After covering two of the phyla within the clade Gnathifera,
it’s time to cover the third and final. The so-called “wheel animals” of phylum Rotifera
account for over 2,000 named mostly free-living species, and an additional
1,200 species of obligate intestinal parasites known as “acanthocephalans”. In this tutorial,
we will focus on some of the free-living, non-parasitic rotifers found all over the world.
Though many people have never heard of them, it’s quite likely that everyone watching this video
has interacted with them in some capacity, numerous times in their lives.
Rotifers have been found in nearly every body of water around the globe, from backyard pools,
to ponds, to puddles, to lakes and streams, and even individual water droplets. They’ve even been
found to form the nucleus necessary for a drop of rain to form. In fact, they’re so common that
simply drinking unchlorinated water is likely to result in the ingestion of a few rotifers. Don’t
worry though, free-living rotifers are harmless and have no deleterious impacts on humans.
Rotifers are microscopic pseudocoelomates. They demonstrate eutely, meaning that they cease
undergoing mitosis once they complete their embryonic development. Some species, such as
Epiphanes senta, also demonstrate cell constancy. All adults of this species always have the same
number of cellular nuclei, 959. Other species vary considerably in their final adult cell count.
This is a bit different than other eutelic animals, like the nematode C. elegans which have
a fixed number of somatic cells but continue to divide their germline, also known as sex cells, as
adults. This is not the case in rotifers, as they seem to stop cell division even in the germline.
Rotifers are considered to be non-segmented, though they do have distinct body regions,
those being the head, which bears their ciliated corona, a trunk,
which holds most of their internal organs, and a posterior foot.
Their bodies lack cilia except for the corona. It is the rhythmic beating of the cilia within the
corona that gives these animals their name. Cilia on the corona beat in synchronous succession,
giving the appearance of a revolving wheel, or pair of wheels. The beating of these cilia aids
in both locomotion and feeding. The mouth is located in the corona on the mid-ventral side.
Within the mouth, rotifers have a muscular mastax and trophi, which form the complex jaw,
characteristic of all animals within clade Gnathifera, though the rotiferan jaw is distinctly
different from those of Micrognathozoans and Gnathostomulids, as well as Chaetognaths.
For this anatomical example we will examine members of the genus Philodina, since they are
perhaps the best known and most commonly studied genus of bdelloid rotifers. Though the trunk
region is elongated in this genus, it is sac-like in others, such as those of the genus Asplanchna.
The trunk contains most of the animal’s organs and is occasionally superficially ringed, which gives
the appearance of segmentation. All rotifers have a fibrous layer that can be quite thick
in some species, and forms a case-like lorica which is often arranged in plates or rings.
The rotifer foot is generally narrower and usually ends in one to four toes, which are
used in attachment to surfaces, since they contain pedal glands that secrete an adhesive material
used by both creeping and sessile forms. Rotifers are able to move by creeping or
“leach-like” movements aided by their foot, or by swimming with the coronal cilia.
The pseudocoelom of rotifers is quite large, and occupies the space between the body wall and
internal organs. It is filled with fluid, occasional muscle bands, and a network of amoeboid
cells. Most rotifers have a complete digestive system. Once food is captured and ground up in
their muscular jaws, it is covered in gastric juices and other digestive enzymes secreted from
the salivary and digestive glands. Absorption of nutrients occurs primarily in the stomach,
a large holding chamber that aids in digestion. The rotifer excretory system is comprised of a
pair of protonephridial tubules that contain several flame cells, much like the free-living
members of phylum Platyhelminthes. These tubules empty into a common bladder that contracts and
empties waste into a bladder-like structure that fills up and expels its contents. Both the anus
and oviducts also empty into the cloaca when present, though not all species have a cloaca.
Rotifers have a rather large bilobed brain, at least in proportion to their body, and this
brain is dorsal to the jaws. They are capable of mechanoreception, through the bristles in their
corona, which allow them to detect and respond to certain kinds of stimuli, most notably touch,
sound and changes in pressure or posture. There are also photoreceptors, in the form
of up to five pit-eyes that respond to light, and chemoreceptors in the form of sensory pits
located in the head region. And that covers some general
features that are shared by all members of phylum Rotifera. But there is quite a bit more to discuss
in the way of the taxonomy within this clade, so let’s move forward and dig a little deeper.