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!
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.
viewed nearly face-on.
viewed from side.
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.
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.
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.
rotifer general anatomy.
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.
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.
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.
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".
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.
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.
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.
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)
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.
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.
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.
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.