Tardigrade

Tardigrades ( ), also known as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär 'little water bear'. In 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means "slow walkers". Tardigrades live in diverse regions of Earth's biosphere: mountaintops, the deep sea, tropical rainforests, and the Antarctic. They are among the most resilient animals known, with individual species able to survive severe conditions — such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation — that would quickly kill most other forms of life. They have survived exposure to outer space. There are about 1,500 known species in the phylum Tardigrada, a part of the superphylum Ecdysozoa. The earliest known fossil is from the Cambrian, some 500 million years ago. They lack several of the Hox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen. Instead, most of their body is homologous to an arthropod's head. Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sticky pads. Tardigrades are prevalent in mosses and lichens and can readily be collected and viewed under a low-power microscope, making them accessible to students and amateur scientists. Their clumsy crawling and their well-known ability to survive extreme conditions have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.

Description

Body structure

Tardigrades have a short plump body with four pairs of hollow unjointed legs. Most range from 0.05 to 0.5 mm (0.002 to 0.02 in) in length, although the largest species may reach 1.3 mm (0.051 in). The body cavity is a haemocoel, an open circulatory system, filled with a colourless fluid. The body covering is a cuticle that is replaced when the animal moults; it contains hardened (sclerotised) proteins and chitin but is not calcified. Each leg ends in one or more claws according to the species; in some species, the claws are modified as sticky pads. In marine species, the legs are telescopic. There are no lungs, gills, or blood vessels, so tardigrades rely on diffusion through the cuticle and body cavity for gas exchange. Tardigrades have been suggested to be eutelic, with a fixed number of body cells at maturity, but there is evidence that they are not.

Nervous system and senses The tardigrade nervous system has a pair of ventral nerve cords with a pair of ganglia serving each pair of legs. The nerve cords end near the mouth at a pair of subpharyngeal (or suboesophageal) ganglia. These are connected by paired commissures (either side of the tube from the mouth to the pharynx) to the dorsally located cerebral ganglion or 'brain'. Also in the head are two eyespots in the brain, and several sensory cirri and pairs of hollow antenna-like clavae which may be chemoreceptors. The tardigrade Dactylobiotus dispar can be trained by classical conditioning to curl up into the defensive 'tun' state in response to a blue light associated with a small electric shock, an aversive stimulus. This demonstrates that tardigrades are capable of learning.

Locomotion Although the body is flexible and fluid-filled, locomotion does not operate mainly hydrostatically. Instead, as in arthropods, the muscles (sometimes just one or a few cells) work in antagonistic pairs that make each leg step backwards and forwards; there are also some flexors that work against hydrostatic pressure of the haemocoel. The claws help to stop the legs sliding during walking, and are used for gripping.

Feeding and excretion Tardigrades feed by sucking animal or plant cell fluids, or on detritus. A pair of stylets composed of the mineral aragonite pierce the prey; the pharynx muscles then pump the fluids from the prey into the gut. A pair of salivary glands secrete a digestive fluid into the mouth, and produce replacement stylets each time the animal moults. Non-marine species have excretory Malpighian tubules where the intestine joins the hindgut. Some species have excretory or other glands between or at the base of the legs.

Reproduction and life cycle

Some species are hermaphroditic and self-fertilizing, but most tardigrade species have both male and female individuals, which copulate by a variety of methods. The females lay eggs; those of Austeruseus faeroensis are spherical, 80 μm in diameter, with a knobbled surface. In other species, the eggs can be ovoid, as in Hypsibius annulatus, or may be spherical with pyramidal or bottle-shaped surface ornamentation. Some species appear to have no males, suggesting that parthenogenesis is common. Both sexes have a single gonad (an ovary or testis) located above the intestine. A pair of ducts run from the testis, opening through a single gonopore in front of the anus. Females have a single oviduct opening either just above the anus or directly into the rectum, which forms a cloaca. The male may place his sperm into the cloaca, or may penetrate the female's cuticle and place the sperm straight into her body cavity, for it to fertilise the eggs directly in the ovary. A third mechanism in species such as H. annulatus is for the male to place the sperm under the female's cuticle; when she moults, she lays eggs into the cast cuticle, where they are fertilised. Courtship occurs in some aquatic tardigrades, with the male stroking his partner with his cirri to stimulate her to lay eggs; fertilisation is then external. Up to 30 eggs are laid, depending on the species. Terrestrial tardigrade eggs have drought-resistant shells. Aquatic species either glue their eggs to a substrate or leave them in a cast cuticle. The eggs hatch within 14 days, and the hatchlings use their stylets to open their egg shells.

… excerpt ends here. Continue reading the full article.