a metatarsus, the two external claws inserting directly on the foot. The claws are compound with accessory hooks. A lunular structure (peduncle) is present at the base of the external claw. The stylets and placoids are of the Echiniscoides-type (Echiniscoidea) without stylet supports. In the females of the three known species of Archechiniscus, the seminal receptacles vary from long tubular structure (A. minutus, A. marci) to small pouches around the female rosette-shaped gonopore (A. pacifici).

The systematic position of the Archechiniscidae has been unstable, because they lack the median cirrus, the key character for Echiniscoidea, but otherwise they have many arthrotardigrade-like characters. They have been included in the terrestrial Echiniscoidea (Oreellidae) or in the Arthrotardigrada (Halechiniscidae — see Guidetti & Bertolani 2005). We believe that separate family status is warranted, although Archechiniscidae definitely share apomorphic character states with the tidal family Echiniscoididae. Nonetheless, the arrangement adopted by Guidetti & Bertolani (2005) for Florarctus, with assignment to a subfamily of Halechiniscidae is followed here.

Australian archechiniscids are found in two quite different habitats. The common species is found associated with barnacles together with the echiniscoid tardigrade Echiniscoides sigismundi. The type locality for the type species Archechiniscus marci was the Pacific coast of Central America. The type material no longer exists, but it was collected from barnacles, and Echiniscoides was found in the same sample. The Australian form from barnacles has all sensory structures reduced, including the length of cirri A and E, as in the type species. We have therefore assigned the most common tidal tardigrades on Australian barnacles to A. marci. In the Coral Sea and at Heron Island, a species of Archechiniscus is found subtidally in coralline sand. This species has long cirri A and E and fits the description of A. pacifici from New Caledonia. Unfortunately, Renaud-Mornant (1967) synonymised her own species with A. marci in her otherwise carefully described A. pacifici. A closely related third species is A. minutus from the Ionian Sea. This species is also found in coralligenous debris from water 20-35 m deep.

Ecological Descriptors

Marine.

General References

Renaud-Mornant, J. 1967. Tardigrades de la Baie Saint-Vincent, Nouvelle-Calédonie. Expédition Française sur les récifs coralliens de la Nouvelle-Calédonie 2: 103-118

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