Bdelloid Rotifers: Female Filter Feeders

Text and photography copyright Thomas Webster 2003. All rights reserved.

Bdelloid rotifer in the Family Philodinidae.

I was struggling with a bad case of "writer's block" in my attempt to write this article. I finally decided, "Oh to heck with it!" and went out to my birdbath to make sure the drip system was still working. Just for the heck of it I gathered a water sample from the birdbath, dropped a few drops on a depression slide, and tossed the slide under the 'scope. The first thing that struck me was, "Where're all the free-swimming protozoa?". It didn't take me long to find out. Three drops of birdbath water contained more than 50 bdelloid rotifers! Good grief! With this many filter-feeders in the birdbath no wonder there weren't many free-swimming protozoa!

Rotifers are some of the smallest animals, if not the smallest animals, to be found. Rotifers are true animals (Kingdom: Animalia) as they are multicellular in structure. Most rotifers are composed of an average of about 1,000 cells. Rotifers occur in both freshwater and saltwater environments in a variety of habitats. Many rotifers inhabit moist soils surviving in the thin film of water surrounding soil particles. Rotifers have even been found in the waters of Antarctica! Rotifers occupy their own Phylum, Rotifera, and there are currently over 2,000 recognized species contained in 3 classes and 120 genera. There are approximately 360 species of bdelloid rotifers occupying the Class, Bdelloidea. Oddly enough, there are no known male bdelloid rotifers, which we'll discuss later on in this article.

Coronae viewed nearly face-on.
Coronae viewed from side.

Rotifers were one of the first organisms observed by Antony Van Leeuwenhoek in the late 1600's. He named these organisms, "wheel animalcules", in response to the swirling, rotating cilia contained on the coronae. In actuality, the cilia do not rotate but wave back and forth in a sequential pattern giving rise to the illusion of rotation. The cilia are arranged around the coronae in double rows contained in an inner ring and an outer ring ("trochal discs"). I don't care how many times you watch a rotifer filter-feeding, the illusion is always of the cilia rotating around the coronae. The term "bdelloid" means "leach-like". Bdelloid rotifers acquired their name from their leach-like appearance and the manner in which they can locomote. Rotifers can also swim with the aid of the ciliated coronae, using the coronae to pull them through the water much like an airplane propeller.

Bdelloid rotifer general anatomy.

Bdelloid rotifers are divided into three main regions: head, trunk, and foot. The head carries the coronae with the mouth opening between the coronae. Actually, the coronae do not represent the anterior end of the head. The coronae can be retracted to reveal the "rostrum", a sort of a snout. The mouth empties into a muscular pharynx called the "mastax". The mastax contains chitenous jaws, "trophi", that crush food particles before passing them on into the stomach.

All digestive and reproductive processes take place in the trunk. The trunk is dominated by the large stomach which terminates in a short intestine with an anal opening. On either side of the stomach lie the "germovitellaria". These are the gonads that produce the eggs ("germarium") and the yolk ("vitellaria"). The eggs produced are largish and oval in shape. This particular species of bdelloid rotifer has 2 germovitellaria but there are species that have but one germovitellaria.

Posterior to the trunk is the "foot". The foot terminates in 4 claws that the bdelloid rotifer uses to attach itself temporarily to the substrate. Just above the 4 claws are two obvious spurs. The toes are also used for locomotion. When traveling from one location to another, bdelloid rotifers anchor their toes to the substrate, extend their bodies fully, anchor the rostrum to the substrate, and then release the toes and pull the rest of the body forward to the rostrum. In this manner bdelloids look like tiny inch worms. Besides using the claws to anchor themselves to the substrate, rotifers can also exude a sticky substance from the toes to act as a glue to glue themselves to the substrate.

The coronae may be retracted to reveal the "rostrum", or snout, of the rotifer. The rostrum is still able to grap onto the substrate allowing the rotifer to inch along like an "inchworm".
The body of a bdelloid rotifer is covered by a semi-rigid, "lorica", or shell. The lorica is divided by 16 rings, or "annuli", to allow flexibility of the rotifer's body. The annuli act as pleats allowing the rotifer to contract within itself.
The head region contains a pair of red eyespots just in front of the mastax. The eyespots help the rotifer distinguish between day and night and help to regulate the rotifers internal clock.
Bdelloid rotifers are not strictly filter feeders and can spend a considerable amount of time foraging for food particles, too. Often, the cilia on the trochal discs are used to scrape bacteria and small algae from the substrate.
Also contained on the head is a single protruding sensory antenna. The antenna can only be seen when the rotifer is viewed in side profile. Other authors have referred to this appendage as a siphon. After having viewed hundreds of rotifer videos I, too, believe this appendage is a siphon. In a couple of my videos you can see cilia at the end of the siphon. (Updated 12/2004)
Sometimes Scyphidia is mistaken for a rotifer. Scyphidia are actually single-celled Protists in the Family Vorticellidae. Although filter feeders themselves, Scyphidia are composed of a single cell and are approximately 1/2 to 1/3 the size of a rotifer.

Probably the most interesting fact about bdelloid rotifers are the manner in which they reproduce. There are no known bdelloid rotifer males. All bdelloid rotifers are females. This is not entirely unheard of in the animal kingdom. There are many, many insect populations that consist entirely of females. Even larger animals, such as some Teiid lizards (Whiptail Lizards), have no known males.

Reproduction by bdelloid rotifers is accomplished through a process known as obligate parthenogenesis (parthenogenesis in which only females are produced). Because (for most species) parthenogenesis produces exact copies of the reproducing female, genetic diversity suffers and any genetic diversity must be induced by mutation. There is danger involved in this type of reproduction. Because of a lack of genetic diversity, the animal populations are very prone to react negatively to sudden changes in their environment. However, there are advantages to parthenogenesis in that only one individual animal is needed to start a new population. Also, parthenogenetic females do not need to expend energy looking for a mate and can take this unused energy and apply it to produce more robust eggs. Often times you will find parthenogenetic species rapidly able to occupy habitats that would be considered marginal for other similar species.

So, how does parthenogenesis work? View the interactive Macromedia Flash® movie, to the right, for the explanation.

Bdelloid rotifers are very valuable to the aquatic environments they inhabit. Through efficient filter-feeding and foraging activity, bdelloid rotifers keep the populations of other aquatic microorganisms in check. Without efficient predators, an aquatic environment could become overrun rather quickly. Bdelloid rotifer populations can run as high as 1,000/liter of pond water making bdelloid rotifers one of the most prolific and numerous of the Metazoans. Bdelloid rotifers are the second-most numerous form of multi-cellular life on our planet. Only nematode worms are more abundant. Additionally, where they occur in an aquatic environment, bdelloid rotifers comprise an important food source for fish fry and fish larvae. A newly hatched fish's initial diet is comprised almost entirely of microorganisms. At 1,000 rotifers/liter of pond water, bdelloid rotifers significantly impact the successful survival of many fish species.
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