Phylum Rotifera

Phylum Rotifera
Rotifera (ro-tif´e-ra) (L. rota, wheel, + fera, those that bear) derive their name from the characteristic ciliated crown, or corona, that, when beating, often gives the impression of rotating wheels. Rotifers range from 40 µm to 3 mm in length, but most are between 100 and 500 µm long. Some have beautiful colors, although most are transparent, and some have bizarre shapes (Figure 15-2). Their shapes are often correlated with their mode of life. Floaters are usually globular and saclike; creepers and swimmers are somewhat elongated and wormlike; and sessile types are commonly vaselike, with a thickened outer epidermis (lorica). Some are colonial. One of the best-known genera is Philodina (Gr. philos, fond of, + dinos, whirling) (Figure 15-3), which is often used for study.

Rotifers are a cosmopolitan group of about 1800 species, some of which are found throughout the world. Most species are freshwater inhabitants, a few are marine, some are terrestrial, and some are epizoic (live on the body of another animal) or parasitic.

Rotifers are adapted to many kinds of ecological conditions. Most species are benthic, living on the bottom or in vegetation of ponds or along the shores of freshwater lakes where they swim or creep about on the vegetation. A large proportion of the species that live in the water film between sand grains of beaches (meiofauna) are rotifers. Pelagic forms (Figure 15-2B) are common in surface waters of freshwater lakes and ponds, and they may exhibit cyclomorphosis, variations in body form resulting from seasonal or nutritional changes.

Many species of rotifers can endure long periods of desiccation, during which they resemble grains of sand. While in a desiccated condition, rotifers are very tolerant of temperature variations, especially those rotifers thatdwell in mosses. True encystment occurs in only a few rotifers. On addition of water, desiccated rotifers resume their activity.

Strictly marine species are rather few in number. Some littoral (intertidal) species of the sea may be freshwater ones that are able to adapt to seawater.

Form and Function
External Features
The body of a rotifer is composed of a head bearing a ciliated corona, a trunk, and a posterior tail, or foot. It is covered with a cuticle and is nonciliated except for the corona.

The ciliated corona, or crown, surrounds a nonciliated central area of the head, which may bear sensory bristles or papillae. The appearance of the head end depends on which of the several types of corona it has—usually a circlet of some sort, or a pair of trochal (coronal) discs (the term trochal comes from a Greek word meaning wheel). Cilia on the corona beat in succession, giving the appearance of a revolving wheel or pair of wheels. The mouth is located in the corona on the midventral side. Coronal cilia are used in both locomotion and feeding.

The trunk may be elongated, as in Philodina (Figure 15-3), or saccular in shape (see Figure 15-2). It contains the visceral organs and often bears sensory antennae. The body wall of many species is superficially ringed so as to simulate segmentation. Though some rotifers have a true, secreted cuticle, all have a fibrous layer within their epidermis. The fibrous layer in some is quite thick and forms a caselike lorica, which is often arranged in plates or rings. The foot is narrower and usually bears one to four toes. Its cuticle may be ringed so that it is telescopically retractile. It is tapered gradually in some forms (Figure 15-3) and sharply set off in others (see Figure 15-2). The foot is an attachment organ and contains pedal glands that secrete an adhesive material used by both sessile and creeping forms. In swimming pelagic forms, the foot is usually reduced. Rotifers move by creeping with leechlike movements aided by the foot, or by swimming with the coronal cilia, or both.

Characteristics of Pseudocoelomate Phyla

1. Symmetry bilateral; unsegmented; triploblastic (three germ layers)
2. Body cavity a pseudocoel.
3. Size mostly small; some microscopic; a few a meter or more in length
4. Body vermiform; body wall a syncytialor cellular epidermis with thickened cuticle, sometimes molted; muscular layers mostly of longitudinal fibers; cilia absent in several phyla
5. Digestive system (lacking in acanthocephalans) complete with mouth, enteron, and anus; pharynx muscular and well developed: tubewithin- a-tube arrangement; digestive tract usually only an epithelial tube with no definite muscle layer
6. Circulatory and respiratory organs lacking
7. Excretory system of canals and protonephridia in some; cloaca that receives excretory, reproductive, and digestive products may be present
8. Nervous system of cerebral ganglia or of a circumenteric nerve ring connected to anterior and posterior nerves; sense organs of ciliated pits, papillae, bristles, and some eyespots
9. Reproductive system of gonads and ducts that may be single or double; sexes nearly always separate, with males usually smaller than females; eggs microscopic with shell often containing chitin
10. Development may be direct or within a complicated life history; cleavage mostly mosaic; constancy in number of cells or nuclei common

Internal Features
Underneath the cuticle is a syncytial epidermis, which secretes the cuticle, and bands of subepidermal muscles, some circular, some longitudinal, and some running through the pseudocoel to the visceral organs. The pseudocoel is large, occupying the space between the body wall and the viscera. It is filled with fluid, some of the muscle bands, and a network of mesenchymal ameboid cells.

