The rotifers constitute a phylum of animals that are characterized by presenting an elongated body, which has at its anterior end a double ring of cilia that, when vibrating, give the impression of being rotating.
The name of this group comes from the union of two words, broken (wheel) and fera (wear). They were first described in 1798 by the French naturalist Georges Cuvier and cover approximately 2,000 species.
This type of animal can be planktonic or benthic and, sometimes, they can establish colonies among them. They have a defense mechanism against the adverse conditions of the very curious environment: they can form resistance cysts, which can last for a long time in hostile conditions..
Article index
Rotifers are small eukaryotic, multicellular animals (some even microscopic). Its DNA is packaged within the cell nucleus conforming to chromosomes and is made up of cells that have undergone a specialization process that fulfill specific functions..
During its embryonic development, the presence of the three germ layers is appreciated: ectoderm, endoderm and mesoderm, which is why they are called tripoblastic animals. Various specialized fabrics are generated from each layer.
The type of symmetry that these animals have is bilateral, since they are made up of two exactly equal halves.
The members of this phylum are dioecious, that is, there are female individuals and male individuals. It is important to mention that in some species the sexual dimorphism is quite marked, since the males tend to be smaller than the females..
The taxonomic classification of rotifers is as follows:
Domain: Eukarya.
Animalia Kingdom.
Subkingdom: Eumetazoa.
Edge: Rotiferous.
Animals belonging to the rotiferous phylum generally have a tubular and cylindrical shape. Their body is transparent and they measure between 1 mm and 3 mm.
The body of rotifers is covered by a kind of cuticle whose functions include maintaining the shape of the body. Likewise, this cuticle is responsible for protecting the animal, through some protrusions such as thorns or tubers..
In addition to the cuticle, the body wall is also made up of a layer of musculature and a basement membrane made up of syncytial epithelium. In turn, it presents a kind of cavity called a pseudocele, in which there is a fluid that contains cells called amebocytes. Likewise, the body is divided into three regions: head, trunk and foot..
One of the most characteristic elements of the head is the crown. This presents the oral area, where the mouth opens and an area around the mouth in which there are two rings of cilia.
In current species, the area where the mouth is located is triangular in shape and the cilia are distributed around in a double ring.
It is important to note that the cilia are in constant motion due to the currents of water. That movement makes it look like they have a spinning wheel. Because of this, this complex is known as the rotator apparatus..
In the head you can also find other structures, such as the eyes, and a type of palpiform extensions that have specific functions..
The trunk represents the largest percentage of the animal's body. Specialists consider that it is the most important part of the body, since, among other things, it contains all the organs.
Like the rest of the body, it is covered by a cuticle, which in this specific area is even more developed. Here a structure called loriga appears, which is a thick intracellular layer that serves as protection.
Likewise, in the trunk there are some sensory organs in the form of antennae. These can be located in a dorsal or lateral position.
It is the terminal portion of the body of the rotifers. Its shape and structure is not standard, since it depends on the lifestyle of the animal. Rotifers are known to include some who are swimmers and others who have a sessile lifestyle..
In the case of free-living rotifers, the foot is practically non-existent. In contrast, in sessile rotifers, the foot divides into two structures called caudal rims. The ducts of some glands that secrete a substance of mucous consistency whose function is to promote the fixation of the animal to the substrate flow into these..
The digestive system of rotifers is complete. It begins with the mouth, which opens into an oral cavity. Immediately afterwards, there is a small tube known as the buccal tube, which connects directly with the pharynx, which in rotifers is called the mastax. This tube has a series of cilia.
The mast is followed by a short esophagus that communicates with the stomach. Later there is an intestine that is also short in length, which ends in the anus.
It is worth noting that the digestive system has attached glands. In the first place, at the level of the mastax there are the salivary glands that secrete digestive enzymes and in the stomach are the gastric glands that also secrete enzymes.
Rotifers present a nervous system made up mainly of nerve ganglia and fibers that emerge from these ganglia..
