TARDIGRADES: CHARACTERISTICS, TYPES, HABITAT, NUTRITION - BIOLOGY - 2025

The tardigrades are microscopic animals length between 0.05 and 0.5 mm, but have been reported "giant" 1.7mm. They are invertebrates, segmented protostomes, with the appearance of tiny bears with four pairs of thick legs with claws, and locomotion from side to side with heaviness.

They were described for the first time by Johann A. Ephrain Goeze in 1773 and baptized as water bears by Lázzaro Spallanzani in 1777. Although they have been little studied, at present there are more than 800 described species, inhabitants of semi-aqueous environments, in almost all kinds of environments.

Figure 1. Adult tardigrade. Source: Goldstein lab - tardigrades, via Wikimedia Commons

Although their phylogenetic relationships remain in dispute because they present combined annelid and arthropod characteristics, they can be considered to belong to the phylum Tardigrada.

Like arthropods, tardigrades have a thin protective outer cuticle, which they shed periodically (a process mediated by the pro-steroid ecdysome hormone), allowing them to survive desiccation. However, they have non-articulated appendages with clamps, unlike arthropods, which do have joints.

General characteristics

Body shape

Tardigrades present a body with bilateral symmetry, generally with a rounded and flattened back, with four pairs of ventral legs that culminate in claws whose characteristic shapes are important for their classification.

Body segmentation is not outwardly distinguishable, but the head is followed by three trunk segments, each with a pair of legs, in addition to the last caudal segment, with the fourth pair of legs projecting rearward.

The body is covered by a thin layer of cuticle that they shed and many species have dorsal and lateral plates.

Non-marine adult tardigrades can be colorful, exhibiting shades of pink, green, purple, yellow, red, gray, and black.

Musculature

Tardigrades have smooth and striated muscles, with most muscle bands consisting of a single cell or a few large cells. These form antagonistic sets of muscles that control your locomotion step by step.

Gas exchange

The exchange of gases, like oxygen, depends on diffusion through your body.

Digestive system

The digestive system of tardigrades consists of a buccal tube, a bulbous muscular pharynx, and a pair of calcareous stylets that they use to pierce plants, or other small animal bodies, and then suck out their contents.

Carnivorous and omnivorous tardigrades have an anterior terminal mouth, while herbivores and detritivores have a ventral mouth.

The pharynx communicates with the esophagus, which in turn opens into a middle large intestine and short large intestine (cloaca or rectum), eventually leading to a terminal anus.

Figure 2. Tardigrade. Source: Frank Fox at

Nervous system

The nervous system of tardigrades is metameric, similar to that of annelids and arthropods.

They present a large lobulated dorsal brain ganglion, connected to a subesophageal ganglion. This in turn, extends into a pair of posterior ventral nerve cords, which connect a chain of four pairs of ganglia that run through the legs.

Tardigrades often have a pair of sensory eye spots, each containing five cells, one of which is sensitive to light.

Adaptive strategies

Anabiosis and cyst formation

Tardigrades have the ability to enter a state of latency that implies a very reduced metabolic activity, during environmental conditions unfavorable for their survival.

In periods of drought, as the vegetation inhabited by terrestrial tardigrades dries up, they curl up by pulling their legs, lose water from their body and secrete a double-walled cuticular sheath that covers their entire wrinkled body.

These cysts maintain a very low (but still detectable) basal metabolism, a state called anabiosis.

Tardigrades have also been reported to form cysts under abnormally high CO ₂, hydrogen sulfide, and potassium cyanide conditions.

Cryptobiosis and barrel stage

Cryptobiosis is an extreme state of anabiosis, in which all signs of metabolic activity are totally absent. Due to this ability to enter this state, many species of tardigrades survive extreme environmental conditions.

Under extreme environmental conditions, tardigrades contract their legs and form a particular type of single-walled cyst, shaped like a “wine barrel” (called “tun” in English).

In this barrel state, the body's metabolism is undetectable, being considered cryptobiotic. Thus, they protect themselves from extremely adverse conditions, covering their body and reducing the interaction surface with the environment.

Anhydrobiosis

Anhydrobiosis is a desiccation tolerance strategy that allows many species of tardigrades (and other invertebrates, rotifers and nematodes) to resist the state of dehydration due to external conditions of freezing water or drought.

