Tardigrade Ecology

Ecology

Figure 3: “Adaptations in various marine tardigrades. (a) vermiform body shape in the deep water, mud-dwelling Coronarctus tenellus, (b) buoyant bodies in Tanarctus bubulubus, (c, d) cuticular extensions or “alae” in (c) Florarctus and (d) Actinarctus, (e) enlarged Malpighian tubules (dotted lines) in Halobiotus crispae.” [1].

Marine Tardigrades
Composing almost all species in the Heterotardigrada class, including the order Arthrotardigrada and the family Echiniscoididae, marine tardigrades are found in all seas, from intertidal (exposed at low tide) and subtidal (covered at low tide) shores, to manganese nodules, abyssal mud, and deep-sea ooze on ocean floors 5730 m below sea level(Hansen 2003; J. Hansen pers. comm). Marine tardigrades are either interstitial, living between grains of sand in the soil or aquatic sediment, alongside others in the psammon community, or anchor to different substrates, detritus, or other organisms. They are interstitial among coarser sands, often found in intertidal but also subtidal zones, and tend to be epibenthic (on sediment surface) in finer sands and mud due to lower oxygenation. Marine tardigrades in microhabitats that are both intertidal and interstitial are usually found within the first few centimeters of the substrate, but they also live in communities up to 180 cm deep. Additionally, some semibenthic species in the family Halechiniscidae sometimes drift or weakly swim above their substrate in order to settle on a new location(Kristensen and Renaud-Mornant 1983).

In some marine tardigrade species, zonation has been found. In one study, for example, the species composition shifted from littoral (near the shore) to submarine to deep sea caves(Grimaldi de Zio 1984; Grimaldi de Zio and Gallo D’Addabbo 2001). Intertidal tardigrades, like other meiofauna, migrate both horizontally and vertically with the tides(Giere 2009), which dynamically stratifiy beach environments with respect to water saturation and oxygen content. Additionally, competitive interactions between marine tardigrade species have been suggested based on non-overlapping distributions, impacting beach tardigrades(Martinex 1975) and vertical distribution patterns of barnacle-dependent species(Kristensen and Hallas 1980).

Generally smaller, marine tardigrades have telescopic legs. They can have either up to 13 claws or 4 tows with complex claws. Alternatively, interstitial species have 4 to 6 toes with an adhesive round or rod-shaped disk on each that allows them to tightly adhere to shifting sand grains. Tardigrades living in deep-sea muds have cylindrical, wormlike bodies with reduced legs (Fig. 3). Epibenthic species and those on algae often have elongated appendages and claws with multiple hooks. And the eutardigrade Halobioutus crispae possesses enlarged Malpighian tubules attributed to a secondary shift to seawater(Crisp and Kistensen 1983;Mobjerg and Dahl 1996) (Fig. 3). Many marine tardigrades are found the world over, suggesting substantial dispersal capabilities, but the mechanisms for dispersal are poorly understood. Passive dispersal may be utilized. Semibenthic species, like Halechiniscidae semibenthics, have a diversity of structures that facilitate swimming and drifting. Some semibenthic and interstitial species have cuticular extensions that increase surface area and may increase passive dispersion(Grimaldi de Zio 1984; Jorgensen and Kristensen 2001; Giere 2001; Kristensen and Sorensen 2004) (Fig. 3). Tantarctus bubulubus has 18-20 balloon or float-like appendages attached to the fourth pair of legs (Jorgensen and Kristensen 2001) (Fig. 3). However marine species often lack cryptobiotic states, which would seemingly limit passive dispersion. Additionally, tardigrades possess weak swimming ability, calling into question their aptitude for active dispersion in a marine environment. Eggs of one species were found in the exuvium (shed skin) of its host barnacle, suggesting that this is its means of dispersal(Kristensen and Hallas, 1980). They may also spread by the ballast water from or barnacles and algal lawns beneath marine vessels (Giere 2009) as well as various forms of free-floating vegetation and plastic-anchored barnacles(Giere 2009; Arroyo 2006).

