Chaetognaths are an important food source for many fishes, squids, and other marine planktivores (Margulis and Chapman 2010). They are themselves predatory on a variety of other animals, including planktonic crustaceans (especially copepods), small fishes, and even other other chaetognaths (Brusca and Brusca 2003; Margulis and Chapman 2010).
Benthic (bottom-dwelling) chaetognaths such as Spadella are ambush predators. They affix themselves to a substrate using adhesive secretions, raise the head, and protrude the mouth, flaring the spines around the mouth. Potential prey swimming nearby are detected (probably by vibration receptors) and then captured by a rapid movement of the head while the rest of the body remains attached to the substratum. The spines grasp and manipulate the prey, which is swallowed whole. (Brusca and Brusca 2003)
Planktonic chaetognaths dart forward to grab prey with their grasping spines. Many, perhaps most, species inject their prey with a potent neurotoxin and studies have suggested that this toxin is synthesized by a commensal Vibrio bacterium inhabiting the chaetognath's head or gut region. Some large Sagitta chaetognaths can capture and consume fishes as large as themselves. (Brusca and Brusca 2003)
Given its ecological importance, the phylum Chaetognatha is surprisingly poorly known. Chaetognaths (commonly known as "arrow worms") are marine predators that typically locate their prey by detecting vibrations produced by copepods and other zooplankton, then use sharp hooks and teeth at the front of the body to grab their victims and immobilize them with neurotoxins. Chaetognaths, most of which are distinctly transparent, are important predators in many marine food webs. Around 120 to 125 species of chaetognaths are known. Most are planktonic, but a small number of species are benthic or live just above the ocean floor. Although species diversity is low, chaetognaths can be very abundant, sometimes dominating the biomass in mid-water plankton sampling tows. Many chaetognaths undergo daily vertical migrations, rising to surface waters at night and sinking downward during the day, possibly to avoid predators. These vertical movements are facilitated by ammonia-filled vacuolated cells in the trunk, which regulate buoyancy. (Brusca and Brusca 2003; Margulis and Chapman 2010; Jennings et al. 2010) At least one chaetognath species (the cosmopolitan Caecosagitta macrocephala, which is generally found below 700 meters) is bioluminescent (Haddock and Case 1994).
Development of chaetognaths is direct, with no larval stage or metamorphosis. Embryo development from zygote (fertilized egg) stage to hatching as a juvenile is about 48 hours. (Brusca and Brusca 2003)
Chaetognaths may occur in large numbers in polar waters, tropical lagoons, and the deep sea. Although most species are planktonic, about a fifth are benthic (bottom-dwelling). (Margulis and Chapman 2010).
Jennings et al. (2010) used mitochondrial cytochrome oxidase I (COI) DNA sequences to examine the correspondence between species delineations based on often very difficult morphological characters and those suggested by COI. They analyzed 52 specimens of 14 nominal species, collected mainly from the Atlantic Ocean. COI was highly successful at discriminating described species of chaetognaths across the phylum (i.e., sequences clustered by species in all cases). The authors note that additional geographic sampling could help clarify the complex biogeographic history of chaetognaths. Many putative species have nearly global distributions, spanning the same latitudinal bands in all ocean basins, while others present disjunct ranges, in some cases with the same species apparently found at both poles. (Jennings et al. 2010).
Chaetognaths lack circulatory, respiratory, and excretory organs. Cilia circulate fluid within each of the three body cavities, transporting nutrients and dissolved waste. Oxygen and carbon dioxide diffuse directly across the body wall. (Margulis and Chapman 2010)
Chaetognaths are hermaphrodites and reproduce sexually. 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, although some species may self-fertilize. Fertilization is internal and depending on the species fertilized eggs may be brooded, deposited on the seafloor, or released into the ocean. Embryos undergo direct development, i.e., they develop into tiny adults, with no larval stage. (Margulis and Chapman 2010).
Chaetognaths range in size from 1 mm to 15 cm (Margulis and Chapman 2009).
The phylogenetic placement of the phylum Chaetognatha with respect to other animal phyla has been a subject of longstanding controversy and this controversy has continued in the era of modern methods of phylogenetic analysis using molecular markers (e.g., Paps et al. 2009 and references therein; Jennings et al. 2010 and references therein). As of 2010, evidence seems to indicate that the Chaetognatha are in the superclade Ecdysozoa, along with the arthropods, nematodes, and several smaller phyla.
