Class HETEROTARDIGRADA

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Class HETEROTARDIGRADA

Compiler and date details

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

Introduction

The class Heterotardigrada consists of two orders: Arthrotardigrada and Echiniscoidea. The Arthrotardigrada are, with the exception of the homothermic spring species, Styraconyx hallasi, all marine. The Echiniscoidea are marine, limnic and terrestrial, but the order is not found in the deep-sea. The autapomorphies for the class are gonopore separate, anus closed and three lobed, and Malpighian tubes absent. The complex shape of seminal receptacles has been used recently as one of the best systematic characters in Arthrotardigrada (Kristensen & Higgins 1984a; Pollock 1995). Seminal receptacles are lacking in all terrestrial Echiniscoidea except for species in the genus Oreella.

Heterotardigrades have cephalic appendages of two types — cirri (mechanoreceptors) and clavae (chemoreceptors). The plesiomorphic condition is to have one median cirrus and three lateral sets of both cephalic cirri and clavae, but the external part of the median cirrus is always reduced or totally lacking in the order Echiniscoidea, and usually one or two pairs of clavae are lacking in both Arthrotardigrada and Echiniscoidea. The scape of trunk (cirri B-E) and leg (sense organs I-IV) appendages has a high systematic value in all heterotardigrades.

The cuticle varies enormously within the Heterotardigrada. The plesiomorphic condition in this class seems to be the presence of dorsal segmental plates. This is found in the exclusively eumarine arthrotardigrade families, Stygarctidae, Renaudarctidae and Neostygarctidae, and in the echiniscoid family, Echiniscidae. A very thin cuticle without dorsal plates seems to be the apomorphic condition developed by convergence both in arthrotardigrade families, (Halechniscidae, Orzeliscidae, Batillipedidae, Coronarctidae, Archechiniscidae), and in echiniscoid families (Echiniscoididae, Oreellidae, Carphanidae). Having ventral plates may be apomorphic in Renaudarctidae and two Australian species of the genus Halechiniscus and in several genera of Echiniscidae (Bryochoerus/Bryodelphax, Testechiniscus and Antechiniscus).

The pillars in the heterotardigrade epicuticle have a high systematic value in distinguishing species and genera of all Arthrotardigrada, e.g. long pillars occur in the genera Arctinarctus, Rhomboarctus, Raiarctus and Opydorscusand short pillars in nearly all other arthrotardigrades. Epicuticular structures such as alae (wing-shaped stabilisator organs) are found in Australian coralline tardigrades, Wingstrandarctus and Florarctus; in the semi-benthic genus Tholoarctus the outer epicuticle forms a swimming-bell.

Among Heterotardigrada, the character most commonly used in the description of genera and families is the shape of the foot. The presence of toes with claws or sucking discs is considered to be an autapomorphy developed only in the arthrotardigrade genera, while a foot with four single claws inserted directly on the tarsus is considered to be a plesiomorphic character found in both dorsal-armoured Arthrotardigrada (Stygarctidae) and Echiniscoidea (Echiniscidae). The basic pattern for the number of claws is two for the first instar larva and four for the adult. Neotenic species with only two claws in the adult are known among Arthrotardigrada (e.g. Parastygarctus biungulatus). In the Arthrotardigrada, superdigitation is present only in the family Batillipedidae, the adults of which have six toes with sucking discs. In the tidal species of Echiniscoidea, the numbers of claws can vary from three/four (Anisonyches) to twelve/thirteen (Echiniscoides). Usually the fourth legs have one fewer claw than the first three pairs. The very aberrant limnic genus Carphania has only one claw on the fourth legs and two on the first three pairs of legs.

The bucco-pharyngeal apparatus is a highly complex structure which only recently has been used in construction of a cladogram for the terrestrial family Echiniscidae (Kristensen 1987). This is surprising, because this structure is use as a key character in the Eutardigrada (see later). Morphology of the stylets, stylets supports and the placoids vary at the generic and family levels, especially in the Arthrotardigrada and could be used as key characters for defining the evolutionary trends within all heterotardigrade families. Calcium carbonate structures will disappear after a few days in specimens mounted in glycerol or Hoyers medium so many of these structures can only be seen in live or well-fixed specimens. For this reason they have not been used as key characters until now.

Marcus (1927) proposed two suborders of Heterotardigrada, Arthrotardigrada and Echiniscoidea, which are now generally accepted as orders. The key diagnostic character is median cirrus, present in Arthrotardigrada and absent in Echiniscoidea. Unfortunately, a reduced median cirrus was recently found in Anisonyches (Echiniscoididae, Echiniscoidea) while Archechiniscus (Archechiniscidae, Arthrotardigrada) lacks the median cirrus. These two families share several apomorphic features in the bucco-pharyngeal apparatus and sensory organs (black eyes, shape of clavae/IV leg papilla). The deep sea family Coronarctidae also shares some apomorphies with the Echiniscoididae. In the cladogram of the Heterotardigrada, Kristensen & Higgins (1984b) had questioned the systematic position of the three families Coronarctidae, Archechiniscidae and Echiniscoididae. Both Kristensen & Higgins (1984b) and Renaud-Mornant (1987b) suggested that a third order (suborder) be erected for these three marine families, but they also stated that it was necessary to totally revise the taxonomic criteria used to separated related families of all Heterotardigrada. Here we therefore use the more conservative classification of heterotardigrade families and orders.