Very little is known about trophic interactions in marine species. Tardigrades comprise a minuscule portion of marine meiofauna, and so their specific roles in marine ecosystems have not been studied. However, polychaetes , bivalves, various crustaceans, fish, and birds rely heavily on meiofauna, sometimes depending solely on them at least during some phase of their life cycle(Coull 1990, 1999). Intertidal interstitial tardigrades live alongside meiofauna including nematodes, harpacticoid copepods, and turbellarians in lower relative concentration, as is often but not always the case. 402 individuals of the commensal Echinoscoides sigismundi species have been found on a single barnacle(Kristensen and Hallas 1980). Most marine tardigrades probably feed on algal cells, including macroalgae and diatoms, using their paired piercing stylets and a muscular, sucking pharynx. Others may be detritivores, bacterivores or ectoparasites (on the surface of the host)(Kristensen and Sorensen 2004). Some species are associated with organic slime growing on algae(Giere 2009).
Several commensal relationships have been observed among tardigrades, but they are likely facultative (circumstantial rather than necessary), as the same species are also found living freely in interstitial or algal habitats. Some marine tardigrades live between the plates of barnacles, likely feeding on associated algae. Crevices of barnacle plates provide physical protection and shelter from temperature fluctuations especially when exposed by low tides(Faurby 2012). Ectoparasites include facultative parasitism on the pleopods (limbs) of the isopod Ligmoria lignorum and obligate parasites with specific adaptations, like Echiniscoides hoepneri, which feeds on embryos in barnacles’ brood chamber and Tetrakentron synaptae, which only lives on the tentacles of the sea cucumber Leptosunapa galliennei. Most species of Floractus and Eingstrandarctus genus have epicuticular vesicles associated with the buccal apparatus that house symbiotic bacteria, which, in their clean coral sand microhabitats, may provide their primary food source(Kristensen 1984). One final interspecific interaction has been seen in the epibenthic Tanarctus bubulubus, the balloon-appendaged tardigrade from earlier, whose entire dorsal side is covered in mucous, apparently providing adhesion for shed calcerous platelets which form the spherical shells of coccolithophores, perhaps providing chemical or mechanical camouflage(Jørgensen and Kristensen 2001).

Freshwater Tardigrades
Freshwater tardigrades, composed almost entirely of the class Eutardigrada, are not well studied and make up the smallest group of described tardigrades. Freshwater (limnic) habitats are divided into three categories, lotic habitats, defined by flowing water and including rivers, streams, and springs, lentic habitats, defined by standing water and including lakes, ponds, wetlands, and temporary or ephemeral ponds, and subterranean habitats. The best lentic microhabitats are in benthic sediments with sufficient particle size and oxygenation. However, other interesting environments have been found to house freshwater tardigrades including activated sludge ponds in sewage treatment plants where they, along with other microorganisms, break down dissolved organic matter and cryoconite holes, formed when solar radiation is absorbed by the accumulation of dark particles on the surface of ice, resulting in sporadic melting, in which they, alongside rotifers, feed on entrapped microflora.(Kaczmarek 2015). Little is known about population distribution and dynamics of freshwater tardigrades due to lack of replicate samples and patchiness. However, invertebrate communities, like tardigrades, vary in density along bodies of moving water depending on a variety of conditions, the most important being the physical properties of water flow which determines the location of benthic habitats.(Statzner and Higler 1986) Furthermore, from what has been observed, freshwater tardigrades generally inhabit each of the various zones in lotic and lentic ecosystems.(Strayer 1994; Strayer and Findlay 2010)

