Chaetognaths are hermaphroditic, and may undergo reciprocal, nonreciprocal, or self-fertilization. Chaetognaths may be found in marine and some estuarine environments throughout the world, including polar and tropical regions.
Brusca and Brusca, ; Margulis and Chapman, ; Shapiro, Chaetognaths are mainly planktonic organisms in marine and estuarine environments. About a fifth of the total species are benthic, some living just above the deep ocean floor. They are often found in great numbers, particularly in mid-water and neritic waters, and may be found in rock pools or associated with certain oceanic currents.
Brusca and Brusca, ; Margulis and Chapman, ; Ramel, Since first being recorded in , chaetognaths have been classified many ways, including as molluscs , arthropods , and nematodes. They were elevated to their own phylum by the German zoologist Rudolf Leuckart in , a decision which is supported by numerous morphological and embryological autapomorphies, indicated below.
The monophyly of this phylum is widely accepted, and molecular phylogenetic analyses of 28S ribosomal DNA have supported the monophyly of its two constituent orders, Phragmophora and Aphragmophora without the traditionally included family Pterosagittidae. Brusca and Brusca, ; Leuckart, ; Littlewood, et al. Chaetognaths were originally considered deuterostomes , based on embryological observations.
All molecular phylogenetic analyses performed thus far, however, strongly support the placement of Chaetognatha within protostome animals.
The precise phylogenetic placement of Chaetognatha within Protostomata remains somewhat confused today, as molecular analyses have suggested Chaetognatha is a sister group to all other protostomes, Spiralia , or phylum Priapulida within the superphylum Ecdysozoa. Their potential identity as a basal sister group to such a large metazoan group makes further examination of their phylogenetic placement essential, as it has the potential to greatly contribute to our understanding of morphological and developmental characters found in some of the earliest bilaterian organisms.
Brands, ; Brusca and Brusca, ; Dunn, et al. Chaetognaths range from 1 mm to 12 cm in length and are typically transparent, although some deep-water species may be orange in color carotenoid pigmentation , and phragmophorids may be opaque, due to their musculature.
They are bilaterally symmetrical and have long, streamlined bodies, which may be divided into head, trunk, and tail regions. They have paired lateral fins and a single tail fin. The mouth is located ventrally on the head, and is set into a vestibule; this structure is typically associated with grasping spines or hooks, located laterally to the mouth, as well as teeth, which are in located at the front of the mouth. A hood anterolateral body wall fold may be pulled over the head to enclose the vestibule.
Chaetognaths are covered in a thin, flexible cuticle on top of the epidermis. Epidermal cells are mainly squamous and have interlocking margins; they may be stratified. Epidermal cells covering the fins are elongated and the cells lining the vestibule are columnar rather than squamous. The cuticle is not continuous and, where it is not present, there are many secretory cells in the epidermis.
There is a basement membrane present between the epidermis and body wall; the body wall is made up of four quadrants of dorsolateral and ventrolateral longitudinal muscles. The body cavities are most likely derived from enterocoelic cavities, which form during development.
The body cavity has a tripartite arrangement, with a head cavity protocoel, reduced in space by the cephalic musculature , and paired trunk and tail coeloms with dorsal and ventral longitudinal mesenteries, which correspond to the mesocoel and matcoel, respectively.
Transverse septa separate the body regions. The body fluid has a variety of cells and cell-like structures, although their functions are largely unknown.
The fluid-filled coeloms, body wall, basement membrane, and cuticle all provide support to the body. They do not have circulatory, respiratory, or excretory organs; gases are diffused across the body wall and fluid transport is via cilial action within the body cavities. A few species of deep sea chaetognaths, including Eukrohnia fowleri and Caecosagitta macrocephala , are bioluminescent.
Thuesen, et al. Chaetognaths are hermaphroditic. Cross-fertilization is most common, although some species will self-fertilize. Fertilization is typically internal and eggs may be released into the water, deposited on the sea floor or other substrate, or brooded in pouches near the tail.
