Ctenophora
(
Bioluminescent Ctenophores
)
Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that marine habitats worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are the largest animals to swim with the help of cilia.
Depending on the species, adult ctenophores range from a few to in size. 186 living species are recognised.
Their bodies consist of a mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped Cydippida with a pair of retractable tentacles that capture prey, the flat, generally combless Platyctenida, and the large-mouthed Beroidae, which prey on other ctenophores.
Almost all ctenophores function as , taking prey ranging from microscopic and to the adults of small ; the exceptions are juveniles of two species, which live as parasites on the on which adults of their species feed.
Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in Lagerstätten (well-preserved fossil beds) dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with being the sister-group to all other multicellular animals ( Porifera sister hypothesis). Other biologists contend that ctenophores diverged earlier than sponges ( Ctenophora sister hypothesis), which themselves appeared before the split between and . Pisani et al. reanalyzed the data and suggested that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species. Follow up analysis by Whelan et al. (2017) yielded further support for the 'Ctenophora sister' hypothesis; the issue remains a matter of taxonomic dispute. Schultz et al. (2023) found irreversible changes in synteny in the sister of the Ctenophora, the Myriazoa, consisting of the rest of the animals.
Etymology
Distinguishing features
Like cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians and ctenophores; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the mesoderm, and as a result some text books classify ctenophores as triploblastic, while others still regard them as diploblastic. The comb jellies have more than 80 different , exceeding the numbers from other groups like placozoans, sponges, cnidarians, and some deep-branching bilaterians.
Ranging from about to in size, ctenophores are the largest non-colonial animals that use cilia as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. The name "ctenophora" means "comb-bearing", from the Greek Language κτείς (stem-form κτεν-) meaning "comb" and the Greek suffix -φορος meaning "carrying".
Since the body of many species is almost radial symmetry, the main axis is Mouth to aboral (from the mouth to the opposite end). However, since only two of the canals near the statocyst terminate in anus pores, ctenophores have no mirror-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks the same as when it started.
The outer layer of the epidermis (outer skin) consists of: sensory cells; cells that secrete mucus, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body, the outer layer contains along the surface of tentacles, used in capturing prey, or cells bearing multiple large cilia for locomotion. The inner layer of the epidermis contains a nerve net, and myoepithelial cells that act as .
The internal cavity forms: a mouth that can usually be closed by muscles; a pharynx ("throat"); a wider area in the center that acts as a stomach; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal. The inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in (internal compartments), that produce eggs or sperm, and photocytes that produce bioluminescence. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores surrounded by double whorls of cilia and connected to the mesoglea.
Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis may help to remove wastes from the mesoglea, and may also help to adjust the animal's buoyancy by pumping water into or out of the mesoglea.
It is uncertain how ctenophores control their buoyancy, but some species rely on osmotic pressure to adapt to water of different densities. Their body fluids are normally as concentration as seawater. If they enter less dense brackish water, the ciliary rosettes may pump this into the mesoglea to maintain buoyancy. Conversely, if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea.
In addition, there is a less organized mesogleal nerve net consisting of single neurites. The largest single sensory feature is the aboral organ (at the opposite end from the mouth), which is underlined with its own nerve net. Did the ctenophore nervous system evolve independently? This organ's main component is a statocyst, a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it often puts some comb rows into reverse, spinning the mouth towards the prey.
The ciliated larvae in cnidarians and bilaterians appear to share an ancient and common origin. The larvae's apical organ is involved in the formation of the nervous system. The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin.
Ctenophore nerve cells and nervous system have distinctive biochemistry. They lack the genes and enzymes required to manufacture neurotransmitters like serotonin, dopamine, nitric oxide, octopamine, noradrenaline, and others, seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing. Monofunctional catalase (CAT), one of the three major families of antioxidant enzymes that target hydrogen peroxide, an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss. They use L-glutamate as a neurotransmitter, and have a distinctively high number of ionotropic glutamate receptors and genes for glutamate synthesis and transport. The genomic content of the nervous system is the smallest of any animal, and could represent the minimum genetic requirements for a functional nervous system.
The last common ancestor (LCA) of the ctenophores was hermaphrodite. Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. There is no metamorphosis. At least three species are known to have evolved separate sexes (dioecy); Ocyropsis crystallina and Ocyropsis maculata in the genus Ocyropsis and Bathocyroe fosteri in the genus Bathocyroe. The are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is generally external, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and most hermaphroditic species are presumed to be self-fertile.
Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally , and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus Beroe, however, the juveniles have large mouths and, like the adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different ecological niche from their parents, only attaining the adult form by a more radical ontogeny after dropping to the sea-floor.
At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features enable ctenophores to increase their populations very quickly. Members of the Lobata and Cydippida have a reproduction form called dissogeny; two sexually mature stages, first as larva and later as juveniles and adults. During their time as larvae they release gametes periodically. After their first reproductive period is over they do not produce more gametes until later. A population of Mertensia ovum in the central Baltic Sea have become Neoteny, and consist solely of sexually mature larvae less than 1.6 mm.
In Mnemiopsis leidyi, nitric oxide (NO) signaling is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms. The mature form of the same species is also able to revert back to the cydippid stage when triggered by environmental stressors. This Benjamin Button-like Jellyfish Can Age in Reverse, From Adult to Juvenile
The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence but by the diffraction as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness. However some significant groups, including all known platyctenids and the cydippid genus Pleurobrachia, are incapable of bioluminescence.
When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlation between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.
In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named photoproteins in cells called photocytes, which are often confined to the meridional canals that underlie the eight comb rows. In the genome of Mnemiopsis leidyi ten genes encode photoproteins. These genes are co-expressed with opsin genes in the developing photocytes of Mnemiopsis leidyi, raising the possibility that light production and light detection may be working together in these animals.
In 2013 Mnemiopsis was recorded in lake Birket Qarun, and in 2014 in lake El Rayan II, both near Faiyum in Egypt, where they were accidentally introduced by the transport of fish (mullet) fry. Though many species prefer brackish waters like estuaries and coastal lagoons in open connection with the sea, this was the first record from an inland environment. Both lakes are saline, with Birket Qarun being hypersaline, and shows that some ctenophores can establish themselves in saline limnic environments without connection to the ocean. In the long run, it is not expected the populations will survive. The two limiting factors in saline lakes are availability of food and a varied diet, and high temperatures during hot summers. Because a parasitic isopod, Livoneca redmanii, was introduced at the same time, it is difficult to say how much of the ecological impact of invasive species is caused by the ctenophore alone.
The two-tentacled "cydippid" Lampea feeds exclusively on , close relatives of sea-squirts that form large chain-like floating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of the cydippid genus Pleurobrachia and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachias long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as and mollusc and crustacean larvae.
It is often difficult to identify the remains of ctenophores in the guts of possible predators as they are broken down quickly, although the combs sometimes remain intact long enough to provide a clue. Chum salmon, Oncorhynchus keta, digest ctenophores 20 times as fast as an equal weight of ; ctenophores can provide the fish with a good diet if there are enough of them around. Some jellyfish and eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists and may have a greater effect on populations than specialist jelly-eaters. Herbivorous fishes deliberately feed on gelatinous zooplankton during blooms in the Red Sea. The larvae of some are parasites on ctenophores, as are the larvae of some that parasitize fish when they reach adulthood.
On the other hand, in the late 1980s the Western Atlantic ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea and Sea of Azov via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. Mnemiopsis is well equipped to invade new territories (although this was not predicted until after it so successfully colonized the Black Sea), as it can breed very rapidly and tolerate a wide range of water temperatures and salinity. The impact was increased by chronic overfishing, and by eutrophication that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal – and above all by the absence of efficient predators on these introduced ctenophores. Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the Mnemiopsis-eating North American ctenophore Beroe ovata,
In the late 1990s Mnemiopsis appeared in the Caspian Sea. Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis there. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North Sea and Baltic Sea.
The Tentaculata are divided into the following eight orders:
The Ediacaran Eoandromeda could putatively represent a comb jelly. It has eightfold symmetry, with eight spiral arms resembling the comblike rows of a ctenophore. If it is indeed ctenophore, it places the group close to the origin of the Bilateria.
