With thousands of species of worms scattered across the planet, a phylum consisting of less than two hundred species of tiny marine worms that few people have ever heard of might seem insignificant. However, arrow worms, as chaetognaths are popularly called, make up for their lack of diversity with sheer numbers (and a voracious appetite).
In fact, chaetognaths are a major component of plankton. Their vertical migrations might be thought of as a biological counterpoint to the tides, part of the rhythm of life.
Chaetognath classification is a work in progress. In What in Darwin’s Name are Chaetognaths?! (Deep Sea News), Kevin Zelnio writes,
“The chaetognaths are an ancient lineage of invertebrates that shares some characteristics with just about every other major invertebrate phylum and has consequently puzzled taxonomists ever since its original description in 1769. Darwin described chaetognaths as ‘remarkable for the obscurity of their affinities’ and they have puzzled a succession of eminent zoologists ever since.”
The above article is a good read if you really want to wade into chaetognath classification. (Warning: It’s complex!)
Many sources state there are about 125 known species of chaetognaths. However, the Catalogue of Life listed 178 species as of June 16, 2012.
Below is a scrollable table that lists the species listed by the Catalogue of Life, with the number of species in each family listed. However, the families don’t necessarily match the families listed by other sources. Chaetognaths are commonly divided into two orders, Aphragmophora and Phragmophora, though the Catalogue of Life currently lists no orders.
Chaetognaths are strictly marine but range around the world. Many individual species appear to have nearly global distributions, spanning the same latitudinal bands in all ocean basins, while others have disjunct ranges; in some cases, the same species apparently occurs at both poles. Paraspadella anops is known only from a single cave in the Bahamas.
Most chaetognaths are planktonic, but about a fifth are benthic (bottom-dwelling).
As invertebrates go, chaetognaths look pretty cool. True to their common name, arrow worms resemble a cross between a worm and an arrow, with a fish’s fins. Or one might describe an arrow worm as a cross between a long, skinny fish and a squid, with its grasping spines resembling the squid’s arms and tentacles. (Then again, the grasping spines aren’t always readily visible.)
The diagram on the right illustrates the basic chaetognath body plan, from its rounded to sub-triangular head to its paddle-like tail. You can get a more detailed view of the head anatomy here. Note that some species have just one pair of fins.
Adding to the cool factor is their color: most chaetognaths are transparent or translucent, and at least two deep sea species are bioluminescent.
The mouth opens into a muscular pharynx, which is surrounded by a ganglionated nerve ring. Nerves extend from the ganglia along the length of the body. Sensory organs include a pair of compound eyes and bristles that detect vibrations.
According to Erik Thuesen, the biggest chaetognath is Pseudosagitta gazellae, a four-inch (105 mm) denizen of the Southern Ocean, ranging to the shores of Antarctica. The smallest is Spadella boucheri - .05 inch (1.3 mm) - which lives between grains of sand around the tropical island of Miyakojima in southern Japan.
However, the Encyclopedia of Life says chaetognaths range in size from about .04 inch to six inches (1 mm to 15 cm).
Chaetognaths lack circulatory, respiratory, and excretory organs. Fluid is circulated within the three body cavities by cilia, while oxygen and carbon dioxide diffuse directly across the body wall.
The arrow worm’s bacteria-derived venom is the same nerve toxin used by the blue-ringed octopus. (However, arrow worms aren’t considered dangerous to humans.)
At least two deep sea species (Caecosagitta macrocephala and Eukrohnia fowleri) are bioluminescent. The luminescent organ is located on the ventral edge of each anterior lateral fin in Caecosagitta macrocephala, whereas that of Eukrohnia fowleri runs across the center of the tail fin on both dorsal and ventral sides.
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, with the tail assisting in propulsion and the fins acting as stabilizers. When swimming, chaetognaths cover their grasping spines with a hood, making them more streamlined.
Vertical movements are facilitated by ammonia-filled vacuolated cells in the trunk, which regulate buoyancy.