Cleavage is radial, holoblastic, and equal, leading to a coeloblastula. Development is direct and accomplished quickly, typically from zygote to juvenile within 48 hours. Brusca and Brusca, ; Margulis and Chapman, Chaetognaths may undergo reciprocal, nonreciprocal, or self-fertilization.
Some benthic species have been documented performing a mating "dance," with an individual depositing balls of sperm onto a mate. Brusca and Brusca, ; Goto and Yoshida, Chaetognaths have paired ovaries located in their trunks and paired testes located in their tails. Sperm mature before eggs which makes self-fertilization less likely , and are stored in coelomic cavities within the tail until they are released in clusters outside the body via a pair of seminal vesicles.
Ovaries have oviducts, which lead to genital pores located near the trunk-tail junction. In populations of at least a few species, breeding occurs twice a year, and hatching occurs from April to June and late September to December typically fewer hatchlings. Brusca and Brusca, ; Ghirardelli, ; Ramel, ; Zo, Outside of a few species such as members of genus Eukronhnia that brood their young until they are ready to swim, chaetognaths exhibit no parental investment beyond the production of gametes.
Brusca and Brusca, Most cold water chaetognaths have a longer life expectancy than those in tropical waters, two years versus six weeks, respectively. Many species within this phylum are known to undergo daily vertical migrations, rising to the surface at night to follow prey and sinking during the day, which provides protection from predators.
These worms have ammonia-filled vacuolated cells in their trunks, which help them to regulate their depth in the water column. Pelagic chaetognaths move by contracting the longitudinal muscles of their right and left sides alternately, creating forward, darting motions. Fins do not appear to aid in locomotion, instead acting as stabilizers. Chaetognaths have a central nervous system with a large cerebral ganglion, dorsal to the pharynx. Additional ganglia, which serve muscles and sensory organs of the head, arise from this structure.
They also have a pair of circumenteric connective nerves, emerging from the rear of the cerebral ganglion and extending posterioventrally to meet in a ventral ganglion in the epidermis of the trunk. This ganglion controls swimming motion and also gives rise to many pairs of nerves, which create a subepidermal nerve plexus. Chaetognaths have a pair of compound eyes below the epidermis, on the head.
They are made up of five inverted pigment-cup ocelli, one large ocellus directed laterally and four smaller ones directed medially; this gives these worms a nearly uninterrupted field of vision. Their eyes do not typically have lenses and likely do not form images, but are used for light reception and body orientation. The ocelli also contain ciliated receptor cells. Bundles of sperm are transferred from one individual to another, and fertilized eggs are subsequently released into the sea.
The life expectancy of chaetognaths ranges from just six weeks in tropical waters to over two years in colder waters, such as the Arctic. Arrow Worms — Phylum Chaetognatha Chaetognaths, also known as arrow worms, are found in the open waters of every ocean. They all have long, narrow bodies without legs.
All worms also have tissues, organs, and organ systems. Worms have bilateral symmetry. Unlike sponges or cnidarians, worms have distinct head and tail ends. Skip to content How do arrow worms find and capture prey?
Do arrow worms have symmetry? Characteristics of Chaetognatha: Bilaterally symmetrical and vermiform. Body has more than two cell layers, tissues and organs. What do arrow worms feed on?
Arrow worms eat plankton, including tiny crustaceans, fish larvae, and other arrow worms. What size are most arrow worms? Do arrow worms have a complete digestive tract? Why do some chaetognaths undergo daily vertical migrations? What do arrow worms look like? Does Chaetognatha have notochord? Are arrow worms protostomes? Are Chaetognatha segmented?
Are arrow worms protostomes or deuterostomes? Why is the phylogenetic placement of Chaetognaths uncertain? Arrow worms that live near the surface are clear, which helps them avoid predators. The intestines of species that live in middle depths often are yellow or red because they eat prey of those colors. Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Popular articles What does the arrow worms eat?
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