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Ctenophora

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Phylum Ctenophora

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Ctenophora is a phylum of invertebrate creatures which live in marine environments all over the world. They are the largest species to swim with the aid of cilia, and they are known for the groups of cilia they use for swimming (typically called the "combs"). Adult ctenophores vary in size from a few millimetres to 1.5 metres, depending on the species. Only about 100 to 150 species have been confirmed, with another 25 or so yet to be fully identified and named. Cydippids, with egg-shaped bodies and retractable tentacles fringed with tentilla which are coated by colloblasts, sticky cells which trap prey, are textbook examples.


Their bodies are made up of a jelly mass with a two-cell thick layer on the outside and another covering the interior cavity. The egg-shaped cydippids with retractable tentacles that catch prey, the flat usually combless platyctenids, and the large-mouthed beroids that prey on many other ctenophores, are all members of the phylum. Except for juveniles of two species that live as parasites on the salps on which adults of their species feed, mostly all ctenophores are predators, eating everything from microscopic larvae and rotifers to the adults of small crustaceans.


Considering their delicate, gelatinous bodies, ctenophores have been found in lagerstätten dating back to the early Cambrian, around 525 million years ago. In molecular phylogenetics research, the role of ctenophores in the "tree of life" has long been discussed. Ctenophores are thought to be the second-oldest branching animal lineage, with sponges serving as the sister group to many other multicellular organisms, according to biologists. Many biologists previously thought that ctenophores emerged before sponges, which appeared well before split amongst cnidarians and bilaterians. The existence of unique ctenophore genes which have been significantly different from that of other organisms deceived the computer algorithms used for analysis, according to a reanalysis of the results.


The common ancestor of modern ctenophores was cydippid-like, descending from different cydippids after the Cretaceous–Paleogene extinction event 66 million years ago, according to molecular phylogenetic studies. Comb jellies, according to a 2020 report, are older than sponges. However, the most recent research, published in 2021, confirmed that sponges have become the oldest species on the planet.


Ctenophora Examples With Names: Mertensia, Thalassocalyce inconstans, Pleurobrachia, Ctenoplana, Coeloplana, Cestum, Hormiphora, Mnemiopsis, Bolinopsis, Velamen and several other represents Ctenophora examples with names. 

Ctenophora Classification

Kingdom: Animalia

Subkingdom: Eumetazoa

Phylum: Ctenophora

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Description

The phylum Ctenophora have a diverse variety of body plans for a phylum of just a few species. Coastal species must be able to withstand waves and swirling sediment particles, although some oceanic species are so delicate that capturing them intact for research is difficult. Furthermore, since oceanic organisms do not preserve well, they are only identified through photos and observations. As a result, till lately, the majority of attention was focused on three coastal genera: Pleurobrachia, Beroe, and Mnemiopsis. The cydippid Pleurobrachia is used in at least two textbooks to describe ctenophores. As several species' bodies are nearly radially symmetrical, the main axis is oral to aboral.


Common Features: The flattened, deep-sea platyctenids, wherein the adults of all other species lack combs, and the coastal beroids, that do not possess tentacles and feed on certain ctenophores with massive mouths armed with groups of thick, stiffened cilia that serve as teeth, are both members of the Ctenophora phylum.


Body Layers: Ctenophores' bodies, such as that of cnidarians, are made up of a jelly-like mesoglea placed between two epithelia, which are membranes of cells connected by inter-cellular links and a fibrous basement membrane which they secrete. Ctenophores comprise two layers of epithelia instead of one, and that some of the cells in the upper layer have multiple cilia in each cell.


Feeding, excretion and respiration: When prey is ingested, enzymes and pharyngeal muscle contractions liquefy it in the pharynx. The cilia beat, as well as the resulting slurry, is wafted via the canal system and metabolised by the nutritive cells. The canals' ciliary rosettes might aid in the transportation of materials to the mesoglea's muscles.


Locomotion: The outermost layer generally has eight comb rows, referred to as swimming plates, that are being used for swimming. The rows stretch from near the mouth (the "oral pole") to the opposite side and are distributed almost uniformly across the body, though spacing patterns differ by species, and most species' comb rows just span a portion of the distance from the aboral pole to the mouth.


Nervous System and Senses: Ctenophores lack a brain or central nervous system, rather having a nerve net (similar to a cobweb) which creates a ring around the mouth and is densest around the comb rows, pharynx, tentacles (if present), and sensory complex furthest from the mouth. The nerve cells are generated by the same progenitor cells as colloblasts.


