Class EUTARDIGRADA

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Class EUTARDIGRADA

Compiler and date details

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

Introduction

The class Eutardigrada consists of two orders: Parachela and Apochela. The Parachela are a diverse order of almost totally terrestrial/ limnic species. The order Apochela consists of the single family Milnesiidae which contains only terrestrial species.

Eutardigrades do not have cephalic or trunk cirri. The lateral cephalic papillae in the eumarine genus Halobiotus and in the family Milnesiidae may be homologous with the clavae in heterotardigrades. Usually eutardigrades have double claws (or diploclaws) which consist of a primary and a secondary branch. The animals are generally elongate and caterpillar-shaped, indistinctly segmented and lack dorsal plates. The cuticle may be smooth or textured with reticulate, granular or large excrescences including spines, although these structures are of little value in discriminating genera. The presence or absence of pores or pillars in the cuticle, or granulation, particularly around the claws is, however, species specific.

A pair of branched claws is normally present on each leg. The secondary branch is attached to the leg and the primary branch arises from the secondary branch (except in Milnesiidae). The sequence of claw branches with respect to the median plane of the leg is, in some genera, alternate (2-1-2-1 or secondary-primary-secondary-primary) while in other genera the two primary branches are adjacent to each other (2-1-1-2). Lunulae may occur at the base of the claws and may be ornamented with teeth. Accessory claws occur near the distal ends of primary branches and their size and shape are important in species determination. Both lunulae and accessory claws have often been neglected in older descriptions.

In eutardigrades, the most utilised characters in the determination of genera and families are the form of the claw and the type of bucco-pharyngeal apparatus (Pilato & Binda 1990). Pilato (1969) constructed a phylogenetic arrangement based primarily on claw structures, proposing a subdivision into four families. He based his arrangement on the observation that the structure of the eutardigrade claw has been much less modified than that of the bucco-pharyngeal apparatus during the course of evolution. Schuster et al. (1980) based their classification on a highly complex structure — the buccal apparatus. They took into consideration peribuccal structures (previously poorly utilised) such as lamellae, lobes and papulae and proposed the erection of three new genera Pseudobiotus, Dactylobiotus and Minibiotus. They also proposed the two orders of Eutardigrada which have been generally accepted, however, their arrangements have now been superceded (see Marley et al. 2011 and Degma et al. 2013).