The digestive system is complete. Some rotifers feed by sweeping minute organic particles or algae toward the mouth by the beating of the coronal cilia. The cilia are able to dispose of larger unsuitable particles. The pharynx (mastax) is fitted with a muscular portion that is equipped with hard jaws (trophi) for sucking in and grinding up food particles. The constantly chewing pharynx is often a distinguishing feature of these tiny animals. Carnivorous species feed on protozoa and small metazoans, which they capture by trapping or grasping. Trappers have a funnel-shaped area around the mouth. When small prey swim into the funnel, the lobes fold inward to capture and hold them until they are drawn into the mouth and pharynx. Hunters have trophi that can be projected and used like forceps to seize prey, bring it back into the pharynx, and then pierce it or break it up so that edible parts can be sucked out and the rest discarded. The salivary and gastric glands are believed to secrete enzymes for extracellular digestion. Absorption occurs in the stomach.

The excretory system typically consists of a pair of protonephridial tubules, each with several flame cells, that empty into a common bladder. The bladder, by pulsating, empties into a cloaca—into which the intestine and oviducts also empty. The fairly rapid pulsation of the protonephridia— one to four times per minute—would indicate that protonephridia are important osmoregulatory organs. Water apparently enters through the mouth rather than across the epidermis; even marine species empty their bladder at frequent intervals.

The nervous system consists of a bilobed brain, dorsal to the mastax, that sends paired nerves to the sense organs, mastax, muscles, and viscera. Sensory organs include paired eyespots (in some species such as Philodina), sensory bristles and papillae, and ciliated pits and dorsal antennae.

Reproduction
Rotifers are dioecious, and males are usually smaller than females. In the class Bdelloidea males are entirely unknown, and in Monogononta they seem to occur only for a few weeks of the year.

The female reproductive system in the Bdelloidea and Monogononta consists of combined ovaries and yolk glands (germovitellaria) and oviducts that open into the cloaca. Yolk is supplied to developing ova by way of flow-through cytoplasmic bridges, rather than as separate yolk cells as in ectolecithal Platythelminthes.

In Bdelloidea (Philodina, for example), all females are parthenogenetic and produce diploid eggs that hatch into diploid females. These females reach maturity in a few days. In class Seisonidea females produce haploid eggs that must be fertilized and that develop into either males or females. In Monogononta, however, females produce two kinds of eggs (Figure 15-4). During most of the year diploid females produce thin-shelled, diploid amictic eggs. These eggs develop parthenogenetically into diploid amictic females. However, such rotifers often live in temporary ponds or streams and are cyclic in their reproductive patterns. Any one of several environmental factors—for example, crowding, diet, or photoperiod (according to species)—may induce amictic eggs to develop into diploid mictic females that will produce thin-shelled haploid mictic eggs. If these eggs are not fertilized, they develop into haploid males. But if fertilized, the eggs develop a thick, resistant shell and become dormant. They survive over winter (“winter eggs”) or until environmental conditions are again suitable, at which time they hatch into diploid females. Dormant eggs are often dispersed by winds or birds, which may account for the peculiar distribution patterns of rotifers.

The male reproductive system includes a single testis and a ciliated sperm duct that runs to a genital pore (males usually lack a cloaca). The end of the sperm duct is specialized as a copulatory organ. Copulation is usually by hypodermic impregnation; the penis can penetrate any part of the female body wall and inject the sperm directly into the pseudocoel.

Females hatch with adult features, needing only a few days’ growth to reach maturity. Males often do not grow and are sexually mature at hatching.

Nuclear Constancy
Most structures in rotifers are syncytial, but nuclei in the various organs are said to show a remarkable constancy in numbers in any given species (eutely). For example, E. Martini (1912) reported that in one species of rotifer he always found 183 nuclei in the brain, 39 in the stomach, 172 in the corona epithelium, and so on. Organisms with eutely show a highly precise genetic control of nuclear division and differentiation. Nuclei are programmed to differentiate and divide an exact number of times, then halt when the appointed number is reached.

Classification of Phylum Rotifera
Class Seisonidea (sy´son-id´e-a) (Gr. seison, earthen vessel, + eidos, form). Marine; elongated form; corona vestigial; sexes similar in size and form; females with pair of ovaries and no vitellaria; single genus (Seison) with two species; epizoic on gills of a crustacean (Nebalia).
Class Bdelloidea (del-oyd´e-a) (Gr. bdella, leech, + eidos, form). Swimming or creeping forms; anterior end retractile; corona usually with pair of trochal discs; males unknown; parthenogenetic; two germovitellaria. Examples: Philodina (Figure 15-3), Rotaria.

Class Monogononta (mon´o-gonon ´ta) (Gr. monos, one, + gonos, primary sex gland). Swimming or sessile forms; single germovitellarium; males reduced in size; eggs of three types (amictic, mictic, dormant). Examples: Asplanchna (Figure 15-2B), Epiphanes.