Phylum Rotifera

Colonial rotifers from a lake in Germany

Since I didn't have much to say about the last phylum, I decided to move on to the next phylum on my list: Rotifera. This is actually one of my favorites of the invertebrate phyla.

Rotifera comes from the Greek for "wheel bearer," and reflects one of the common names: wheel animals. This rather unusual-sounding common name comes from the ciliated corona that surrounds the mouth and is used to capture food. Due to cilia action, the corona appear to rotate like wheels when viewed under a microscope. There are approximately 1850 species known, with the vast majority of those species being found in freshwater. These animals are actually a common sight in pond water samples, especially those with a fair amount of debris. Samples of lake and pond average between forty and five hundred rotifers per liter. Most rotifers are free living, with parasitism being unusual in this phylum. Though some species are colonial, most are not. They aren't long-lived animals, with five weeks being an unusually long lifespan and one or two weeks being more typical.

The mastax is seen in the middle left

The corona is not the only interesting-looking moving structure found in rotifers. When viewed under a microscope, there is something that looks rather a lot like a beating heart. However, this is actually a unique muscular pharynx known as a mastax containing structures known as trophi. Trophi are used for grinding food, sucking food in, or grabbing prey. Interestingly enough, the trophi are so varied in shape between species that they play a very important part in species identification.

Rotifers were one part of the former Phylum Aschelminthes which has since been broken down into several smaller groups. One characteristics that almost all of the aschelminths share is a developmental trait known as eutely. Eutely is when cells stop dividing at a certain point in development and any further growth in the size of the animal happens because the cells that are present increase in size.

Looping movement in some rotifers

Movement in free-living species often involves swiming using the cilia in the corona. Rotifers have a foot with up to four toes. Glands opening in the toes secrete a substance that certain species will use to attach to surfaces temporarily. Feeding will often occur while the individual is attached, with coronal cilia being used to sweep food into the waiting mouth. Other species are constantly swimming and never attach to a surface.

The cementing substance that is secreted by the toes can also be used in the form of movement seen at left, where a combination of muscular contractions and attachment at the toes will draw the animal across a surface. This method is often called looping due to the body's resemblance to a U-shaped loop. This form of movement does not require swimming via the corona.

A member of Class Bdelloidea

Rotifer reproduction is one of the major characteristics to break the phylum into classes. Class Seisonidea includes only parasitic species  and all species reproduce sexually. Class Bdelloidea has only free-living species and all of those species reproduce asexually, with all known individuals being female. This class is also capable of forming into a cyst capable of withstanding long-term extreme conditions. Class Monogononta is the largest of the three and all of the species included have a rather interesting life cycle. Usually, reproduction is asexual via of the creation of amictic eggs that contain only the genes of the mother. This reproduction is very quick, with populations capable of doubling in less than a day. When conditions do not favor this, some of the eggs being produced are a bit different. They hatch into females that produce only mictic eggs that are like the eggs we produce: having only half of the normal amount of genetic information. If not fertilized, the egg will hatch into a small male. Males cannot feed and their only purpose in life is to fertilize mictic eggs. The fertilized eggs then go into a resting stage which can handle the harsh conditions that triggered sexual reproduction to occur. Overall, the vast majority of rotifers that you will find are females due to the characteristics of these three classes.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or copyright free: one, two, three, four.