Ecology - Distribution and Habitats
Figure 1. Tardigrade eating nematode
The tardigrade can live in extreme temperatures over 150 degrees C and as low as -272.8 degrees C. They inhabit every continent, latitude and almost every altitude as long as they have the right habitat. Even though they can live without water for more than three decades, the tardigrade habitat is made up of mosses, lichen and algae. There are over 1000 species of tardigrade and each species has the ability to live in a specific niche environment, over 150 of them are marine. These tardigrade species live in intertidal and subtidal spaces, the areas in between coarse sediment or as deep as 7 km below sea level! This page will show you how tardigrades are dispersed all over the world, where they can live and what role they play in the food chain.
The habitats of tardigrades are in many different climates and biomes. The fact that tardigrades have been found in all seven continents proves that they can live and reproduce almost anywhere just as long as the habitat has some water and an appropriate food source. Tardigrades need water in order to do their everyday processes like development, gas exchanges and reproduction. They can get that water by piercing the cell walls of moss or algae or any plant that it lives in or nearby. They do this by using a part of their mouth called a stylet which is designed to pierce the cell to provide access to its water supply (“Tardigrades: Size, Lifespan, Diet, and Other Shocking Facts” 2019). Some species of tardigrade are carnivorous and get their water from other small animals like nematodes and plankton in their own environment. The carnivorous tardigrades can sometimes be cannibalistic too.
One main role of tardigrades in the ecosystem is to control the population of mosses and other plants but also detritus and small animals like plankton. Tardigrades are part of a complicated food web, they feed off of mosses, lichen and in some species other animals or even tardigrades. They are eaten by crustaceans, arthropods and earthworms along with the microorganisms in their habitat. Nematodes and other microorganisms compete with carnivorous tardigrades. This means that nematodes can consume smaller tardigrades and tardigrades can eat nematodes. Carnivorous tardigrades can feed on the same microscopic animals as their predators. In some cases, tardigrades feed primarily on nematodes, drastically affecting their population (Wright n.d.).
Some other scientists have found tardigrades infected with a fungus (Pohlad and Bernard 1978). This specific fungus couldn’t be grown outside of its original habitat so it is unknown what the fungus does to other animals . Scientists are still investigating more interspecies relationships between bacteria. Some of these investigations found bacteria in a certain family of tardigrade. Halechiniscidae has the most in detail account of bacteria affecting tardigrades. This family has shown to hold bacteria in their cephalic vesicles. It has been observed that bacteria produce small particles that make their way into the tardigrade body. It is hypothesized that this transfer could be a way that tardigrades get energy when there is a lack of resources (Vecchi et al. 2016). This energy exchange has been viewed in other invertebrates in similar habitats with low food supplies.
Tardigrades can be found all over the world from kelp gull nests in Antarctica to remote volcanic islands off of Japan and Hawaii. Researchers do not exactly know how tardigrades traveled to these places but multiple studies offer informed guesses. One explanation for this is that tardigrades and other meiofauna tend to disperse passively. In some cases, birds can carry pieces of moss or kelp across bodies of water to transport tardigrades to different islands. It is also known that in their cryptobiotic state, (see cryptobiotic page for more details) they are light enough to be blown by the wind. It is possible that tardigrade eggs can be moved in the same way. Certain stream currents could move the tardigrades downstream and a heavy rainstorm or melting snow could push them into the stream in the first place. It is also possible that insects could carry them over smaller distances since they can feed on tardigrades.
The population density of a tardigrade habitat has to do with a lot of factors and some of them are unknown. In some cases, two seemingly identical habitats have vastly different population sizes. We don’t know the optimal or minimal conditions for growth of a tardigrade population so it is hard to identify which factors could affect growth as well (Nelson 2002). When faced with tough living conditions, tardigrades don’t migrate unlike other animals (Nelson and Adkins 2001). Instead they undergo anhydrobiosis and wait out the less forgiving elements.
Mosses tend to grow in layers like hair. Over time, this builds up layers of living and dead tissue that tardigrades live in (Carruthers 2018). Different species of moss can live in the same area as well. Habitats with layers of moss rarely have statistically significant variation of population density within the layers of moss. Different species of tardigrade were also found to have similar densities when looking across layers of moss or moisture conditions (Nelson and Adkins 2001). The distribution of tardigrade species in most remote places are proportional to the amount of papers and studies done at these remote locations so more species and populations are yet to be discovered as more areas in the world are studied (Zawierucha et al. 2013).
Figure 2. Tardigrades in Different Substances. Samples collected by the Woods Lo lab (Summer 2020).
References