Limnic eutardigrades usually have long legs and claws and undergo limited or no cryptobiosis. Limnoterrestrial (freshwater or terrestrial) tardigrades can be predators, prey, or primary consumers in food webs.(Nelson 2015) Predators of aquatic tardigrades include oligochaetes, nematodes, rotifers, and insect larvae. Some species of tardigrades prey on other micrometazoans, mainly nematodes and rotifers but also other tardigrade species. And Lecophagus antarcticus, a fungus found in Antarctic lakes, preys on bdelloid rotifers and freshwater tardigrades when the decaying biomass they normally consume grows scarce.(McInnes 2003) Other fungi and protozoa also infect tardigrades as parasites, but little is known about their role. Freshwater tardigrades are mainly herbivorous, consuming algae and mosses or bacteria and detritus. Though little is known about the specific role tardigrades play in the ecosystem, stream meiofauna in general increase trophic web complexity because many stream invertebrates feed on meiofauna and organic matter.(Schmid-Araya and Schmid 2000; Schmid 2002)
Knowledge of the dispersal methods of freshwater species is, again, limited, but flowing water is a top contender for lotic habitats, and it is possible that large animals may contribute to their transportation. Similarly, after their apparent disappearance from aquatic habitats in the summer or after human habitat disturbances, tardigrades consistently repopulate, but their mechanism for doing so is unknown.(Nelson 1987)

Terrestrial Tardigrades
Tardigrades endemic to land microhabits, referred to as semiterrestrial or limno-terrestrial, comprise 80% of described species and most eutardigrade genera.(Degma 2009-2015) These microhabitats include mosses, lichens and liverworts on rocks, soil, tree trunks, and man-made constructions as well as ferns, flowering plants, leaf litter, and soil. Terrestrial habits generally must provide sufficient oxygenation, alternating wet and dry conditions, and sufficient food.(Ramazzotti and Maucci 1983) Due to factors not yet understood, the density of individuals in terrestrial tardigrade populations is highly variable. Estimates on density of soil tardigrades have ranged from 8050 to 75,500 individuals per square meter(Ito 1999) as well as much higher and much lower. Soil tardigrade densities are highest at depths of 4-6 cm cm, but are also found up to 40 cm depending on groundwater levels.(Ito and Abe 2001) On a macro level, tardigrade habitats are very separated. Though few studies have been conducted on zonation on the microscale level, within a moss cushion, for example, variables such as availability of food, hardness of leaves, or water retention capacity could influence tardigrade micro distribution.(Greven and Schuttler 2001; Degma 2011). Fluctuations in terrestrial tardigrade population densities have been attributed to environmental variables such as humidity, fungal population dynamics, precipitation, temperature, soil type, habitat and landscape type, and air pollution in various studies.(Franceschi 1962-1963; Morgan 1977; Dastych 1987, 1988; Steiner 1994; Jonsson 2003, 2007; Schuster and Greven 2007, 2013; Guol 2009a; Kaczmarek 2011)

[characterize eutardigrades, what often comes to mind for the tardigrade] Interstitial eutardigrades living in inorganic soils have very small legs and claws and reduced or no hind claws, likely due to the miniscule spaces available in interstitial soil habitats. These adaptations have evolved several times as indicated by their presence in different superfamilies. (Dabert 2013; Guil 2013; Bertolani 2014b) Terrestrial tardigrades are prey for nematodes, collembolans, mites, spiders, and insect larvae. Some terrestrial species are carnivores, feeding on protists, nematodes, rotifers, and other tardigrade species.(Doncaster and Hooper 1961; Morgan 1977) Others are herbivores or detritivores. Terrestrial tardigrades have numerous symbiotic relationships with fungi, protozoans, and bacteria(Vecchi 2016) and are frequently parasitized by fungi, especially in moist habitats. The protist Pyxidium tardigradum has been found on the integument of a number of eutardigrade species. Rather than feeding on the host itself it feeds on bacteria and other organisms in the surrounding water.(Vincente 2008) The tardigrade Echiniscus mollusorum has been found in the feces of land snails, (Fox and Garcia-Moll 1962) but was also found living in the mold growing on a cement wall.(Fox 1966) Dispersal of terrestrial tardigrades is dependent on a variety of factors––their ability to be transported passively by wind, rain, plants and animals (including humans) as well as the size of the individual and their eggs and their cryptobiotic abilities, which allows transportation through any condition and for long periods of time.(Guil 2011) Active dispersal is limited due to their size. The success of pioneers depends on their dispersibility and ability to survive and reproduce upon introduction.

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Edited by Zachary Spivack, student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2022, Kenyon College.