Exposed to drought conditions, it loses water (which in the active state constitutes 85% of its weight), until it reaches less than 2% of its body weight and its metabolic activity decreases to almost imperceptible levels, being able to enter the barrel stage.

Resistance to extreme conditions

Among the extreme physical conditions to which many species of tardigrades survive in the late barrel stage, are:

• Very high temperatures (149 ° C) and very low (-272 ° C).
• High atmospheric pressure (up to 6000 atm).
• Intense levels of ionizing radiation.
• Exposure to vacuum.
• Long periods of total absence of oxygen.

In addition, some species have recovered after immersion of their barrels in toxic substances such as brine, ether, absolute alcohol and even liquid helium.

After favorable conditions for their active state are re-established (particularly the availability of water), the animals swell and reactivate their metabolism within a few hours.

Ecological role of encystment and barrel stage

Cysts and barrel stages represent survival strategies in space and time.

In the temporal aspect, years can pass in these encysted stages until the environmental conditions (in particular of humidity) return to be favorable.

In the spatial field, encystment also represents a means for its geographical dispersion, either by the dispersive action of the wind, or by being in dry mud adhering to locomotion waterfowl.

Due to the alternation between active and encysted periods, the life expectancy of tardigrades can vary from less than a year to more than 100 years.

Figure 3. Active adult tardigrade (a) and its encysted form (b). Source: Takuma Hashimoto, Daiki D. Horikawa, Yuki Saito, Hirokazu Kuwahara, Hiroko Kozuka-Hata, Tadasu Shin-I, Yohei Minakuchi, Kazuko Ohishi, Ayuko Motoyama, Tomoyuki Aizu, Atsushi Enomoto, Koyuki Kondo, Sae Tanaraka, Yuichiro, Yuichiroka Shigeyuki Koshikawa, Hiroshi Sagara, Toru Miura, Shin-ichi Yokobori, Kiyoshi Miyagawa, Yutaka Suzuki et al., via Wikimedia Commons

Habitats

Tardigrades are free-living or symbiotic (even parasitic) animals with a wide geographical distribution, inhabitants of extreme or highly variable environments such as temporary freshwater ponds.

Water availability

The limiting factor for these microorganisms is the availability of water, although in the absence of this (under freezing or drought conditions), tardigrades dehydrate, forming cysts or barrel stages, as previously mentioned.

Terrestrial species share their microhabitats with other organisms such as rotifers, nematodes, bacteria, protozoa, mites, and small insect larvae.

Wide geographic distribution

Information on the geographic distribution of tardigrades is limited by the lack of their extended study, and by the scarcity of collections of specimens from different critical regions of the planet.

However, its wide geographical distribution is favored by its dispersal through cysts, barrel stages and their eggs.

All these structures are very light and resistant to be transported long distances (either by wind or sand, in mud attached to insects, birds and other animals).

Tardigrades have been found from the Arctic to Antarctica, from beach sands to the abyssal depths (3000 m deep), in natural and artificial bodies of water (pools, rivers, lakes, seas and hot springs), in semi-aquatic habitats, such as the thin layer of water that covers the soil, litter, mosses, liverworts, lichens, algae, and certain vascular plants.

Some species are interstitial (they live between grains of sand), others are epiphytes (they live on the surface of algae and plants), and others are epizoic or commensal (they live on or within other marine invertebrates, such as the mantle of mussels).

Examples of tardigrade species

Most tardigrade species are widely distributed on planet Earth and many are cosmopolitan, such as Milnesium tardigradum (carnivorous diet).

Other species are marine such as Halobiotus crispae, which is commonly found on the brown algae of Greenland. Littoral species, such as Echiniscoides sigismundi in Denmark, have also been studied.

However, there could be apparently endemic species such as Isohypsibius cameruni, found (so far) only in Cameroon (Africa), although this assumption could be due to the fact that it has not been sought in other regions.

Other epizoic species, such as Styraconyx qivitoq, live on ectoproct or bryozoan aquatic animals.

