Water bear

Chapter 18 Tardigrada (Water bears) Classification

    Kingdom 		Animalia - animals
     Subkingdom 	Bilateria
      Branch 		Protostomia

Infrakingdom Ecdysozoa

Superphylum Panarthropoda

         Phylum 		Tardigrada 

Class Heterotardigrada

Class Mesotardigrada

Class Eutardigrada

Water Bears are members of a largely unknown phylum of invertebrate animals, the Tardigrada. The first tardigrades were discovered in 1773 after microscopes were invented. Over 800 species have been described since then. The largest tardigrades grow to a size of just over 1 mm, but they can easily be seen with microscopes. Tardigrade bodies are divided into segments, roughly cylindrical and posses four pairs of lobopodial limbs (poorly articulated limbs without joints, which are found in soft bodied animals like Onycophorans). The limbs terminate in four to eight claws or discs. They crawl about with a bear-like pawing motion of the legs (that originated the name water bears) over sand grains in the seas, soil, lichen or pieces of plant material etc. A number of morphological and molecular studies have sought to resolve the relationship of tardigrades to other lineages of ecdysozoan animals. Two plausible placements have been recovered: tardigrades most closely related to Arthropoda - Onychophora (a common result of morphological studies) or tardigrades most closely related to nematodes (found in some molecular analyses). The latter hypothesis has been rejected by recent microRNA and expressed sequence tag analyses. Apparently, the grouping of tardigrades with nematodes found in a number of molecular studies is a LongBranchattraction artifact. Within the arthropod group (called panarthropoda and comprising onychophora, tardigrades and euarthropoda), three patterns of relationship are possible: tardigrades sister to onychophora plus arthropods (the lobopodia hypothesis); onychophora sister to tardigrades plus arthropods (the tactopoda hypothesis); and onychophora sister to tardigrades. Recent analyses indicate that the panarthropoda group is monophyletic, and that tardigrades are a sister group of lobopodia, the lineage consisting of arthropods and Onychophora.

Panarthropoda Water bears (Tardigrada)

Lobopoda Velvet worms (Onychophora)

Arthropods (Arthropoda)

TarTardigrades are strictly aquatic animals. They are commonest in streams especially in high altitudes. Also, the droplets and film of wateron terrestrial wet mosses, liverworts and certain angiosperms are the likelyhabitats of tardigrada. Tardigrades are called water bears because they resemble miniature bears. The body is nearly one mm in lengthand shows head and four indistinct trunk segments. There are four pairs of stumpy legs on the trunk segments. Each leg possesses two pairs of terminal double claws. The cuticle of fresh water species is thicker than that of the marine species. The body surface may be smooth sculptured or with long cirri. The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1773.The name water bear comes from the way they walk, reminiscent of a bear's gait. The biggest adults may reach a body length of 1.5 millimetres (0.059 in), the smallest below 0.1 mm. Freshly hatched tardigrades may be smaller than 0.05 mm. About 1,150 species of tardigrades have been described. Tardigrades occur throughout the world, from the Himalayas(above 6,000 metres (20,000 ft)), to the deep sea (below 4,000 metres (13,000 ft)) and from the polar regions to the equator. The most convenient place to find tardigrades is on lichens and mosses. Other environments are dunes, beaches, soil, and marine or freshwater sediments, where they may occur quite frequently (up to 25,000 animals per liter). Tardigrades often can be found by soaking a piece of moss in spring water. Generally, tardigrades are plant feeders. There are two protrusible calcareous stylets to cut the plant surface. The pharynx is divided into two parts: the tubular pharynx which is narrow and sclerotizedcontinues into globular sucking pharynx. The anus is a longitudinal or tram verse slit. The faeces, many times are released into the old cuticle during;molting. In the class Eutardigrada there are three types of glands at the junction of midgut and hindgut which are, most probably, excretory in function. Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are predatory (e.g., Milnesium tardigradum). Muscular system is well developed. There is no special circulatory 01 respiratory system. Nervous system consists of a large brain, subpharyngeal ganglion, four trunk ganglia and their connectives. The water bears crawl on the substratum. In many species the males are unknown and so the development of the eggs is by parthenogenesis. In many species from which the males are known, sexual dimorphism is conspicuous. The gonads are unpaired. In Eutardigrada, the genital dud opens in the rectum which thus acts as cloaca. Two types of eggs, thin and thick shelled are produced by tardigrades. Thick shelled eggs overcome unfavourable conditions. It ispossible that thin shelled eggs are parthenogenetic and thick shelled are fertilized one. In many cases external fertilization takes place, lit the typical semi-aquatic species, internal fertilization takes place. Generally one to thirty eggs are contained in the newly shed cuticle. In few cases they are deposited freely. The young ones come out of the egg by breaking it. The mature adult is formed alter two to three molts. In general there are four to six molts during the life history. The tardigrades are often associated with mushrooms and are especially common in wet weather. They are found in slow flowing rain water films in the hills and mountainous tracts. They live in places where mosses, algal mattsand liverworts grow abundantly. Tardigrades are active as long as the moss is damp. If the moss dries tardigrades live in inactive abiotic state which may extend for several years. Sometimes under abnormal ecological conditions the cysts are formed. The oxygen deficient condition is overcome by asphyxia. During this condition the body swells and becomes turgid and rigid. The animal cannot survive more than five days in asphyxia.

Figure: This is how water bear looks if one is observed under low power in compound microscope.

Prospects Tardigrada are easy to rear in the laboratory and are abundant in moist, mossy hills. Active efforts should be made to search for these curious animals in our monsoon-enriched locales like the Western Ghats and the Nilgiris.