The aboral organ seems to be the biggest single sensory function (at the opposite end from the mouth). A statocyst is a balance sensor made up of a statolith, a small particle of calcium carbonate, and four packages of cilia called "balancers'' which feel its orientation. A transparent dome composed of large, immobile cilia protects the statocyst. Rather, the animal's "mood," or the condition of the nervous system as a whole, determines its response. When a ctenophore with trailing tentacles catches prey, for instance, it will sometimes reverse several comb rows, turning the face towards the prey.

Cydippids

Cydippid ctenophores include rounded bodies, often nearly spherical, certain times cylindrical or egg-shaped; the typical coastal "sea gooseberry," Pleurobrachia, does have an egg-shaped body with the face there at narrow end, however, some individuals are much more generally round. A set of large, slender tentacles spread from opposite sides of the body, each housed in a sheath into something which can be retracted. Some cydippid species include flattened bodies to varying degrees, making them broader in the plane of the tentacles.


Tentilla ("little tentacles') are commonly found on the tentacles of cydippid ctenophores, though several genera include simple tentacles without such side branches. Microscopic colloblasts surround the tentacles and tentilla, allowing them to adhere to prey and capture it. Colloblasts are mushroom-shaped cells in the epidermis' outermost surface that have three major aspects: a domed head with adhesive-filled vesicles (chambers); a stalk that anchors the cell inside the epidermis' lower layer or in the mesoglea; and a spiral thread that coils around the stalk and is connected to the head and the base of the stalk.


The spiral thread's purpose is unknown, but it can sustain stress as prey attempts to flee, preventing the collobast from being broken apart. In contrast to colloblasts, species of the genus Haeckelia, which rely primarily on jellyfish, integrate their victims' stinging nematocytes within their own tentacles for defence; several cnidaria-eating nudibranchs do the same.


Euplokamis tentilla vary from that of other cydippids in two ways: they comprise striated muscle, a type of cell previously unknown within phylum Ctenophora, and they have been coiled when relaxed, whereas all other established ctenophores' tentilla elongate once relaxed. Euplokamis' tentilla can flick out quite rapidly (in 40 to 60 milliseconds); they might wriggle, which can entice prey by acting like tiny planktonic worms; and they can wrap around prey. The specific flicking is an uncoiling movement fueled by striated muscle contraction. Smooth muscles, but that of a highly specialised kind, create the wriggling motion. The return of the tentilla to their inactive state is primarily responsible for coiling across prey, however, the coils can be strengthened by smooth muscle.

Reproduction and Development

Adults of most organisms can regenerate tissues that have been weakened or destroyed, but platyctenids have been the only ones who reproduce through cloning, breaking off pieces of their flat bodies that grow into new individuals. The ctenophores' last common ancestor (LCA) has been hermaphroditic. Simultaneous hermaphrodites can develop both sperm and eggs around the same time, whereas sequential hermaphrodites mature their sperm and eggs at various times.


Ocyropsis maculata and Ocyropsis crystallina in the genus Ocyropsis, and Bathocyroe fosteri in the genus Bathocyroe, are believed to have developed different sexes (dioecy). The gonads are found underneath the comb rows in the internal canal network, and sperm and eggs are expelled through openings in the epidermis. External fertilisation is common, but platyctenids fertilise their eggs internally and hold them in brood chambers before they hatch. Self-fertilization was being observed in Mnemiopsis species on rare occasions, and perhaps most hermaphroditic species are considered to be self-fertile. 


The fertilised eggs develop directly; there seems to be no separate larval shape. Most juveniles are planktonic, and so most species resemble miniature adult cydippids as they mature, progressively forming their adult body shapes. Juveniles throughout the genus Beroe, on the other hand, have big mouths and are observed to lack both tentacles as well as tentacle sheaths, much like adults. The juveniles of certain platyctenid families, like the flat, bottom-dwelling platyctenids, behave somewhat like true larvae. They live among some of the plankton and therefore inhabit a diverse ecological niche than their kin, achieving adulthood only after falling to the seafloor through a more drastic metamorphosis.


