11.2: Phylum Tardigrada

Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the features of Tardigrades

 

The tardigrades ("slow-steppers") comprise a phylum of inconspicuous little animals living in marine, freshwater, or damp terrestrial environments throughout the world. They are commonly called "water bears" because of their plump bodies and the large claws on their stubby legs. There are over 1,000 species, most of which are less than 1 mm in length. A chitinous cuticle covers the body surface and may be divided into plates (Figure 28.35). Tardigrades are known for their ability to enter a state called cryptobiosis, which provides them with are resistance to multiple environmental challenges, including desiccation, very low temperatures, vacuum, high pressure, and radiation. They can suspend their metabolic activity for years, and survive the loss of up to 99% of their water content. Their remarkable resistance has recently been attributed to unique proteins that replace water in their cells and protect their internal cell structure and their DNA from damage.

Figure 28.35 Scanning electronmicrograph of Milnesium tardigradum. (credit: Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) -https://commons.wikimedia.org/w/index.php?curid=22716809)

 

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Morphology and Physiology

Tardigrades have cylindrical bodies, with four pairs of legs terminating in a number of claws. The cuticle is periodically shed, including the cuticular covering of the claws. The first three pairs of legs are used for walking, and the posterior pair for clinging to the substrate. Bands of single muscle cells are attached to the various points of the epidermis and extend into the legs to provide ambulatory movement.

A circular mouth leads to a muscular pharynx and salivary glands. Tardigrades feed on plants, algae, or small animals. Plant cells are pierced with a chitinous stylet and the cellular contents are then sucked into the gut by the muscular pharynx. The major body cavity is a hemocoel, but there are no specialized circulatory structures for moving the blood, nor are there specialized respiratory structures. A dorsal brain is connected to a ventral nerve cord with segmental ganglia associated with the appendages. Sensory structures are greatly reduced, but there is a pair of simple eyespots on the head, and sensory cilia or bristles concentrated toward the head end of the animal.

Malpighian tubules in the hemocoel remove metabolic wastes and transport them to the gut. Malpighian tubules are also found lining the gut of some species of arthropods, such as the bee illustrated in Figure 41.11. They are usually found in pairs and the number of tubules varies by species. Malpighian tubules are convoluted, which increases their surface area, and they are lined with microvilli for reabsorption and maintenance of osmotic balance. Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered as in the case of nephridia seen in annelids and some molluscs; urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph. Metabolic wastes like uric acid freely diffuse into the tubules. There are exchange pumps lining the tubules, which actively transport H⁺ ions into the cell and K⁺ or Na⁺ ions out; water passively follows to form urine. The secretion of ions alters the osmotic pressure which draws water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments, and uric acid is excreted as a thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water; this is especially important for life in dry environments.

Figure 41.11 Malpighian tubules of tardigrades, insects and other terrestrial arthropods remove nitrogenous wastes and other solutes from the hemolymph. Na⁺ and/or K⁺ ions are actively transported into the lumen of the tubules. Water then enters the tubules via osmosis, forming urine. The urine passes through the intestine, and into the rectum. There, nutrients diffuse back into the hemolymph. Na⁺ and/or K⁺ ions are pumped into the hemolymph, and water follows. The concentrated waste is then excreted.

 

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Reproduction

Most tardigrades are dioecious, and males and females each have a single gonad. Mating usually occurs at the time of a molt and fertilization is external. Eggs may be deposited in a molted cuticle or attached to other objects. Development is direct, and the animal may molt a dozen times during its lifetime. In tardigrades, like nematodes, development produces a fixed number of cells, with the actual number of cells being dependent on the species. Further growth occurs by enlarging the cells, not by multiplying them.

 

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This page titled 11.2: Phylum Tardigrada is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license.