Ctenophora - WikiHQ

Ctenophora (; ) or ctenophores (), commonly known as comb jellies, is a basal phylum of marine invertebrate animals notable for their unique aquatic locomotion using groups of cilia (commonly referred to as "combs"). They inhabit marine habitats worldwide, with 186 recognised living species.

Ctenophores are the largest animals to swim with cilia, with adult ctenophores range from a few millimeters to in size depending on the species. Their bodies consist of a mass of jelly (mesoglea) with a two-cell-thick layer on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with a pair of retractable tentacles that capture prey; the flat, generally combless platyctenids; and the large-mouthed beroids, which prey on other ctenophores.

Almost all ctenophores are predators, feeding on prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps 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 sponges 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 cnidarians and bilaterians. 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. However, follow up analysis by Whelan et al. (2017) yielded 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.

thumb|Spotted comb jelly

Etymology

The New Latin

Distinguishing features

thumb|upright=1.5| <br/>a [[Beroe ovata, b  unidentified cydippid, c "Tortugas red" cydippid, d Bathocyroe fosteri, e Mnemiopsis leidyi, and f Ocyropsis sp.]]

Ctenophores are distinguished from all other animals by having colloblasts, which are sticky and adhere to prey, although a few ctenophore species lack them. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, and as a result some text books classify ctenophores as triploblastic,

Ranging from about to in size, ctenophores are the largest non-colonial animals that use cilia as their main method of locomotion.

{| class="wikitable" style="margin-left:4px"

|+ Comparison with other major animal groups

! Sponges

! Cnidarians

! Ctenophores

! Bilateria

| No

|- align="center"

! Digestive and circulatory organs

|colspan="3"| No

| Yes

|- align="center"

! Anal pores

|colspan="2"| No

| Yes

| Mostly Yes

|- align="center"

! Number of main cell layers

|colspan="2"| Two, with jelly-like layer between them

| Debate about whether two

|colspan="3"| Yes: Inter-cell connections; basement membranes

|- align="center"

! Sensory organs

| No

|colspan="3"| Yes

|- align="center"

! Eyes<br/>

| <blockquote>Larval forms have light sensing organs</blockquote>

| Yes

| No

| Yes

|- align="center"

! Apical organ

| No

|colspan="2"| Yes

| In species with primary ciliated larvae

|- align="center"

! Cell abundance<br/>

| Many

|colspan="2"| Few

|rowspan="2"|

|- align="center"

! Outer layer cells

| can move inwards and change functions

|colspan="2"| do not move or change

|- align="center"

! Nervous system

| No

|colspan="2"| Yes, simple

| Simple to complex

|- align="center"

! Muscles

| None

| Mostly epitheliomuscular

| Mostly myoepithelial

| Mostly myocytes

Description

thumb|Comb jelly, [[Shedd Aquarium, Chicago]]

For a phylum with relatively few species, ctenophores have a wide range of body plans. Hence most attention has until recently concentrated on three coastal genera – Pleurobrachia, Beroe and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid Pleurobrachia.

Body layers

thumb|alt=Anatomy of Cydippid Ctenophore|Anatomy of Cydippid Ctenophore

Like those of cnidarians, (jellyfish, sea anemones, etc.), ctenophores' bodies consist of a relatively thick, jelly-like mesoglea sandwiched between two epithelia, layers of cells bound by inter-cell connections and by a fibrous basement membrane that they secrete. These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is eating, unlike jellyfish. Their body fluids are normally as concentrated 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. Fossils show that Cambrian species had a more complex nervous system, with long nerves which connected with a ring around the mouth. The only ctenophores with long nerves today is Euplokamis in the order Cydippida. Their nerve cells arise from the same progenitor cells as the colloblasts.

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. 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 presence of directly fused neurons without synapses suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently.

Reproduction and development

thumb|Cydippid larva of Bolinopsis sp., a few millimetres long

Adults of most species can regenerate tissues that are damaged or removed, although only platyctenids reproduce by cloning, splitting off from the edges of their flat bodies fragments that develop into new individuals.

The last common ancestor (LCA) of the ctenophores was hermaphroditic. 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 gonads 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,

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 to the cydippid stage when triggered by environmental stressors.

Colors and bioluminescence

thumb|Light [[diffraction|diffracting along the comb rows of a Mertensia ovum, left tentacle deployed, right tentacle retracted]]

Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" (see illustration here), which has not yet been formally described. Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness.

When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same wavelengths 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.

Ecology

thumb|"Tortugas red", with trailing tentacles and clearly visible sidebranches, or tentilla

Distribution

Ctenophores are found in most marine environments: from polar waters at −2 °C to the tropics at 30 °C; near coasts and in mid-ocean; from the surface waters to the ocean depths at more than 7000 meters. The best-understood are the genera Pleurobrachia, Beroe and Mnemiopsis, as these planktonic coastal forms are among the most likely to be collected near shore.

Prey and predators

Almost all ctenophores are predators – there are no herbivores and only one genus that is partly parasitic. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods, amphipods, and even krill. Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts.

The two-tentacled "cydippid" Lampea feeds exclusively on salps, 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.

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 shrimps; ctenophores can provide the fish with a good diet if there are enough of them around. Some jellyfish and turtles 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 sea anemones are parasites on ctenophores, as are the larvae of some flatworms that parasitize fish when they reach adulthood.

Ecological impacts

thumb|right|Beroe ovata at the surface on the Black Sea coast

Ctenophores may balance marine ecosystems by preventing an over-abundance of copepods from eating all the phytoplankton (planktonic plants), which are the dominant marine producers of organic matter from non-organic ingredients.

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. 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. and by a cooling of the local climate from 1991 to 1993,

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. Genomic studies have suggested that the neurons of Ctenophora, which differ in many ways from other animal neurons, evolved independently from those of the other animals.

Modern taxonomy

thumb| [[Lobata sp., with paired thick lobes]]

The traditional classification divides ctenophores into two classes, those with tentacles (Tentaculata) and those without (Nuda). The Nuda contains only one order (Beroida) and family (Beroidae), and two genera, Beroe (several species) and Neis (one species).

Fossil record

Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in Lagerstätten, places where the environment was exceptionally suited to the preservation of soft tissue. Until the mid-1990s, only two specimens good enough for analysis were known, both members of the crown group, from the early Devonian (Emsian) period. Three additional putative species were then found in the Burgess Shale and other Canadian rocks of similar age, about in the mid-Cambrian period. All three lacked tentacles but had between 24 and 80 comb rows, far more than the eight typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. The youngest fossil of a species outside the crown group is Daihuoides from the late Devonian, which belongs to a basal group that had been assumed to have gone extinct more than 140 million years earlier.

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 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 cladistic 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 arrow worms and could have been an agile swimmer. sister to the Cnidaria, Placozoa, and Bilateria, and sister to all other animals.

Walter Garstang in his book Larval Forms and Other Zoological Verses (Mülleria and the Ctenophore) even expressed a theory that ctenophores 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., homeoboxes, nuclear receptors, 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. They also have extremely high rates of mitochondrial 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.

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. 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 Hox genes. Innexin genes, which code for proteins used for intercellular communication in animals, also appears to have evolved independently in ctenophores.

Internal phylogeny

</div>Relationships within Ctenophora (2001).

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.