Tardigrades are small, aquatic invertebrates that are ubiquitous in all marine, freshwater, and limnoterrestrial habitats on earth. They are categorized into two main classes, the unarmored eutardigrades, and the armored heterotardigrades and the number of documented species is ca. 1300 and counting (1). Whereas some tardigrade species are cosmopolitan, others have endemic distributions (2, 3). Tardigrades are often called the “toughest animals on earth” due to their abilities to survive extreme environmental conditions well beyond the limits of other animals (4). In response to desiccation and other types of rapidly deteriorating environments, many freshwater and limnoterrestrial tardigrade species are able to enter a volume reduced, ametabolic state known as a “tun'’ exemplified by the near complete loss of all intracellular water and a number of molecular modifications to protect cellular components in this state (5, 6).
The ability to rear tardigrades under laboratory conditions enables replicable experiments to study various aspects of their fascinating biology (7). There is however, no unified protocol for culturing all tardigrade taxa, and all available protocols proposed to date concern eutardigrades only (3, 8). The published methods differ depending on the specific environmental requirements of different species (9–11). Furthermore, many culturing protocols have been proposed by different authors for different tardigrade species or even different methods for the same species (2, 12–15). These methods differ primarily in the type of food they use, walking substrate, photoperiod, and ambient temperature (8, 16). Aside from having a suitable, oxygenated water source, the most important requirement for successfully culturing tardigrades appears to be their food source (2, 17). However, ecological studies concerning their dietary habits and other behaviors are rare (3, 18). Current studies have indicated that carnivorous tardigrades mostly feed on nematodes, rotifers, other small invertebrates, and even other tardigrades, while a herbivorous diet consists of algae or bryophyte leaf cells (1, 19). Some tardigrade species are omnivorous, and others detritivores (3). Studying tardigrade feeding behavior furthermore allows us to understand their distributions and roles in the food web, e.g. prey-predator interactions (17).
Von Wenck performed the first attempt of culturing tardigrades in 1914 by keeping them in an aquarium (20). His study provided useful information about their mating behavior and embryology. Some of the other early studies during the 19th century used algae and agar for providing food and a walking substrate in glass petri dishes, distilled, tap, or demineralized water as a medium, and Chlorella, Chlorococcum, blue-green algae, nematodes, or diatoms as their food (21–25). More recent studies contain more detailed culturing protocols for eutardigrades and provide information about their life cycles. Kagoshima et al used 1.8% agar, 1x Bold Modified Basal Freshwater Nutrient Media (BMBFN), and 5 µg mL–1 cholesterol with distilled water in petri dishes for culturing Acutuncus antarcticus. Cyanobacteria and green algae isolated from the moss they were found in were used as their food. Tsujimoto et al cultured A. antarcticus in individual wells of tissue culture plates coated with 1.5% agar gel on the bottom using Chlorella sp. algae as a food source, a 2 mm layer of Volvic water, and keeping the cultures at 15°C in the dark (26). For culturing Ramazzottius varieornatus, 2% Bacto agar (Difco) gel, Volvic mineral water, and C. vulgaris algae were used. Animals were transferred to new petri dishes every week, and eggs and juveniles were examined to study life-history traits (27). Tumanov cultured Hypsibius pallidoides for a redescription study. He used a mixture of distilled and filtered tap water as a medium. Instead of agar gel, the petri dishes were scratched with sandpaper to provide a suitable substrate for tardigrade locomotion. They were fed with unicellular Chlorella sp. algae and kept at 16°C in the lab (28). For culturing Paramacrobiotus sp., Suma et al used 2% agarose with KCM solution as a medium. Cultures were kept in the dark at 20°C and animals were fed with C. elegans (29). Paramacrobiotus tardigrades were also cultured with different protocols in Sugiura et al study by using a 1.2% agar gel for coating the bottom of 30 or 90 mm petri dishes. Lecane inermis rotifers and C. vulgaris algae were used as their food source and mineral water as a medium (30).
Culturing heterotardigrades has proved more challenging, and as far as we are aware, there is no published culturing protocol available for them despite decades of culturing attempts. This is likely because they are more dependent on the specific conditions of their microhabitats to survive, thrive and reproduce. The limnoterrestrial heterotardigrades studied here are generally found in bryophytes or lichens growing in drier and sunnier habitats (Pienaar, pers. obs.). Whereas all of the existing tardigrade culturing protocols are wet cultures with a continuous film of water, and animals are always active in these protocols (8, 19, 31), limnoterrestrial heterotardigrades appear to spend a lot of their time in a dried state. Studying heterotardigrades diet preferences will also help to identify suitable food sources for them under laboratory conditions. For example, Schill et al. identified the moss Grimmiaceae in the gut of Echiniscus granulatus (31). Some other studies recorded moss chlorophyll organelles as the Echiniscidae family’s main source of food; however, both algae and fungi could be part of their diet (19). Here, based on observations of natural feeding habits and wet / dry conditions, and making use of what has been learned from the eutardigrade culturing literature, we aimed to experimentally determine a viable culturing protocol for limnoterrestrail tardigrades.