Chaetognaths are marine predators that feed on a variety of other animals, including planktonic crustaceans (especially copepods), small fishes, and even other chaetognaths.
After detecting vibrations produced by potential prey, planktonic chaetognaths dart forward and grab their victims with sharp hooks or grasping spines at the front of the body, dispatching them with their teeth. (See a video of a chaetognath feeding.) Many immobilize their prey with neurotoxin. (Studies have suggested this toxin is synthesized by a commensal Vibrio bacterium inhabiting the chaetognath’s head or gut region.)
Some large chaetognaths of the genus Sagitta can capture and consume fishes as large as themselves. Voracious eaters, chaetognaths sometimes choke to death on prey that’s too big to swallow.
Benthic (bottom-dwelling) chaetognaths such as members of the genus Spadella are ambush predators. They affix themselves to a substrate using adhesive secretions, then raise the head and protrude the mouth, flaring the spines around the mouth. Prey that swim within range are then captured with a rapid movement of the head while the rest of the body remains anchored to the seafloor. The spines grasp and manipulate the prey, which is swallowed whole.
Many chaetognaths undergo daily vertical migrations, rising to surface waters at night and sinking downward during the day, possibly to avoid predators and/or follow prey. At least some species display courtship behavior.
Chaetognaths reproduce sexually...but it may not be the type of sex you’re thinking of. Chaetognaths are hermaphrodites; in other words, an individual animal has both male and female sex organs. Better yet, they’re simultaneous hermaphrodites, species that have male and female sex organs at the same time.
Sperm in the tail cavity mature before the eggs in the trunk cavity. Sperm are released outside the body, and cross-fertilization between individuals is typical. However, some species may fertilize themselves. At least one species, Paraspadella gotoi, engages in courtship behavior in what might be referred to as a ritualistic dance before exchanging a sperm packet. (See pictures)
Depending on the species, fertilized eggs may be brooded, deposited on the seafloor, or released into the sea. It takes about forty-eight hours for a zygote (fertilized egg) to hatch into a juvenile. Embryos develop directly into adults, with no larval stage or metamorphosis.
Despite their relative lack of diversity, arrow worms are of enormous ecological importance. Often very abundant - sometimes dominating the biomass in mid-water plankton sampling tows - they are an important link in many marine food webs, serving as an important food source for many fishes, squids, and other marine planktivores. Chaetognaths are typically the most abundant planktonic predators, sometimes accounting for more than ten percent of zooplankton biomass and being outnumbered only by their major prey, copepods.
With their soft bodies, chaetognaths fossilize poorly. However, the fossilized remains of the chitinous grasping apparatus - commonly known as protoconodonts - are quite common in rocks of late Proterozoic to Early Ordovican age. These fossils suggest that chaetognaths originated half a billion years ago during the Cambrian Period.
The oldest chaetognaths that have been formally described (Eognathacantha ercainella and Protosagitta spinosa) come from the Lower Cambrian Maotianshan shales of Yunnan, China. However, some authorities caution that these fossils can’t be positively identified as chaetognaths.
Older paleontology texts commonly cited Amiskwia sagittiformis, from the Middle Cambrian Burgess Shale of British Columbia as the only known fossilized chaetognath. However, it is now believed that another species from the same formation, Oesia disjuncta, is in fact most closely related to living chaetognaths. The living genera that most resemble Oesia are the hyperbenthic (living above the seafloor) Archeterokrohnia and Heterokrohnia, which some authorities consider the most primitive.
A more recent chaetognath, Paucijaculum samamithion, has been described from Illinois’ Mazon Creek formation, which preserves plants and animals that lived during the Pennsylvanian Period.
• What in Darwin’s Name are Chaetognaths?! - Kevin Zelnio, Deep Sea News, June 7, 2010
• Cambrian chaetognaths recognized in Burgess Shale fossils (Hubert Szaniawski, Acta Paleontologica Polonica, 2005)