The ultrastructure of the tardigrade cuticle with special attention to marine species

The cuticle of the marine Arthrotardigrada Parastygarctus sterreri, Actinar ctus doryphorus, Actinarctus cf. physophorus, Raiarctus aureolatus, Tholoar ctus natans, Orzeliscus belopus, and Batillipes mirus as well as the marine Eutardigrada Halobiotus crispae was studied by scanning (SEM) and transmis sion electron microscopy (TEM). The body cuticle of all Arthrotardigrada st udied here consisted of an amorphous basal cuticle, an inner, membrane-like trilaminate structure, pillars, and a membrane-like outer epicuticle. The dorsal plates of P. sterreri were formed by the basal cuticle which consist ed of alternating layers of electron-dense and lucent amorphous material; e picuticular pillars were missing dorsally. In H. crispae the dorsal amorpho us basal cuticle was composed of three sublayers whereas a single layer occ urred ventrally. Wheat germ agglutinin (WGA) coupled with colloidal gold wa s used for TEM localization of chitin in the body cuticle of A. physophorus and T. natans as well as in the body and pharyngeal cuticle of H. crispae. The basal layers of the body cuticle of all three species and the foregut cuticle of the eutardigrade were labelled intensely. Control experiments co nfirmed that binding of the WGA-complex was with chitin. Previous studies r evealed that the thickening of the plates of terrestrial heterotardigrades (Echiniscidae) occured in the epicuticle. In comparison with our findings o n the arthrotardigrade P. sterreri, it was concluded that the plates of the Arthrotardigrada and of the Echiniscoidea are analogous structures, which evolved independently twice inside the taxon Heterotardigrada. The morpholo gy of the cuticle was compared within the Tardigrada in order to hypothesiz e characteristics in the ground pattern of this taxon and its subgroups. Po ssibly, cuticular pillars evolved several times independently within the Ta rdigrada. Alternatively, a cuticle with a honeycomb layer and pillars might be plesiomorphic, and the "simple" cuticle predominant in eutardigrades mi ght represent the apomorphic condition.