Aquatic animals can live in moss on a tree because their water does not need to form a pond. Films around leaves and small pockets between shoots can supply a usable habitat. When that water disappears, some inhabitants survive by becoming dormant; others cannot tolerate the change.
The useful distinction is between living actively in water and surviving a period without it. A tardigrade’s ability to endure drying does not mean it walks and feeds on dry bark. Moss can alternate between an aquatic neighborhood and a place where activity has to stop.
The water is around the plant
Imagine rain reaching a cushion of moss. Some enters plant cells, while some spreads over surfaces or occupies spaces between leaves and stems. The animals use the external water. They are not swimming inside the moss’s cells.
A study of external water conduction in Syntrichia examined eleven moss species using microscopy and hydration experiments. Capillary spaces help these mosses transport and store water outside their cells. Leaf surfaces, the arrangement of leaves around a stem, and the density of a clump all contribute to that system.
Capillary action needs no muscular pump. Wettable surfaces and narrow spaces allow water to spread through the plant’s architecture. A paper towel offers a familiar analogy for the spreading, although a living moss cushion has a much more varied structure.
The study found differences among species in conduction speed and water storage. A measurement from one moss therefore cannot supply a universal timetable for how long any tree’s moss stays wet. The relevant question for an animal is whether usable liquid remains where it needs to move and feed.
A small animal can use a small stretch of water
A habitat that looks negligible to us can surround an entire microscopic animal. This changes the meaning of “aquatic”: it describes the conditions around the animal rather than the landscape visible from a few steps away.
Bdelloid rotifers make that especially clear. Richard Fox’s anatomy account describes these multicellular animals in periodically wet habitats, including moss. Their characteristic feeding structures carry beating cilia. Coordinated beating can look like turning wheels, but the structures do not rotate like mechanical wheels.
The cilia create currents that carry food toward the mouth. A foot with adhesive toes helps the animal attach to a surface; stretching and contracting can move it onward. Water supplies both the surrounding medium and the material its feeding apparatus moves.
This is a useful correction to a microscope impression: a tiny moving shape is not necessarily a single cell. Rotifers have tissues and organs despite their small size. For a comparison with other microscopic residents, see cells and animals in pond water.
Drying removes routes as well as water
Picture two wet pockets joined by a narrow film. Drying that film cuts the route between them while both pockets may still contain water. For an animal trying to reach food, habitat can become disconnected before the moss looks completely dry. The cushion is a network of usable spaces, not one tank with a single water level.
The distinction also helps separate shelter from activity. A crevice might protect an animal from immediate exposure while no longer providing enough water for ordinary feeding. Finding an animal in moss does not establish what it was doing before the sample became wet.
Nor should a scene containing a rotifer, a roundworm, and a tardigrade be read as a census of every cushion. Species composition and local conditions matter. The video’s enlarged thumbnail is an illustration of the habitat, not a microscope photograph documenting those animals together.
Becoming a tun takes biological work
Anhydrobiosis is a survival state associated with extreme water loss and greatly reduced metabolism. Some tardigrades reorganize into a compact tun, retracting their legs and shortening the body. That visible shape is part of a larger response to dehydration.
A 2013 experiment on Richtersius coronifer tested whether tun formation was merely passive shriveling. Treatments interfering with energy production or muscle function disrupted the formation of viable tuns. The results support an active role for muscular reorganization in this species.
A compact outline is consequently not enough to prove successful preparation or future recovery. The structures inside must remain capable of restarting. The study concerns one tardigrade species and particular experimental treatments; it does not establish identical requirements for every moss animal. The companion on how tardigrades survive drying examines the cellular side in more detail.
Preparation time changes the outcome
Roundworms provide another reason to avoid a single rule. A study of twenty-four Panagrolaimus strains compared desiccation survival after different periods in very humid conditions, followed by drying over silica gel.
Some strains tolerated immediate severe drying. Others performed much better after a preparation period, while some remained sensitive. The authors distinguished these responses rather than treating all nematodes as equally resistant.
These were controlled laboratory conditions. They show why drying history matters; they cannot predict the survival percentage in a particular patch of bark. “It can survive desiccation” leaves out the species, preparation, duration, and conditions needed to interpret the claim.
Nine dry years did not mean nine active years
In a 2002 study of stored moss and lichen, researchers examined sixty-three samples kept dry for nine to 138 years. They recovered living Mniobia rotifers from one nine-year-old sample. Tardigrades also hatched from eggs in that sample.
Keep those two findings separate: the tardigrade result involved eggs, and the discovery was not evidence that every animal survived every storage period. The paper also discusses contamination and sample treatment as problems in interpreting long-term revival reports.
Rehydration reveals which survivors can resume life under the new conditions. It does not show that they spent the interval eating or moving, and it does not revive an animal that has irreversibly died.
Rain therefore does more than provide a drink. For surviving inhabitants it can restore the surrounding medium that makes movement and feeding possible. The moss’s watery spaces explain how an aquatic life can fit on a tree; dormancy explains how some residents bridge the times when those spaces disappear.