Adult ctenophores generate eggs and sperm for almost as long as they have enough food, at minimum in certain species. Juvenile ctenophores are able to produce minimal quantities of eggs and sperm when they are well under adult size, and adults generate sperm or eggs as often as they have enough food. They bring a pause to the production of eggs and sperm and shrink in size when they run out of food. When the food supply increases, they regain their natural size and begin reproducing again. Because of these characteristics, ctenophores can rapidly expand their populations. Members of the Lobata and Cydippida utilize a mode of reproduction known as dissogeny, which involves two sexually mature stages: larva then juveniles and later as adults.


They are likely to release gametes on a regular basis when they are larvae. They would not develop more gametes till after the metamorphosis, ever since their reproductive larval cycle has ended. Mertensia ovum populations in the central Baltic Sea are becoming paedogenetic, consisting primarily of sexually mature larvae with a length of less than 1.6 mm.

Distribution

Ctenophores can be present in a wide range of marine habitats, from polar to tropical waters, close to coasts and in the middle of the ocean, but from the bottom to the depths of the ocean. Pleurobrachia, Beroe, and Mnemiopsis are one of the best-studied genera since these planktonic coastal types are by far the most probable to be found near the sea. In freshwater, no ctenophores were being discovered. 

Mnemiopsis leidyi, a marine ctenophore, was inadvertently introduced into a lake in Egypt in 2013, by the transport of fish (mullet) fry; it was the first record from a true lake, while other species can be identified in the brackish water of estuaries and coastal lagoons.


Ctenophores are typical and hard to identify in certain coastal areas during the summer months, although they are rare and hard to identify in others.

Ctenophores can regulate the populations of tiny zooplanktonic organisms including copepods in bays in which they are abundant, that would otherwise wash out phytoplankton, which is an important component of marine food chains.

Prey and Predators

Mostly all ctenophores are predators; no vegetarians exist, and therefore only one species is partially parasitic. They will eat 10 times their entire mass a day if food is abundant.


Certain surface-water organisms feed on zooplankton (planktonic animals) varying sizes from microscopic mollusc and fish larvae to small adult crustaceans including amphipods, copepods, and even krill, whereas Beroe primarily feeds on other ctenophores. Rather than colloblasts, members of the genus Haeckelia eat jellyfish and insert their prey's nematocysts (stinging cells) within their own tentacles.


Ctenophores were contrasted to spiders in terms of their wide variety of prey capture techniques: certain hang motionless inside the water employing their tentacles as "webs," others are ambush predators such as Salticidae jumping spiders, as well as some dangle a sticky droplet just at end of a fine string like bolas spiders. This diversity describes why there are so many different body types in a phylum of so few species. Lampea juveniles bind itself like parasites to salps which are too large for them to swallow, and the two-tentacled "cydippid" Lampea depends solely on salps, family members of sea-squirts which produce larger chain-like floating colonies.


Since they specialise in distinct forms of prey, members of the lobate genus Bolinopsis and cydippid genus Pleurobrachia frequently achieve large population densities at the very same location and time. Pleurobrachia's long tentacles catch relatively strong swimmers like adult copepods, whereas Bolinopsis eats tiny, poorer swimmers like mollusc and rotifers and crustacean larvae.

Ctenophora Digestive System

The Ctenophora digestive system breaks down food using various organs. They consume other ctenophores and planktonic species with a pair of branched and sticky tentacles. When food reaches their mouth, it travels through the cilla to the pharynx, in which it is broken down by muscular constriction. The food eventually moves to the wider intestine, whereby enzymes gradually break it down. It travels from the stomach to the anal pore, which is not really a true anus but does secrete certain particles; several others escape through the mouth.

Ctenophora Facts

Below Mentioned are Some of the Ctenophora Facts:-

  • Ctenophores are a group of animals of less than a hundred species.

  • Ctenophores are similar to Cnidaria, but they don't have nematocysts.

  • They're often seen as iridescent ball-like shapes rolling in the waves throughout the day, and intensely phosphorescent balls at night.

  • Ctenophores can be identified in the seas between Greenland and Long Island, as well as off the coasts of North and South America.

FAQ (Frequently Asked Questions)

Q1. In Ctenophora, What are the Functions of Comb Plates?

Ans. The ciliary appendages used in animals are known as comb plates. These ciliated comb plates are arranged in eight rows on the outside. They are important for locomotion because these Ctenophores are marine animals, and their comb plates help them swim.

Q2. Which Mechanism is Missing in Ctenophora?

Ans. The skeletal system is missing in Ctenophora.