Arrow worms, also known as chaetognaths, are a group of free-living, predatory marine invertebrates that belong to the phylum Chaetognatha. They have distinct head, trunk, and tail segments, with paired lateral fins and a single caudal fin. Their name originated from their transparent or translucent, arrow-shaped bodies, which range between 1 millimeter to over 12 centimeters. However, species in colder waters are generally larger.
They are a major part of the planktonic community and can be found in all marine environments, from shallow tide pools to the deep sea and polar regions. Most arrow worms are nektonic, actively swimming through the water column, while about 20% of species are benthic, attaching to substrates like algae and rocks.
Despite their relatively low species diversity, currently, around 133 species are classified under two orders, Phragmophora and Aphragmophora.
They are typically 0.1 to 4.7 in (2 to 120 mm) long.
Their bodies are bilaterally symmetrical, transparent or translucent, and streamlined into an arrow or dart shape. The body is divided into three distinct segments – head, trunk, and tail, separated by transverse septa and covered by a thin, flexible cuticle. The cuticle is discontinuous, interspersed with secretory cells of the epidermis. A basement membrane lies sandwiched between the body wall and the epidermal layer.
On each side of the head, there are four to fourteen hooked, chitinized spines that assist in grasping prey. These spines are made of α-chitin and flank, a hollow vestibule that houses the mouth.
The trunk features one or two pairs of lateral fins, which resemble the fin rays of fish, though they are not homologous. Unlike fish fins, these fins are composed of a thick basement membrane that originates from the epidermis. Additionally, a single caudal fin covers the tail segment. The epidermal cells covering each fin are elongated.
The body cavity, or coelom, which most likely originated from enterocoelic cavities, is lined by a membranous peritoneum and is divided into compartments: one on each side of the trunk, with additional compartments in the head and tail.
Approximately 80% of an arrow worm’s body consists of four quadrants of dorsolateral and ventrolateral longitudinal muscles, which aid in their movement.
Although arrow worms lack distinct circulatory and respiratory units, they possess other integral organ systems.
The mouth, lined with one or two rows of tiny teeth, leads to a muscular pharynx equipped with glands that secrete digestive and lubricating fluids. The pharynx connects to the intestine, which serves as the primary site for digestion and runs along the trunk to the anus. The entire digestive tract is lined with glandular and absorptive cells, and the action of cilia facilitates the movement of food particles. Metabolic waste is expelled either through the anus or directly through the skin.
The nervous system is simple and characterized by a large ganglionated nerve ring (cerebral ganglion) surrounding the pharynx. A pair of circumenteric connective nerves originating from the rear part of the cerebral ganglion meets a ventral ganglion in the trunk epidermis. This ventral ganglion gives rise to multiple pairs of nerves, forming the subepidermal nerve plexus.
The brain has two distinct regions: the anterior neuropil domain and the posterior neuropil domain. While the former most likely controls the head muscles, the latter is linked to the corona ciliata (chemoreceptor organ) and the eyes.
There are two ovaries attached to the sides of the trunk, while the testes are located in the tail cavity. A sperm duct or vas deferens connects the testes to a sperm-storing structure called the seminal vesicle.
The first known description of these invertebrates was published in the 1770s by Dutch naturalist Martinus Slabber, who also coined the term ‘arrow worms’. The other name, chaetognaths, was coined in 1856 by Rudolf Leuckart.
In 1844, Charles Darwin spotted and took notes on these animals during his great voyage of the Beagle. A year later, German zoologist August David Krohn published an anatomical description of Sagitta bipunctata, the first species to be described in this group.
The modern systematics of the phylum Chaetognatha began in 1911 with Ritter-Záhony, followed by the Japanese zoologist Takasi Tokioka (1965) and Robert Bieri (1991). Currently, there are two orders (as classified by Tokioka), Phragmophora and Aphragmophora, containing 133 species under 9 families containing over 26 genera. While members of Phragmophora possess ventral transverse muscle bands, those in the group Aphragmophora lack them.