In the central area it presents a main ganglion that is bilobed. From this emerge nerve fibers that innervate the different structures of the head. Other ganglia that make up the nervous system are the mastax ganglion, the geniculate ganglia, and the anterior and posterior ganglion..
It also has pharyngeal nerves, some motor fibers and two stomatogastric cords.
Rotifers do not have a proper circulatory system. The fluid that circulates in these animals is the pseudocoelomatic fluid. As there are no blood vessels or anything like that, this fluid circulates with the help of body movement and muscle contractions.
The excretory system of rotifers is quite rudimentary. It is made up of two collecting tubes into which several pairs of nephridiums flow. Later, these tubes unite to form an excretory vesicle, the duct of which leads directly into the cloaca of the animal..
Rotifers are organisms that are widely distributed throughout the world geography. Due to their characteristics, they need to be in habitats where there is a great availability of water..
These can be found in both freshwater and saltwater ecosystems. Likewise, members of the rotifer phylum can be found on all continents and, generally, there are no species that are specific to a geographic location. On the contrary, it is common to find the same species on different continents.
It is important to emphasize that, despite their small size, rotifers constitute an element of transcendental importance in the ecosystems in which they are found..
This is because they constitute one of the links in the different trophic chains. In them they occupy the place of consumers, since they are known predators of the environments in which they develop.
Rotifers are heterotrophic animals. This means that they are not able to synthesize their own nutrients. Due to this, they must feed on other living beings, detritus and also eggs.
Likewise, depending on the characteristics of the crown and the mast of the rotifer, different ways of feeding can be found..
First of all, there are the rotifers that are reptorian, which what they do to feed is scrape the food.
On the other hand, free-floating rotifers feed on food particles that remain suspended in the water. These types of animals use their cilia to create water currents and take advantage of redirecting those currents towards the mouth opening and in this way ingesting the available food.
In another vein, there is a group of rotifers that have a symbiote lifestyle. They live in a symbiotic relationship with certain crustaceans. These rotifers feed on detritus, that is, on the remains released by the crustacean to which they remain fixed. Likewise, they also feed on their eggs.
In rotifers there are two types of reproduction: sexual and asexual. The first one involves the union or fusion of sexual gametes, one female and the other male. While in asexual reproduction, the intervention of organisms of both sexes is not necessary, since it does not involve the union of sexual cells..
The most observed asexual reproduction mechanism in rotifers is parthenogenesis. It is worth mentioning that some of the species in which this occurs are those in which there is no presence of male specimens..
There are species of rotifers whose fixed reproduction mechanism is parthenogenesis, while there are others in which the climatic seasons determine whether or not this occurs..
Basically, parthenogenesis consists of the generation of a new individual from a female sex cell (ovum). What happens here is that the ovum begins to undergo successive divisions until it becomes an adult individual..
Now, this process is not so simple, but it has certain peculiarities. During the summer time, the eggs that are produced by the females are known as amícticos, while the eggs that are produced during the winter are called micticos..
Amyctic eggs develop through parthenogenesis and always give rise to female individuals. However, this does not always happen this way, since when some environmental change is generated, mimic eggs are originated, from which females are formed. The peculiarity is that these females lay eggs, which, if they are not fertilized, give rise to individuals of the male sex.
On the contrary, if these eggs are fertilized, they will form highly resistant eggs to adverse environmental conditions, which can remain dormant for a long period of time..
This process involves copulation between a female and a male specimen. In this process, the male introduces his copulatory organ into the female's cloaca so that fertilization can occur..
When copulation itself does not occur, the male simply injects the female with sperm in various parts of her anatomy, although this process can be hampered by the thickness and resistance of the cuticle that covers these animals..
Once fertilization occurs, there may be two cases: the female expels the eggs that will develop outside her body, or they are kept inside.
Rotifers generally have direct development. This means that the individuals that hatch from the eggs present characteristics similar to those of the adult specimens..
Yet No Comments