Moving along in our journey through Spiralia, we arrive at the phylum Chaetognatha,
or bristle-jaws. The animals that make up this phylum are commonly known as “arrow worms.” They
are famous, or perhaps infamous, for a number of reasons. First, they are morphologically distinct,
voracious predators with unique methods of hunting and feeding.
Second, they play an important ecological role that is often overlooked or outright completely
ignored in most biology courses, though they are commonly addressed as important members of the
food web in more advanced planktology courses. And three, they are some of the most phylogenetically
confounding creatures on our planet. Let’s start with their physical features and well-known
appetites, and then work towards their taxonomy. Arrow worms are bilaterally symmetrical coelomates
with transparent, or translucent bodies. They are dart or arrow-shaped worm-like animals
partially protected by a collagenous cuticle, capable of short bursts of speed, hence the name,
“arrow worms”. Their bodies can be divided into three distinct regions:
the head, trunk, and tail. These body regions are separated both externally and internally.
Their head region terminates with the neck, under which the animal bears a mouth that is
surrounded by sets of grasping spines which are used in capturing prey. This mouth is located on
the ventral side of the head, which along with the entire head, can be enclosed by a protective hood.
Like the flatworms, they lack any type of discrete gas exchange or urinary systems, and instead rely
on diffusion achieved by the epidermis and gut system. However, like most bilaterians, they have
a complete gut and a ventral anus. They have a set of cup-shaped eyes and a peculiar central nervous
system consisting of a dorsal brain and ganglionic rings encircling different parts of the gut
system. The first ring is confined to the head and contains paired vestibular and esophageal ganglia,
as well as an unpaired suboesophageal ganglion surrounding the foregut. The second ring encloses
the midgut and connects the brain with the ventral ganglion, which regulates the twitchy swimming
motion which is typical for chaetognaths. All known species of arrow-worms are hermaphroditic.
They have their testes in their tail region, and their ovaries are located at the posterior end
of their trunk. Some species are bioluminescent, some live deep in the ocean on benthic sediments,
and some inhabit shallow water sediments such as sand. Some have demonstrated unique mating dances,
and others like Spadella cephaloptera, spend most of the day attached to seagrass leaves.
Chaetognaths carry a variety of sense organs in their thick, multilayered epidermis. They have
eyes, but the biological role of vision for them is not yet known. What is well known, however,
is that they rely mainly on mechanical vibrations for information about their environment and,
thus, can hunt in total darkness. They do this by sensing vibrations through their
fans of mechanoreceptive hairs, and at least one species, Spadella cephaloptera,
will attack a metal or glass probe vibrating at an appropriate frequency and amplitude,
with the same ferocity as it will attack a copepod, its primary food source.
Speaking of food sources, arrow worms have been documented to swallow their prey whole
and consume anything they can fit in their mouths. After sensing prey,
an arrow worm will suddenly shoot forward and use its chitinous spines and teeth to capture it.
In addition to their imposing jaws, quick bursts of speed, and sensory fans, some species of arrow
worms have been found to harbor the extremely potent neurotoxin known as tetrodotoxin. However,
it is not yet clear if the arrow worms are capable of injecting small doses of the toxin from their
chaetae, or if the bacteria are simply symbionts that live in and around the arrow worm spines.
The neurotoxin seems to be produced by Vibrio alginolyticus, the very same pathogen infamously
present in the bodies of pufferfish that makes them a deadly meal if not properly prepared.
In warm waters, arrow worms are often the most abundant planktonic predators, sometimes
accounting for more than 10% of zooplankton biomass. With such a high abundance, it is
perhaps unsurprising that they play an important ecological role. Like many planktonic organisms,
chaetognaths migrate up and down in the water column. They come to the surface to feed at night,
then retire to deeper waters as the sun rises. Unlike other planktonic organisms, though,
some chaetognaths migrate annually. They live in the surface waters during the winter
but sink to deeper waters in summer. As such, they play an important role in the vertical oceanic
carbon flux. Essentially, their migratory life cycle transports carbon up and down in the ocean,
while their fecal pellets transport organic matter downward in the form of “marine snow.”
Thus, they, like most planktonic organisms, are an important part of the oceanic carbon cycle and
even play a role in global carbon sequestration. In fact, it is possible that they have been an
important ecological component of our planet for over 500 million years. The oldest fossil records
of arrow worms known so far date back to the Cambrian period, 541 to 485 million years ago,
making them some of the oldest members of Bilateria, or Metazoa. Interestingly enough,
these specimens that appear in the so-called Burgess shale are extremely morphologically
similar to modern arrow worms, meaning they have changed very little over their half-a-billion-year
tenure on our planet. Currently, over 132 extant species have been discovered,
though some taxonomists put that number closer to 190. This number is very likely to increase
over the coming years, not least because the species diversity of benthic spadellids
is far away from being entirely resolved. Lastly, let’s discuss arrow worm classification.
Looking at our diagram of animal phyla we can see that the arrow worms sit within Spiralia. This,
like the diagram itself, is based on the book The Invertebrate Tree of Life,
by Gonzalo Giribet and Gregory D. Edgecombe. These researchers have placed the arrow worms there
due in part to phylogenomic analyses, namely an analysis of Hox genes across the protostomes,
and the morphological similarities of the grasping spines of the arrow worms to the jaw-like
structures of the gnathiferans, which we will discuss momentarily. So we have them placed here,
as a sister group to the gnathiferans. However, they weren’t always placed like this.
Part of the notoriety of these animals comes from the fact that for quite some time
scientists were unsure which of their traits were simply derived or apomorphic characters, those
that arose since their common ancestry, and which were truly ancestral traits that could be used to
classify them. Charles Darwin himself highlighted two interesting aspects of the arrow worms.
First, their ecological abundance, which we have already covered, and second, their odd
anatomy which makes them difficult to place. Due in part to their apparent three-part body
plan and their post-anal tail, when they were first discovered, they were placed
close to phylum Chordata, and subsequent studies placed them as a phylum within Deuterostomia.
However, their unique method of development cast doubt on that placement. Unlike all deuterostomes,
the blastopore of the arrow worms closes up entirely during embryogenesis, and the anus
forms later in development from the posterior end. This caused researchers to then consider the arrow
worms as members of Bilateria, but as neither protostomes nor deuterostomes. This implied that
chaetognaths were most likely an early offshoot of bilaterians rooting themselves in the tree of life
close by protostomes and deuterostomes. But this thinking continued to be revised.
In 1996, they were considered a sister group to the nematodes. In 1998, to members of Ecdysozoa,
or as a sister group to all protostomes. However, more recent studies have found these placements
to be inaccurate. The most recent studies support the placement of the arrow worms within spiralia,
as a sister group to Gnathifera, though admittedly there are researchers that
still disagree with this placement. Only time will tell if the evidence of subsequent studies
will unify the field around this designation. Such studies are still ongoing, and it is likely that
we will learn much more about these enigmatic creatures in the years to come. But, for now,
let’s leave the arrow worms behind. Let’s make our way into the phyla within Gnathifera.