The early Cambrian sessile frond-like fossil Stromatoveris, from China's Chengjiang fauna lagerstätte and dated to about , is very similar to Vendobionta of the preceding Ediacaran period. De-Gan Shu, Simon Conway Morris, et al. found on its branches what they considered rows of cilia, used for filter feeding. They suggested that Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system. Other Cambrian fossils that support the idea of ctenophores having evolved from sessile forms are Dinomischus, Daihua, Xianguangia and Siphusauctum which also lived on the seafloor, had organic skeletons and cilia-covered tentacles surrounding their mouth, which have been found by cladistics analysis as members of the ctenophore stem-group
520 million-year-old Cambrian fossils also from Chengjiang in China show a now wholly extinct class of ctenophore, named "Scleroctenophora", that had a complex internal skeleton with long spines. The skeleton also supported eight soft-bodied flaps, which could have been used for swimming and possibly feeding. One form, Thaumactena, had a streamlined body resembling that of Chaetognatha and could have been an agile swimmer.
Walter Garstang in his book Larval Forms and Other Zoological Verses ( Mülleria and the Ctenophore) even expressed a theory that were descended from a neotenic Mülleria larva of a polyclad.
A series of studies that looked at the presence and absence of members of gene families and signalling pathways (e.g., , , the Wnt signaling pathway, and sodium channels) suggest that ctenophores are either sister to Cnidaria, Placozoa, and Bilateria or sister to all other animal phyla.
Several more recent studies comparing complete sequenced genomes of ctenophores with other sequenced animal genomes support ctenophores as sister to all other animals. This position would suggest that neural and muscle cell types either were lost in major animal lineages (e.g., Porifera and Placozoa) or evolved independently in the ctenophore lineage.
Other researchers have argued that the placement of Ctenophora as sister to all other animals is a statistical anomaly caused by the high rate of evolution in ctenophore genomes, and that Porifera is the earliest-diverging animal taxon instead (a "sponge sister" topology).They also have extremely high rates of Mitochondrion evolution,and the smallest known RNA/protein content of the mtDNA genome in animals. As such, the Ctenophora appear to be a basal diploblast clade. In agreement with the latter point, the analysis of a very large sequence alignment at the metazoan taxonomic scale (1,719 proteins totalizing acid positions) in Simion et al. (2017) showed that ctenophores emerge as the second-earliest branching animal lineage, and sponges are sister to all other multicellular animals. Research on mucin genes shows that sponges have never had them while all other animals, including comb jellies, do.
Despite all their differences, ctenophoran neurons share the same foundation as cnidarian neurons after findings shows that peptide-expressing neurons are probably ancestral to chemical neurotransmitters.
The issue with the "rate of evolution" counterargument is that it mainly affects analyses based on the sequence of genes, not those based on gene family presence or synteny, both of which have produced results in support of the "Ctenophora sister" theory. Even with sequence-based analyses, the biases can also be corrected for: Whelan et al. (2017), using such an approach, strongly rejects the hypothesis that sponges are the sister group to all other extant animals and establishes the placement of Ctenophora as the sister group to all other animals, and disagreement with Simion et al. (2017) is explained by methodological problems in analyses in that work. Synteny analysis by Schultz et al. (2023) supports the same result.
Neither ctenophores nor Porifera possess HIF pathways, their genome express only a single type of voltage-gated calcium channel unlike other animals which have three types, and they are the only known animal phyla that lack any true . A few species from other phyla; the pilidium larva, the larva of the phoronid species Phoronopsis harmeri and the acorn worm larva Schizocardium californicum, do not depend on Hox genes in their larval development either, but need them during metamorphosis to reach their adult form. — technical introduction and review of genomic research, evolution, and taxonomy Innexin genes, which code for proteins used for intercellular communication in animals, also appears to have evolved independently in ctenophores. Independent Innexin Radiation Shaped Signaling in Ctenophores
Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead, he found that various cydippid families were more similar to members of other ctenophore orders than to other cydippids. He suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like.
A molecular phylogeny analysis in 2001, using 26 species, including four recently discovered ones, confirmed that the cydippids are not monophyletic and concluded that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps survived the Cretaceous–Paleogene extinction event while other lineages perished. When the analysis was broadened to include representatives of other phyla, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain. A 2017 study corroborates the paraphyly of Cydippida but finds that Lobata is paraphyletic with respect to Cestida.
+ Comparison with other major animal groups
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! Sponges
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! Ctenophores
! Bilateria
Description
Body layers
Feeding, excretion and respiration
Locomotion
Nervous system and senses
Reproduction and development
Colors and bioluminescence
Ecology
Distribution
Prey and predators
Ecological impacts
Taxonomy
Early classification
Modern taxonomy
Evolutionary history
Fossil record
Relationship to other animal groups
Internal phylogeny
See also
Further reading
External links
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