The Chaetognatha /kiːˈtɒɡnəθə/ or chaetognaths /ˈkiːtɒɡnæθs/ (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.1 to 4.7 in).
There are more than 120 modern species assigned to over 20 genera.[3] Despite the limited diversity of species, the number of individuals is large.[4]
Arrow worms are usually considered a type of protostome that do not belong to either Ecdysozoa or Lophotrochozoa.
Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. The body is divided into a distinct head, trunk, and tail. There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. All chaetognaths are carnivorous, preying on other planktonic animals.[5] Some species are also reported to be omnivores, feeding on algae and detritus.[6]
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish, with which they are not homologous. Unlike those of vertebrates, these lateral fins are composed of a thickened basement membrane extending from the epidermis. An additional caudal fin covers the post-anal tail.[5] Two chaetognath species, Caecosagitta macrocephala and Eukrohnia fowleri, have bioluminescent organs on their fins.[7][8]
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering.[9] Some species are known to use the neurotoxin tetrodotoxin to subdue prey.[10]
The body cavity is lined by peritoneum, and therefore represents a true coelom, and is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail, all separated completely by septa. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no circulatory, excretory or respiratory systems.[11]
The mouth opens into a muscular pharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end.[5] Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. Eukrohniid species possess an oil vacuole closely associated with the gut. This organ contains wax esters which may assist reproduction and growth outside of the production season for Eukrohnia hamata in Arctic seas.[12] Owing to the position of the oil vacuole in the center of the tractus, the organ may also have implications for buoyancy, trim and locomotion.[13]
The nervous system is reasonably simple, consisting of a ganglionated nerve ring surrounding the pharynx. The dorsal ganglion is the largest, but nerves extend from all the ganglia along the length of the body. Chaetognaths have two compound eyes, each consisting of a number of pigment-cup ocelli fused together. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the lateral line in fish. An additional, curved, band of sensory bristles lies over the head and neck.[5]
The arrow worm rhabdomeres are derived from microtubules 20 nm long and 50 nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a cilium.[14]
All species are hermaphroditic, carrying both eggs and sperm.[4] Each animal possesses a pair of testes within the tail, and a pair of ovaries in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, and then swim through a short duct to a seminal vesicle where they are packaged into a spermatophore.[5]
During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.[5]
The eggs are planktonic, develop in marsupial sacs or attached to algae, and hatch into miniature versions of the adult, without a well-defined larval stage.[5][15]
Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.[16] Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification.[17] The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan.[18] Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.[19] This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.[20] Studies of arrow worms' nervous systems suggests they should be placed within the protostomes.[21][22] According to 2017 and 2019 papers, chaetognaths appear related to gnathiferans.[23][24]
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.[1] Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha ercainella Chen & Huang[25] and Protosagitta spinosa Hu[26]) and the Middle Cambrian Burgess Shale of British Columbia (Oesia disjuncta Walcott[27]), a view challenged by Conway Morris (2009). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, have been shown to be dental elements of vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then.[28]
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a giant virus (Meelsvirus) infecting Adhesisagitta hispida; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped.[29] In 2019, reanalysis of other previous studies has shown that structures that were taken in 1967 for bristles present on the surface of the species Spadella cephaloptera,[30] and in 2003, for bacteria infecting Paraspadella gotoi,[31] were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication.[32] The viral species infecting P. gotoi, whose maximum length is 3.1 μm, has been named Klothovirus casanovai (Klotho being the Greek name for one of the three Fates whose attribute was a spindle, and casanovai, in tribute to Pr J.-P. Casanova who devoted a large part of his scientific life to the study of chaetognaths). The other species has been named Megaklothovirus horridgei (in tribute to Adrian Horridge, the first author of the 1967 article). On a photograph, one of the viruses M. horridgei, although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteria Escherichia coli. Many ribosomes are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infect metazoans are exceptionally rare.
The Chaetognatha /kiːˈtɒɡnəθə/ or chaetognaths /ˈkiːtɒɡnæθs/ (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.1 to 4.7 in).
There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large.
Arrow worms are usually considered a type of protostome that do not belong to either Ecdysozoa or Lophotrochozoa.
