In the previous tutorial we introduced the phylum Rotifera. Rotifers are, by far,
the largest clade of Gnathifera in terms of species count. They are microscopic animals,
that can range in size between four one-hundredths of a millimeter and three millimeters in length,
but most are between a tenth and a half a millimeter. They exist in a wide range of bizarre
shapes that correlate with their mode of life. There are the globular, sac-like, “floaters”.
There are the elongated, worm-like “creepers” and “swimmers”. And there are the sessile types,
which are often very diverse in their morphology. Rotifers are a cosmopolitan group, meaning some
species of rotifers can be found all over the world, though they commonly inhabit freshwater.
Their polygeny is considered to be controversial by some phylogeneticists, and it is possible
that some viewers of this tutorial may disagree with how they are placed in the animal kingdom,
as the sister group to Micrognathozoa. In some phylogenies, rotifers are confined to only the
clades Bdelloidea and Monogononta, represented collectively here as Eurotatoria. In other
phylogenies, the rotifers instead consist of three classes: Bdelloidea, Monogononta, and Seisonidea,
but exclude Acanthocephala. In still other classifications, the Rotifers are split
into the class Hemirotatoria, which includes Bdelloidea, Acanthocephala, and Seisonidea, and
the class Eurotatoria which includes Monogononta. There are other proposed classifications as well.
Though their phylogeny is currently in flux, and may be revised by the time you view this tutorial,
as of the time of its writing, nearly all modern molecular, mitogenomic, and phylogenomic research
has concluded that the phylum Rotifera consists of four major clades: Bdelloidea,
Monogononta, Seisonidea, and Acanthocephala. Clade Bdelloidea is considered to be a class
by some phylogenists and a subclass by others. It contains the bdelloid rotifers
that are found in habitats all over the world. It includes one famous individual discovered
in the Siberian permafrost that is estimated to be at least 24,000 years old. Once thawed,
the rotifer, a member of the genus Adineta, began to reproduce through parthenogenesis.
Some members of this clade have survived temperatures as cold as -272 degrees Celsius
before being successfully revived, and others have survived going into space and back. They do this
through a form of cryptobiosis, by significantly reducing their metabolic activity to an
undetectable level by drying themselves out. The bdelloid rotifers are the most easily recognized
rotifers and have a conserved body plan. Clade Seisonidea is made up of only four genera.
They are the marine rotifers with vestigial or reduced coronae. They have only been found
to live in the gills of marine crustaceans and are peculiar among the rotifers in that males
and females are both present and equal in size. Clade Monogononta consists of 1,570 described
species and is the most diverse rotifer group. They are capable of surviving for long periods
of time as dormant embryos but are unable to enter cryptobiosis like the bdelloids. They are
primarily free-swimming, though some are sessile as adults. About 25 of these sessile species
form aggregations sometimes referred to as “colonies” that can reach up to 3,500 individuals.
Finally, clade Acanthocephala, commonly referred to as “spiny-headed worms” was, for many years,
considered to be a sister phylum to Rotifera, though more recent studies
place them firmly within Rotifera. We will discuss them in more detail in the next tutorial
since they are such fascinating creatures. Rotifers are dioecious, meaning they have
either male or female reproductive organs. They reproduce sexually, parthenogenetically,
or both, which we can call cyclically parthenogenetic. They are sexually dimorphic,
and the females are always larger than the males. In some species, this size difference is small,
but in others the female may be up to thirty times the size of the male. In cyclical
parthenogenetic species, males may be present only at certain times of the year or absent altogether.
Some rotifers have a complex life history. Females in the rotifer clade Monogononta,
such as Brachionus plicatilis, produce two kinds of eggs. For most of the year,
they produce parthenogenetically, meaning that an embryo will develop from an unfertilized egg.
This is represented in the diagram by this loop. Potentially, this process can continue
indefinitely, with amictic females producing amictic eggs that produce amictic females, and
so on. However, since many monogonontan rotifers live in temporary ponds, streams, or even puddles
after a rainstorm, or in habitats that freeze or have otherwise unfavorable conditions,
any number of environmental factors, such as crowding, diet, or species-specific interactions
may result in a mixis stimulus that gives rise to a mictic female, which is
represented in the beginning of the other loop. These mictic females then produce haploid eggs,
meaning they have only half of the genetic information necessary for development.
Remember from our study of meiosis in the biology series that this is how sexual reproduction works.
An animal gets roughly half of its genetic information from an egg cell and the other
half from a sperm cell. If the rotifer egg is unfertilized, it develops into a haploid male.
These diminutive males are short-lived and generally lack a lot of the morphology of the
diploid females. The haploid males produce haploid sperm. If a haploid sperm comes into contact with
a haploid egg, the egg becomes a fertilized mictic egg that develops a thick resistant shell and goes
dormant. These dormant embryos survive winters or other adverse conditions, and are referred to
as “diapausing eggs.” These eggs can survive for extended periods of time and are often dispersed
by wind, water, and birds, until they land in an area with favorable conditions.
Most females hatch with adult features, and are considered to be
“direct developers”. It often takes females a day or so to reach sexual maturity,
while males do not increase in size after hatching and are born sexually mature.
One clade, the Gnesiotrocha, includes species that produce larvae that are morphologically
much different from the adults. These larvae are free-swimming while the adults are sessile.
Let’s now return to something that was mentioned in passing a moment ago
that may have seemed a bit confusing. We said that in some species of rotifers,
males may be absent all together. However, this is not just a few species. As of 2022,
for the over 450 species of bdelloid rotifers, not a single male has ever been reliably documented.
Females of all these species reproduce exclusively through parthenogenesis. Though the clade has been
extensively studied in the lab and field, neither males, nor hermaphrodites, nor meiosis have ever
been reported for a single bdelloid species. First, this would seem to break our rule on
characteristics of animals. In the introduction to this series, we said that in order to qualify
as an animal an organism must be eukaryotic, heterotrophic, capable of aerobic respiration,
sexually reproducing, motile, and grow from a blastula. However, this does not mean that
bdelloid rotifers are not animals. Just like how snakes are considered to be tetrapods,
meaning four-legged creatures, even though they clearly lack legs,
the solution is in their evolutionary history. Remember that the best definition of an animal is,
“an organism descended from the first organism that qualifies as an animal.” Bdelloid rotifers
do match that definition, however, the manner in which a clade that appears to be entirely
dependent on a form of asexual reproduction was able to diversify into over 450 extant species
is currently considered to be an open question, worthy of research. And in fact, some clues are
beginning to emerge. It has recently been reported that there are genomic signatures associated with
sexual reproduction in bdelloids. Perhaps someone watching this tutorial right now will aid in
furthering this intriguing line of inquiry. For now, that wraps up our discussion of the
free-living rotifers. Let’s move forward and finish off this phylum by discussing,
as promised, the parasitic spiny-headed worms from the Rotiferan clade Acanthocephala.