In the previous tutorial, we discussed some recent studies that tentatively place phylum Chaetognatha
as a sister group to the clade Gnathifera. While this designation may change in the future,
let’s now dig into Gnathifera to discuss the three phyla within this clade. Gnathifera consists of
protostomes with a unique type of cuticularized jaws, and the three phyla are Gnathostomulida,
Micrognathozoa, and Rotifera. In this tutorial we will focus on phylum Gnathostomulida, the jaw
worms, named rather unsurprisingly for their jaws, which are impressive despite being microscopic.
Gnathostomulids are exclusively marine, interstitial animals, meaning they live within
and between individual sand grains, though they are more often, and sometimes exclusively found
in sulfide-rich oxygen-poor sands. It is perhaps their affinity for such an anoxic environment that
kept them hidden from discovery for so long. The first individual jaw worm was discovered in 1928
by Dr. Adolf Remane, though they were not formally described in the scientific community until 1956
by Dr. Peter Ax, and they were not recognized as a separate phylum until 1969 by Dr. Rupert Riedl.
Since their official recognition and as of 2021, 102 species have been formally recognized,
72 of which are documented in studies that have been authored or co-authored by Dr. Wolfgang
Sterrer, who has graciously lent his assistance in the writing of this tutorial. If you find the
information within to be fascinating, I highly suggest visiting the links in the description
to learn more about his research and to join him in his quest of “Global Worming”.
Gnathostomulida, technically the “lesser jaw worms,” but often referred to as the
jaw worms for short, are microscopic, unsegmented, entirely monociliated, marine worm-like creatures.
They are slender, sometimes thread-like in appearance,
and most range from half a millimeter to a millimeter in length, with a diameter of between
four and ten hundredths of a millimeter. Most are colorless, white, or yellow,
but a few species display brilliant splotches of crimson, orange, or pink pigmentation.
These organisms are separated into the orders Filospermoidea and Bursovaginoidea. Organisms from
these two orders can generally be morphologically determined based upon the characteristics of
their rostrum, which is a pre-oral extension of the head. Notice that in Filospermoidea
the rostrum is elongated and pointed, while in Bursovaginoidea the rostrum is somewhat bulbous
and head-like, due to a shortening of the “neck” region. Notice that members of Bursovaginoidea
also bear numerous sensory cirri, which are the hair-like structures around their heads.
Like the flatworms of phylum Platyhelminthes, the jaw worms have a ciliated epidermis, but unlike
the flatworms, the jaw worms have monociliated cells, meaning they have only one cilium per cell,
and a host of unique sensory cilia that appear to be their only sensory structures.
Their nervous system is located within their epithelial tissue, and generally consists
of a frontal ganglia, operating essentially as a brain, and ventral longitudinal nerves
that run the length of the body, though this can differ from species to species.
All Gnathostomulids are triploblastic, bilaterally symmetrical, unsegmented acoelomates. Their
bodies are essentially divided into a “head” and “trunk” region. They lack a circulatory system or
any specialized structures for gas exchange, meaning they, like many microscopic animals,
rely entirely on diffusion for gas exchange. Their intestinal system is located ventrally,
beginning with the mouth that is always behind the tip of the rostrum.
The mouth is bordered by the muscular pharynx that gives rise to the esophagus, which widens into the
uniform single-layered cilia-lined intestine. It is this pharynx that gives rise to the most
distinctive feature of this phylum, the jaws. The cross-striated musculature of the pharynx
is complex enough to allow the articulated movement of the basal plate and jaws.
These complex cuticular hard parts consist of a set of forceps-like jaws and a singular ventral
basal plate. The jaws often contain tiny, sharp teeth and come in three general types.
The relatively simple compact type, which is often embedded within the pharyngeal wall,
the open lamellar type, which is embedded more posteriorly in the pharyngeal walls,
and the fused lamellar type which has an additional major muscle attachment site.
These imposing jaws are used in conjunction with a basal plate on the lower surface of the mouth
that bears a comb-like structure which is used to scrape smaller organisms off of the grains of sand
that make up the jaw worm habitat. Gnathostomulids are hermaphroditic,
with relatively complex male and female reproductive systems. They are simultaneous
hermaphrodites, meaning they have both male and female reproductive organs at the same time. They
fertilize each other through copulation, and young develop directly into miniature
versions of their parents without any type of larval stage. Many species are cosmopolitan,
meaning they are found all around the world. In fact, jaw worms have been found in anoxic sediment
from every continent except Antarctica. The very discovery of the diversity of jaw worms in 1970
led to the realization that beneath the oxidized layer of marine sandy bottoms
there existed a sulfide system complete with microscopic animals largely unknown to science.
There still remains much we do not know about these systems,
including the total diversity of jaw worms. It is possible that many hundreds
of species are waiting to be discovered, patiently grazing on chemoautotrophic
sulfur-oxidizing bacteria in environments once thought to be too toxic for life.
But we will have to wait and see what is revealed by researchers in this field over the coming
decades. So that covers the jaw worms, let’s now continue to move through the clade Gnathifera.