In the previous tutorial we began our investigation of the clade Gnathifera by examining
the jaw worms of phylum Gnathostomulida. Now let’s talk about phylum Micrognathozoa. As of 2021, only
one species from this phylum has been formally described, Limnognathia maerski, or L. maerski.
However, there are several newly discovered forms that could soon be confirmed as new species.
L. maerski is a microscopic freshwater animal originally discovered in a single spring on Disko
Island, Greenland, in 1994 and formally described in 2000 by Dr. Reinhardt Møbjerg Kristensen,
who was very generous to assist in writing this tutorial. L. maerski was determined to be a
member of its own phylum only after extensive analyses confirmed that it did not fit within
Phylum Gnathostomulida, nor Phylum Rotifera, which we will examine in the next tutorial,
though it shared some aspects of morphology with both clades. As we can see, it is currently
considered to be a sister taxon to Rotifera. L. maerski is one of the smallest known animals,
with the average individual reaching about one tenth of a millimeter in size.
They are triploblastic, bilaterally symmetrical, nonsegmental acoelomates without circulatory
systems or gas exchange structures, meaning they rely entirely on diffusion for gas exchange.
Their bodies are divided into three segments, the two-part head,
an accordion-like thorax, and an abdomen. They are ventrally ciliated with a dorso-ventral
adhesive pad that consists of ten ciliated cells which produce a sticky glue-like substance.
L. maerski also has a number of sensory organs that include stiff monocilliated cells which
form bristle-like structures similar to those found on the jaw worms. The cilia
surrounding the anterior region of the head create a current that helps move food towards the mouth,
while those on the ventral end and around their sides aid in locomotion. Micrognathozoans have
a rather large anterior ganglion that acts somewhat like a brain,
and paired nerve cords that run ventrally along the lower side of their bodies.
They feed primarily on bacteria, blue-green algae, and diatoms. They do so with a complex set of jaws
that is unlike any other in the animal kingdom. These jaws are extremely complicated,
with four sets of jaw-like elements connected by a series of ligaments and striated muscles.
They can extend part of their jaw outside of their mouths while eating, and even articulate
their jaws entirely outside of their mouths to aid in the regurgitation of indigestible particles.
As of early 2022, there has not yet been a single male micrognathozoan discovered.
All specimens collected thus far have been female. Researchers are unsure as to what this means. It
has long been hypothesized that micrognathozoans reproduced exclusively through parthenogenesis,
where an embryo develops directly from an unfertilized egg. This is a natural form of
asexual reproduction common in some invertebrate animals, like nematodes, tardigrades, water fleas,
aphids, and many others. But it is still not clear if this is the only method of reproduction
available to L. maerski. It is possible that L. maerski is a sequential hermaphrodite like
the jaw worms, meaning that it begins as male and develops into a female. However, this possibility
is not yet supported by experimental data. As we are beginning to understand,
there is still lots of research to do on these mysterious animals. They currently comprise the
only monotypic animal phylum in existence, meaning that it contains only one species.
The phylum Placozoa, which we covered earlier in the series, previously held this distinction, but
more species of placozoans have been discovered since. It is rather likely that many more
species of micrognathozoa will also eventually be described. Since their formal description in 2000,
they have been discovered in multiple localities in the sub-Artic Ile de la Possession, as well
as in the Crozet Islands archipelago, where they are locally abundant in lakes and rivers.
Additional specimens have been confirmed within a stream in southern Wales, and a location in
Berkshire, England. It has not yet been confirmed if these samples constitute new species, or if the
singular L. maerski is simply a cosmopolitan species that is found around the world.
Such revelations will only come with additional research, though Dr. Kristensen himself believes
it is likely that a few of these existing forms many constitute truly new species
previously undescribed by science. So with that we will move on from the micrognathozoans over
to phylum Rotifera, so that we can wrap up our examination of the clade Gnathifera.