title: Cyclomorphosis in Tardigrada: adaptation to environmental constraints.
abstract: Tardigrades exhibit a remarkable resilience against environmental extremes. In the present study, we investigate mechanisms of survival and physiological adaptations associated with sub-zero temperatures and severe osmotic stress in two commonly found cyclomorphic stages of the marine eutardigrade Halobiotus crispae. Our results show that only animals in the so-called pseudosimplex 1 stage are freeze tolerant. In pseudosimplex 1, as well as active-stage animals kept at a salinity of 20 ppt, ice formation proceeds rapidly at a crystallization temperature of around -20 degrees C, revealing extensive supercooling in both stages, while excluding the presence of physiologically relevant ice-nucleating agents. Experiments on osmotic stress tolerance show that the active stage tolerates the largest range of salinities. Changes in body volume and hemolymph osmolality of active-stage specimens (350-500 microm) were measured following salinity transfers from 20 ppt. Hemolymph osmolality at 20 ppt was approximately 950 mOsm kg(-1). Exposure to hypo-osmotic stress in 2 and 10 ppt caused (1) rapid swelling followed by a regulatory volume decrease, with body volume reaching control levels after 48 h and (2) decrease in hemolymph osmolality followed by a stabilization at significantly lower osmolalities. Exposure to hyperosmotic stress in 40 ppt caused (1) rapid volume reduction, followed by a regulatory increase, but with a new steady-state after 24 h below control values and (2) significant increase in hemolymph osmolality. At any investigated external salinity, active-stage H. crispae hyper-regulate, indicating a high water turnover and excretion of dilute urine. This is likely a general feature of eutardigrades.
authors.affiliation: Department of Biology, University of Copenhagen, August Krogh Building, Universitetsparken 13, DK-2100 Copenhagen Ø, Denmark.
authors.forename: Kenneth Agerlin
authors.initials: KA
authors.lastname: Halberg
authors.forename: Dennis
authors.initials: D
authors.lastname: Persson
authors.forename: Hans
authors.initials: H
authors.lastname: Ramløv
authors.forename: Peter
authors.initials: P
authors.lastname: Westh
authors.forename: Reinhardt Møbjerg
authors.initials: RM
authors.lastname: Kristensen
authors.forename: Nadja
authors.initials: N
authors.lastname: Møbjerg
mesh_terms: D000064:Acclimatization; D000818:Animals; D007448:Invertebrates / Q000502:physiology* / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D009994:Osmolar Concentration; D009997:Osmotic Pressure; D013696:Temperature; D014882:Water-Electrolyte Balance