Grevenius granulifer ( Thulin, 1928 )

Species: Grevenius granulifer ( Thulin, 1928) View in CoL .

( Figs. 1 View Fig , 2 View Fig , 3 View Fig , 4 View Fig , 5 View Fig and 6 View Fig , Table 1 with measurements of adults).

Adults. Body large and stocky, clearly bent dorsoventrally ( Fig. 1A View Fig ). Cuticular sculpturing well-developed dorsally throughout dorsum, extending to lateral sides and legs IV; legs I–III with weak wrinkling, mainly in the centromedian portion ( Fig. 1A View Fig ). Ventral cuticle smooth, not sculptured, only with some wrinkling ( Fig. 1B View Fig ). Muscle attachment points (MAPs) visible dorsolaterally on trunk ( Fig. 2A, E View Fig ). Cephalic region with a different type of sculpturing comprising deep wrinkling ( Fig. 2B View Fig ), smoothly passing at the level of legs I into the reticulum covering the rest of dorsum. Reticulum with thickened edges of polygons ( Fig. 2A, C, G View Fig ); corners of polygons in the form of larger tubercles/granules, which sometimes may be more evident than reticulum itself ( Fig. 2D–F View Fig ) .

Bucco-pharyngeal apparatus of the Isohypsibius type ( Fig. 3 View Fig ). OCA visible in light microscope as black dots ( Fig. 3 View Fig , insert); in fact, it comprises two bands: the first containing three to five rows of small conical teeth and the second composed of one row of tear-shaped teeth ( Fig. 4A–D View Fig ). AISMs of the Isohypsibius type ( Figs. 3E View Fig and 4E View Fig ). Buccal tube narrow and terminated with pharyngeal apophyses. Pharynx with three macroplacoids, of which the first and the second are roughly of the same length, and the third is longer ( Fig. 3 View Fig ). Macroplacoids are connected by a thin cuticular list, and usually malformed, with rugged margins ( Fig. 4F–H View Fig ). Especially the terminal portion of the third macroplacoid can be separated from the remaining macroplacoid by a deep groove, causing an impression that a microplacoid is present ( Fig. 4H View Fig ). Peribuccal lobes absent, a continuous peribuccal lamina present ( Fig. 4A, B View Fig ).

Claws of the Pseudobiotus type, with secondary and primary branches similar in height ( br 0.81–1.00, 0.93 on average; this agrees with previous data published in Gąsiorek et al. (2019): br 0.83–0.99, 0.91 on average). Elongated (chaliceshaped) basal tracts and prominent humps on primary branches of internal and anterior claws present ( Fig. 5 View Fig ). Accessory points closely adjacent to primary branches. Pseudolunulae present ( Fig. 5C, E View Fig ), but often weakly developed and not observable in light microscope. Short and faint cuticular bars present under internal claws I–III ( Fig. 5B–D View Fig ), yet usually difficult to identify. Bars are subcuticular since not visible in SEM ( Fig. 5A View Fig ).

Hatchlings. 111–177 μm long, 144 μm on average ( N = 20). Body elongated, not arched. The cuticular sculpturing in the form of delicate, rugose wrinkling ( Fig. 1C View Fig ), which is uniform throughout the dorsum. At first glance, the cuticle may even seem to be smooth. The difference in sculpturing between adults and juveniles has already been noted ( Ramazzotti & Maucci, 1983).

Exuviae. Containing 23–50 oval to roundish eggs; 34 on average ( N = 15). Eggs are densely packed in exuvia ( Fig. 6A View Fig ) and sometimes fall out through the anterior opening made by an individual laying eggs and shedding cuticle. Chorion smooth.

Remarks. Many largest adults, which probably can reach even up to 600 μm (not measured due to unsuitable position on slides), seemed to be emaciated after egg deposition and Grevenius granulifer ( Thulin, 1928) revised: a fresh look at one of the most intensively studied… 35 likely die soon after the last ecdysis. This is supported by analogous behaviour in another freshwater tardigrade, Pseudobiotus megalonyx (pers. observ.). However, it is also possible that animals were exhausted by massive infection of a parasitic fungus from the genus Ballocephala ( Fig. 6B, C View Fig ), as unicellular assimilative hyphae developed in many recently deceased specimens. Ballocephala is a kickxellomycotin previously reported for eutardigrades dwelling in leaf litter and sheep dung ( Drechsler, 1951; Richardson, 1970).