First record of cysts in the tidal tardigrade Echiniscoides sigismundi

Abstract

Tardigrades are microscopic metazoans that withstand environmental extremes by entering dormant states, such as cryptobiosis (latent life). In addition, they may also form cysts. Here, we present the first report of cyst formation in a marine heterotardigrade, i.e., Echiniscoides sigismundi, which constitutes a cryptic species complex present worldwide in tidal zones. The cysts were initially discovered during experimental series constructed to investigate osmotic stress tolerance. The animals, which eventually formed cysts, showed signs of an imminent molt at the beginning of experimentation. We use the term ''cyst'' for stages, where a total of three or more cuticles have been synthesized. Our observations show that encystment in E. sigismundi involves synthesizing of at least two new cuticle layers. Legs with discharged claws are present in connection with the first outer cuticle, as well as the second cuticular layer. In the most developed cyst, a third cuticle lacking claws seems to surround the animal, which is delineated by a fourth cuticle. Many features are shared with the well-studied cysts of eutardigrades. The cysts of E. sigismundi, however, lack pigmentation and have an extra set of claws, and the animal inside retains buccopharyngeal sclerified parts, until discharging the third cuticle. The finding of cysts in a marine heterotardigrade is novel and confirms that encystment also occurs within this major evolutionary lineage.

Tolerance to Gamma Radiation in the Marine Heterotardigrade, Echiniscoides sigismundi

“…E . sigismundi is a marine species adapted to tidal habitats with recurrent exposure to dry conditions on barnacle shells, and with a recently documented ability to tolerate complete desiccation (i.e., anhydrobiosis) [21,22]. In contrast to limno-terrestrial tardigrades it can survive desiccation with or without contracting into a so-called “tun” [22].…”

Data from new taxa inferIsoechiniscoidesgen. nov. and increase the phylogenetic and evolutionary understanding of echiniscoidid tardigrades (Echiniscoidea: Tardigrada)

“…The genus represents the most dominant tardigrades in intertidal habitats, but has also been recorded at 80 m depth (D'Addabbo Gallo et al ., ). Species within Echiniscoides are highly versatile cryptobionts as they may survive freezing, dehydration and extreme osmotic stress (Møbjerg et al ., ; Clausen et al ., ; Hygum et al ., ). Furthermore, Echiniscoides can produce cysts with several layers of cuticle (Clausen et al ., ), a feature better known from the terrestrial eutardigrades (Guidetti et al ., ).…”

“…Species within Echiniscoides are highly versatile cryptobionts as they may survive freezing, dehydration and extreme osmotic stress (Møbjerg et al ., ; Clausen et al ., ; Hygum et al ., ). Furthermore, Echiniscoides can produce cysts with several layers of cuticle (Clausen et al ., ), a feature better known from the terrestrial eutardigrades (Guidetti et al ., ). Interestingly, the interstitial forms in the higginsi ‐group, with isonych claws (identical number of claws on all legs) and pillars in the epicuticle, are considerably different morphologically from other members of the genus.…”

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

“…Dormancy in tardigrades [6] and some other groups of invertebrates such as rotifers [27,28], nematodes [29], bryozoans [30], insects [31], or crustaceans [32] can be seen as an adaptive strategy to survive in unfavorable conditions. Encystment (a form of diapause) in tardigrades is still poorly understood despite the fact that an ability to form cysts was found among several representatives of Heterotardigrada and Eutardigrada classes [14][15][16][17][18][19][20][21][22][23][24][33][34][35][36][37][38][39][40][41][42][43]. Murray [38] also mentioned cysts of the genus Milnesium that are classified into the class Apotardigrada.…”