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The Phylum Rotifera includes common but unnoticed metazoans (size range between 50µm and 2000 µm) inhabiting every type of habitat. They are very abundant in lakes, ponds and rivers, as well at periodically moist habitats such as soil, leaf litter and mosses, some of them live in marine environments such as rock pools and sandy beaches. Rotifera is divided into two classes, the Digononta and the Monogononta. The Digononta female members are characterised by paired ovaries, ramate trophus and the absence of a hard lorica. The class is divided in two orders: Bdelloidea and Seisonidae (marine). Bdelloids inhabit a variety of environments, have a strict parthenogenesis reproduction and are capable of desiccation or anabiosis. The members of the class Monogononta constitute 90% of the known species, they have a species-specific trophus, usually a loricate body and a single ovary.
![Cephalodella gibba (Oberer Seebach, Austria)[Photo Schmid-Araya]](cephalodella.jpg) |
The rotifers are pseudocoelomate where their body cavity, pseudocoel, occupies the space between the mesoderm of the body wall and the endoderm of the gut. Their structure is syncitial, and their development eutelic (they have no secondary body cavity lined with epithelium, the tissues of most organs do not have specific cells). The number of nuclei produced during ontogenetic development is fixed and no further cell divisions occur once the embryo is fully developed. |
The morphology of rotifers is very diverse, but their body is cylindrical or spherical an can be divided into head, trunk and foot. The head bears a ciliated crown or corona, that is used in locomotion and feeding. In most species, the corona or wheel-organ which gives the taxa its name drives fine, particulate food into the mouth. There are many variations to the corona pattern and in bdelloids it is composed of two ciliary rings: the anterior named trochus (divided into right and left each known as trochal disks) and the posterior called cingulum. Posterior to the corona the dorsal surface bears in bdelloids a long median sensory dorsal antenna. At the head region there are tactile and optic sense organs and the oral aperture or mouth. The trunk cavity is filled with body fluids and organs. Here are: a glandular stomach (where food is digested), a short intestine to the sub terminal anus, a pair of nephridial tubules draining a series of flame cells that lead to a bladder that discharges to the exterior. There is also a nervous system that includes a bi-lobed ganglion and complex sense-organs, tactile, visual and chemoreceptory arranged in pairs around the body. Rotifers can have separate sexes, but much of the reproduction is by parthenogenesis. Females have ovary and germovitellaria (combination of ovary and the yolk gland).
Consists of mouth, pharynx armed with mastax, salivary glands, and stomach plus gastric glands. The digestive tract contains conspicuous features that are used for species identification. The structure is the trophus (hard chitinous structures or jaws Figs.1, 2) all in a pouch like sac and with muscles (mastax). The trophus has seven parts: a fulcrum (unpaired), paired rami (singular ramus), paired unciI (singular uncus) and paired manubria (singular manubrium)
 Fig. 1. Diagram of rotifer trophy |
 Fig. 2. S.E.M. photograph of a trophy |
The mastax vary widely from species to species in relation to the feeding habits. Some trophies are particularly adapted for grinding particulate detritus (malleate or ramate trophus), capture and tearing of prey (forcipate trophus), extract fluids from plants or from microscopic organisms (virgate trophus), scraping organic particle surfaces (forcipate, virgate, malleate)
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MALLEATE
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RAMATE
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UNCINATE
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FORCIPATE
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INCUDATE
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VIRGATE
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In sessile and free-living rotifers the coronal cilia plays an important role, by creating currents that concentrate the particles towards the mouth region. The food of bdelloids consists of miscellaneous small particles of organic matter, bacteria, small green algae, and diatoms. These particles are driven into the mouth of the feeding animals by the action of the wheel-organ, only the species of some families (genus Philodinavus, Adineta) seek and scrape the food from the surfaces of plants and organic particles.
The foot marked by the position of the anus that forms a dorsal joint external opening for rectum, bladder & oviduct. The foot is segmented and has usually two toes and glands (pedal glands). However plankton rotifers deviate from this pattern in their body shape, their lorica or appendages and their morphology.
The skin of rotifers is a syncytial epidermis with an intracellular, epidermal lorica. The lorica may be thin and flexible (mostly bdelloids) or thick and rigid (most plankton rotifers) and often looks as it is segmented.
The reproduction of rotifers can be sexual and by parthenogenesis (Fig. 3). The females involved in parthenogenesis are supposed to be amictic and both their body cells and the eggs have diploid number of chromosomes. When conditions in the environments are probably not adequate (i.e. change in the type of food, temperature), the mictic females appear. The mictic female produces eggs that undergo the usual double meiotic division and are haploid. These eggs can then be fertilized by males, if the process is successful then resting-egg are produced (an egg that has with a thick wall). The mechanism is such, that if a mictic female is not fertilized, she deposits eggs that promptly hatch into males. The resting-egg when hatching produces an amictic female. Sexual reproduction occurs in the rotifers of Class Monogononta, while the order Bdelloidea have no records until today of the presence of males, so that only parthenogenesis is the reproductive mechanism.
| Fig. 3. Schematic representation of parthenogenesis reproduction in rotifers. |
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Another characteristic of Bdelloidea (also common in nematodes and tardigrades) is that they undergo a process of anabiosis (or desiccation). The phenomenon occurs mostly when mosses dry, bdelloids draw both ends of the body into the central part of the trunk. The body becomes globular like and losses water. These rotifers can be kept in this stage for long periods of time (even years), when water is added they recover and start to be again active. In addition to gains in life span, anhydrobiotic organisms are capable of enduring astonishing environmental extremes that would be lethal if the organisms were fully hydrated. Rotifers, for example, are master survivalists in their anhydrobiotic state. In the 1920’s P.G. Rahm of the University of Freiburg, Germany, found that they could survive for days at -200°C (-330°F) and for a few minutes at 150°C (300°F), and in the 1950s Henri Becquerel of the University of Paris reported that the same organisms could survive exposure for lengthy periods to temperatures a few thousandths of a degree above absolute zero, -243.15°C (-459.67°F).
The identification of rotifers depends on the characteristics of the lorica, corona, and mastax (trophi) for plankton specimens. In soft body species (i.e. bdelloids) the killing procedure can contract their bodies and make identification quiet tedious. There are several narcotics used for soft body rotifers, but cheapest and convenient is to use very diluted formalin and add it at different time intervals, drop by drop. However, for Bdelloidea there is no adequate narcotic substance and killing with formalin always produces contracted animals. In this latter group, the identification has to be done with live animals based on morphological and behavioural characters (Donner 1965).
The great majority of species present in river sediment surfaces and deeper layers (or associated with mosses) correspond to Bdelloidea. The class Monogononta is represented mostly by the families Proalidae, Notommatidae (genus Cephalodella, Notommata) Dicranophoridae (genus Encentrum, Dicranophorus), Colurellidae (genus Lepadella).
Voigt, M. 1957. Rotatoria. Die Rädertiere Mitteleuropas. I. Textband, II.Tafelwerk 506pp, 115 Taf. (in German)
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Copyright Schmid & Schmid. Last revised: April 2005
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