Analysis of Encystment, Excystment, and Cyst Structure in Freshwater Eutardigrade Thulinius ruffoi (Tardigrada, Isohypsibioidea: Doryphoribiidae)

Abstract

Encystment in tardigrades is relatively poorly understood. It is seen as an adaptive strategy evolved to withstand unfavorable environmental conditions. This process is an example of the epigenetic, phenotypic plasticity which is closely linked to the molting process. Thulinius ruffoi is a freshwater eutardigrade and a representative of one of the biggest eutardigrade orders. This species is able to form cysts. The ovoid-shaped cysts of this species are known from nature, but cysts may also be obtained under laboratory conditions. During encystment, the animals undergo profound morphological changes that result in cyst formation. The animals surround their bodies with cuticles that isolate them from the environment. These cuticles form a cuticular capsule (cyst wall) which is composed of three cuticles. Each cuticle is morphologically distinct. The cuticles that form the cuticular capsule are increasingly simplified. During encystment, only one, unmodified and possibly functional buccal-pharyngeal apparatus was found to be formed. Apart from the feeding apparatus, the encysted specimens also possess a set of claws, and their body is covered with its own cuticle. As a consequence, the encysted animals are fully adapted to the active life after leaving the cyst capsule.

The tardigrade cuticle

“…to protect themselves from unfavourable environmental conditions, such as fluctuations in temperature, pH, oxygen level or osmotic stress. Encystment and cyclomorphosis may also be triggered by unknown endogenous factors (Møbjerg et al 2007;Halberg et al 2013b;Guidetti and Møbjerg 2018;Janelt and Poprawa 2020). In general, encystment is thought to be an optional response to environmental challenges where animals remain immobile inside their cysts (Węglarska 1957;Szymańska 1995;Clausen et al 2014), whereas cyclomorphosis seems to be an obligate part of seasonal changes with animals remaining partially mobile (Kristensen 1982b;Møbjerg et al 2007).…”

“…stage by ejecting the sclerified parts of the buccopharyngeal apparatus (Szymańska 1995;Guidetti et al 2008;Janelt and Poprawa 2020). The newly serially synthesized cuticles (one to three, depending on the species) are, however, retained as protective coats (Węglarska 1957;Guidetti et al 2006).…”

“…The newly serially synthesized cuticles (one to three, depending on the species) are, however, retained as protective coats (Węglarska 1957;Guidetti et al 2006). These cuticles are not attached to the epidermis but are interconnected by a network of cuticular appendages and/or cuticular fibres of varying thickness (Szymańska 1995;Janelt and Poprawa 2020). The newly formed buccopharyngeal apparatus and claws might be structurally different from the previous ones (Guidetti and Møbjerg 2018).…”

Final Note of Special Issue “Tardigrades Taxonomy, Biology and Ecology”

“…Overall, five papers were published, providing novel insights into this fascinating group of invertebrate animals. Janelt and Poprawa [14] analyzed the poorly studied phenomenon of encystment in the freshwater eutardigrade, Thulinius ruffoi (Bertolani [15]). Authors described in detail a process of cyst formation, as well as the morphology of cysts and animals inside the cyst.…”

“…Cuticle layers are increasingly simplified (from outside to inside). Tardigrades inside the cyst are covered with a cuticle that develops like nonencysted specimens of this species and possesses claws and produces only one functional buccal-pharyngeal apparatus [14].…”