Considerations on the claws of the Apochela and a novel detail of the bucco-pharyngeal apparatus of the genus Milnesium (Tardigrada: Apochela: Milnesiidae)

Abstract

This article clarifies two morphological problems, the first regarding all Apochela Schuster, Nelson, Grigarick and Christenberry 1980, and the second the genus Milnesium Doyère, 1840. Among Eutardigrada Richters, 1926, Apochela have four claws per leg, while Parachela Schuster, Nelson, Grigarick and Christenberry 1980 have two double claws per leg. Until now, the four claws per leg of Apochela have been denominated as if they had derived from two parachelan-like double claws undergoing branch separation. However, morphological and molecular data indicate the relative phylogenetic distance of Apochela from Parachela as sister groups; the ancestor of Eutardigrada was probably similar to a primitive heterotardigrade in having four separate claws per leg, which organised and differentiated in couples but without fusion in Apochela, while they fused to form proper double claws only in Parachela. If this hypothesis is correct (Apochela would have retained four separate claws per leg as plesiomorphic character), the denomination of apochelan claws must be adapted to reflect it. The second problem regards the valvular system present between buccal tube and pharyngeal bulb in the genus Milnesium. Until now, only one valvular system type was known, attributed to Milnesium tardigradum (today sensu lato), or to Milnesium cf. tardigradum. We reveal a second type recognisable in many species. The two types are described, discussing their possible correlation with life stage, sex, some morphological and morphometric traits, mounting medium, geographic distribution and currently available phylogenetic trees; the valvular system type is probably species-specific and we attribute to most species of the genus the correct type. Our conclusions are preliminary, but, in any case, the present study indicates that the exact morphology of the discussed portion of the bucco-pharyngeal apparatus in Milnesium is still not well known.

Keywords

• Milnesiidae
• claws
• bucco-pharyngeal apparatus
• valvular system
• morphology