Low population density

Tardigrades are part of the food chain, but in general they present a low population number. Occasionally they can reach densities of up to 300,000 individuals / m ² in the soil and more than 2,000,000 individuals / m ² in the moss.

Types of tardigrades

Phylum Tardigrada

The phylum Tardigrada comprises eight families in three orders that are defined based on the details of the appendages of their heads, the nature of the claws on the legs, and the presence (or absence) of Malpighi tubules.

The three orders of this phylum are: Heterotardigrada, Mesotardigrada, Eutardigrada.

Figure 4. Adult tardigrade. Source: Willow Gabriel, Goldstein Lab, via Wikimedia Commons

Nutrition

Diet

They generally feed on the cellular fluids of plants and animals, piercing cells with their pair of oral stylets.

The tardigrades that inhabit fresh water, are located among the decomposing vegetation, feeding on organic waste, plant cell content (especially mosses), microalgae, protozoa and other small invertebrates such as rotifers.

Tardigrade species that live on the ground, feed on decaying bacteria, algae, and plant matter, or are predators of small invertebrates.

Feeding process

When eating, tardigrades suck their food and produce saliva in the esophagus, which mixes with the ingested material. They also produce digestive secretions that are emptied into the oral cavity.

Food passes from the pharynx to the esophagus, which in turn opens into a middle large intestine, where digestion and absorption of nutrients occurs. Finally the short large intestine (cloaca or rectum) leads to a terminal anus.

Reproduction

Tardigrades are dioecious, presenting a single gonad on the intestine in both sexes, and gonopores near the anus or in the rectum (in the case of some females).

Females have one or two small seminal receptacles that open into the rectum, near the cloaca.

In some genera, males are unknown, but most of the tardigrades studied copulate and lay eggs.

Tardigrade growth comes from the cuticle molts and they reach sexual maturity after three to six stages.

Sexual

In some species the male deposits the sperm directly into the female's seminal receptacle or into the body cavity by cuticular penetration. In the latter case, fertilization occurs directly in the ovary.

In other tardigrades, a particular form of indirect fertilization takes place: the male deposits the sperm under the cuticle of the female before she molts, and fertilization occurs when the female later deposits the eggs in the shed cuticle.

Females lay 1 to 30 eggs at a time (depending on the species). Its development is direct, without presenting larval stages.

Asexual by parthenogenesis

Parthenogenesis (from the Greek, partheno: virgin and genesis: birth) is a reproductive strategy in which unfertilized eggs develop as individual viable adults.

This strategy has the short-term advantage of allowing rapid reproduction. However, in the long term it presents a disadvantage compared to sexual relatives, since their genetic diversity allows them greater flexibility and adaptation to variations in environmental conditions.

In most organisms, parthenogenesis alternates with periods of sexual reproduction.

Eggs

Eggs generally have characteristic surface pores in addition to conical projections.

Figure 5. Details of the Macrobiotus shonaicus egg. Source: Stec, Daniel; Arakawa, Kazuharu; Michalczyk, Łukasz, via Wikimedia Commons

Some species are identified solely by the pattern of their eggs. For example, the species of the genera Macrobiotus and Minibiotus.

Also the size and shape of the pores of the dorsal plates of the eggs, allows to separate species, as in the case of the genus Echiniscus.

References

1. Edward, RE and Robert D. Barnes, RD (1996). Invertebrate Zoology. McGraw - Hill Interamericana. Mexico. pp 1114.
2. Guidetti, R. and Jönsson, KI (2002). Long-term anhydrobiotic survival in semi-terrestrial micrometazoans. Journal of Zoology 257 (2): 181-187. doi: 10.1017 / S095283690200078X
3. Miller, SA and Harley, JP (2004). Zoology. Sixth edition. MacGraw-Hill Higher Education. pp 538.
4. Suzuki, AC (2003). Life history of Milnesium tardigradum Doyere (tardigrada) under a rearing environment. Zoolog Sci 20: 49–57.
5. Watanabe and Masahiko (2006). Anhydrobiosis in invertebrates Appl. Entomol. Zool., 41 (1): 15–31.
6. Wright, J. (2001). Cryptobiosis 300 Years on from van Leuwenhoek: What Have We Learned about Tardigrades? Zoologischer Anzeiger 240: 563–582.