Arrow worms were initially considered deuterostomes (Superphylum Deuterostomia) by Lynn Margulis and K. V. Schwartz because they share similar embryonic characteristics with them. However, molecular phylogeny data revealed they are actually protostomes (clade Protostomia) and share similarities with groups like nematodes (Phylum Nematoda).
Although arrow worms are soft-bodied and rarely fossilize, several fossil specimens have been discovered, providing valuable insights into their evolutionary history. They originated in the Cambrian Period, as evidenced by Lower Cambrian fossils of Eognathacantha ercainella and Protosagitta spinosa found in the Maotianshan shales of China. Additionally, fossils of Capinatator praetermissus from the Middle Cambrian Burgess Shale in British Columbia further highlight their ancient presence. By the Early Cambrian, it is believed that arrow worms were already an integral part of the marine food web. More recent fossil discoveries include the Cambrian stem-group genus Timorebestia and Paucijaculum samamithion, unearthed from the Mazon Creek fossil beds in Illinois.
Arrow worms are abundant in all oceans, ranging from the poles to the tropics and even in brackishwater environments. They are found in hydrothermal vents, seagrass beds, marine caves, and seafloors.
About one-fifth of the total species are benthic, while others live a little above the bottom (epibenthic or meiobenthic). They are also found in rock pools and neritic waters (shallow waters extending from the intertidal zone to the continental slope).
They are voracious carnivores, consuming copepods, euphausiids, fish larvae, cladocerans, amphipods, appendicularians, and other arrow worms. Some species, like Eukrohnia hamata, feed on algae and detritus.
Arrow worms often undergo daily vertical migrations of the water column. They rise to the surface at night for foraging and sink deeper during the day to avoid predators.
Arrow worms typically swim in short bursts by undulating their bodies in a dorsoventral motion. They use their lateral fins to maintain stability, while the tail fin aids in forward propulsion. These worms can also glide by sinking gently, followed by a quick movement of their fins.
Benthic species remain attached to substrates, whereas pelagic arrow worms propel themselves forward by alternately contracting the longitudinal muscles on their right and left sides. In contrast, planktonic arrow worms often need to swim and hop continuously to maintain their position within the water column.
These invertebrates are stealthy ambush predators, patiently waiting for unsuspecting prey to pass by. They rely on sensory bristles and the corona ciliata to detect movement, then propel themselves forward with a sudden, powerful stroke of the tail fin. As the prey comes into range, the arrow worm quickly encloses it by covering its grasping spines with a hood, effectively trapping the prey in a cage before swallowing it whole.
Sometimes, they also paralyze their prey by injecting neurotoxins, like tetrodotoxin, obtained from symbiotic association with bacteria of the genus Vibrio.
Arrow worms are protandrous (male organs mature first) and hermaphroditic (having both testis and ovary in a single individual). Immature sperm move from the testes to the tail cavity, where they mature before swimming to the seminal vesicle through the vas deferens. In the seminal vesicle, the sperm are packaged into a capsule known as a spermatophore.
During mating, the seminal vesicle ruptures, and the individual transfers the spermatophore to the neck of its partner. In some species, such as those in the family Spadellidae, mating may be preceded by courtship rituals, like dancing.
Once inside the partner’s body, the sperm escape from the spermatophore and swim down to a pair of small pores located in front of the tail, where the oviducts open. Fertilization occurs internally within the oviduct, where the sperm meets the already-developed ovum. While cross-fertilization between different individuals is the most common, some arrow worms also self-fertilize.
The fertilized eggs are primarily planktonic and are released into the water, although in some cases, they are deposited on the seafloor (as in Ferosagitta hispida) or brooded in pouches or sacs near the tail (as in members of the genus Eukrohnia). Most arrow worms usually die after spawning.
The eggs undergo radial, holoblastic cleavage and hatch directly into juveniles within 1 to 3 days, bypassing metamorphosis or a distinct larval stage.
The arrow worm Sagitta friderici survived 15 months, which is the longest lifespan recorded to date. Generally, arrow worms in colder waters have longer lifespans than their counterparts in tropical regions.
They are often preyed upon by fishes, squids, and sea birds.
Arrow worms possess ammonia-filled vacuolated cells in their trunks that help maintain their stability at different depths of the water column.
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!
