The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory
marine worm
Any worm that lives in a marine environment is considered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida (segmented worms), Chaetognatha, Hemichordata, and Phoronida. For a ...
s that are a major component of
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
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 .
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
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...
that do not belong to either
Ecdysozoa or
Lophotrochozoa
Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, brachi ...
.
Anatomy
![MEB back](https://upload.wikimedia.org/wikipedia/commons/d/d8/MEB_back.png)
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.
Some species are also reported to be omnivores, feeding on algae and detritus.
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the
fin ray
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...
s of fish, with which they are not
homologous, however: unlike those of vertebrates, these are composed of a thickened
basement membrane extending from the
epidermis. An additional caudal fin covers the post-anal tail.
[ Two chaetognath species, '']Caecosagitta macrocephala
''Caecosagitta macrocephala'' is a deep sea marine chaetognath that is distributed in meso- and bathypelagic layers. It has a very wide distribution that ranges from the Subantarctic to Subarctic Ocean. Miyamoto, H., Machida, R. & Nishida, S. (2 ...
'' and ''Eukrohnia fowleri
''Eukrohnia fowleri'' is a deep-sea marine arrow worm. It is the only known bioluminescent member of the genus '' Eukrohnia'', and one of the two known species of bioluminescent arrow worms, the other being the distantly related ''Caecosagitta ma ...
'', have bioluminescent organs on their fins.
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. Some species are known to use the neurotoxin tetrodotoxin
Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an order that includes pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Although tetrodotoxin was discovere ...
to subdue prey.
The body cavity is lined by peritoneum
The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...
, 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.
The mouth opens into a muscular pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
, 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
In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false.
In medicine, t ...
near the anterior end.[ 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. 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.
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
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
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.[
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
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike project ...
.
Reproduction
All species are hermaphroditic
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have s ...
, carrying both eggs
Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
and sperm. Each animal possesses a pair of testes
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...
within the tail, and a pair of ovaries
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
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
The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen.
The vesicles are 5†...
where they are packaged into a spermatophore
A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophores ...
.[(Sexually and Asexually)
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 ]oviduct
The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
s, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.[
The eggs are planktonic, develop in marsupial sacs or attached to ]alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
e, and hatch into miniature versions of the adult, without a well-defined larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
...
l stage.[
]
Classification
Chaetognaths are traditionally classed as deuterostomes
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...
by embryologists. Lynn Margulis
Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...
and K. V. Schwartz place chaetognaths in the deuterostomes in their ''Five Kingdom'' classification. Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith
Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford.
His research has led to discov ...
places them in the protostomes in his ''Six Kingdom'' classification. The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoa
Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, brachi ...
n. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. 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.
Studies of arrow worms' nervous systems suggests they should be placed within the protostomes. According to 2017 and 2019 papers, chaetognaths appear related to gnathiferans.
Fossil record
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described. Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales
The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
of Yunnan, China
Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...
(''Eognathacantha ercainella'' Chen & Huang and ''Protosagitta spinosa'' Hu) and the Middle Cambrian Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fo ...
of British Columbia ('' Oesia disjuncta'' Walcott), 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
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
. It is now thought that protoconodont elements (e.g., ''Protohertzina
''Protohertzina'' is a genus of conodonts ( protoconodonts or paraconodonts) or, possibly, Chaetognaths, found at the beginning of the Cambrian explosion.
Protoconodonts are an extinct taxonomic group of conodonts.Zooproblematica and mollusca ...
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.
Infection by giant viruses
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a giant virus
A giant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria. All known giant viruses belong to the phylum '' Nucleocytoviricota''.
Description
While the exact criteria as defined in the ...
(''Meelsvirus'') infecting ''Adhesisagitta hispida''; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped. 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'', and in 2003, for bacteria infecting '' Paraspadella gotoi'', were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication.[Roxane-Marie Barthélémy, Eric Faure, Taichiro Goto]
''Serendipitous Discovery in a Marine Invertebrate (Phylum Chaetognatha) of the Longest Giant Viruses Reported till Date''.
In: ''Biology'', 2019
/ref> The viral species infecting ''P. gotoi'', whose maximum length is 3.1 ÎĽm, 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
(George) Adrian Horridge FRS FAA (born 12 December 1927) is an Australian neurobiologist and professor at Australian National University.
Life
Horridge was born in Sheffield, England, to George William Horridge (1897-1981) and Olive (1899-1995 ...
, the first author of the 1967 article). On a photograph, one of the viruses ''M. horridgei'', although truncated, is 3.9 ÎĽm long, corresponding to about twice the length of the bacteria Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
. Many ribosomes
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infect metazoans
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
are exceptionally rare.
Notes
References
External links
Image of ''Pseudosagitta gazellae'' with a krill in its gut from the Tasmanian Aquaculture and Fisheries Institute
Chaetognatha of the World
– last retrieved December 13, 2006
* Eric Fauré, Roxane-Marie Barthélémy
Specific mitochondrial ss-tRNAs in phylum Chaetognatha
In: Journal of Entomology and Zoology Studies 7(3), April 2019, pp. 304-315
hal-02130653
{{Authority control
Planktology
Animal phyla
Extant Cambrian first appearances
Taxa named by Karl Grobben