The marine invertebrate phylum Ctenophora, often known as comb jellies, is widespread throughout the world’s oceans. Their transparent, gelatinous bodies are comprised of eight rows of cilia, or comb-like structures, which they employ for swimming and collecting prey.

Ctenophores, unlike jellyfish, lack stinging cells, or nematocysts. Instead, they utilise colloblasts, or sticky cells, to capture their meal. Certain kinds of ctenophores can grow as long as 1.5 metres in length.

As predators and prey, ctenophores play a vital role in the marine ecosystem. They consume small crustaceans, fish eggs, and other small marine animals, while larger predators like sea turtles and fish consume them.

Despite their significance, scientists have a limited understanding of ctenophores, and additional research is required to completely comprehend their biology and ecology.

Kingdom	Animalia
Subkingdom	Eumetazoa
Phylum	Ctenophora

Ctenophora Definition

Ctenophores are transparent, jelly-like, soft-bodied marine organisms with biradial symmetry, comb-like ciliary plates for motility, lasso cells, but no nematocytes. Also, they are known as sea walnuts and comb jellies.

• The marine invertebrate phylum Ctenophora, often known as comb jellies, is widespread throughout the world’s oceans.
• Ctenophores have transparent, gelatinous bodies that are typically oval or spherical in shape, and they employ their eight rows of cilia for swimming and collecting prey.
• Ctenophores, unlike jellyfish, lack stinging cells, or nematocysts. Instead, they utilise colloblasts, or sticky cells, to capture their meal.
• Certain kinds of ctenophores can grow as long as 1.5 metres in length.
• Several species of ctenophores are bioluminescent, meaning they make their own light, and are found in both shallow and deep ocean environments.
• Important predators in the marine ecology, ctenophores eat on small crustaceans, fish eggs, and other small marine invertebrates.
• Despite their significance, scientists have a limited understanding of ctenophores, and additional research is required to completely comprehend their biology and ecology.
• Certain species of ctenophores are capable of both sexual and asexual reproduction, while others are hermaphrodite.
• Some ctenophore species are deemed invasive in specific locations and can negatively effect local ecosystems.
• Ctenophores have existed for more than 500 million years and are regarded as one of the first animal groups to evolve on Earth.

Phylum Ctenophora General characteristics

• All are marine entirely.
• All are solitary, free-swimming creatures.
• Demonstrate tissue level organisation.
• The majority of body parts are organised radially, whereas tentacles are paired and so exhibit biradial symmetry.
• Eight longitudinal rows of fused cilia-bearing transverse plates, known as comb plates, aid in movement. Ctenophores are the largest non-colonial creatures that employ cilia for locomotion (absent in Ctenoplana and Coeloplana).
• A pair of retractable, long, solid tentacles are present in the aboral area. On the surface of the tentacles are specialised glue cells termed colloblasts or lasso cells, which exude an adhesive substance that aids in capturing small prey.
• The body is composed of both ectoderm and endoderm. The body has an exterior epidermis and an inner endodermis, a jelly-like mesoglea with dispersed cells in the centre, and muscle fibres. Hence, Ctenophores are similarly triploblastic.
• The pharynx makes up the gastrovascular cavity. Stomach with a system of gastrovascular canals, in addition to two aboral and oral apertures.
• Both external and intracellular digestion occurs.
• Nematocysts are lacking in their entirety.
• There are no respiratory, circulatory, skeletal, or excretory systems.
• The nervous system is of the diffuse type, and the aboral end contains the statocyst sense organ.
• There is no asexual reproduction or alternation of generations.
• There is only sexual reproduction.
• All are monoecious (Hermaphrodite).
• Gonads originate from endoderm.
• Fertilization is an external process.
• Development is determined and indirect, with larvae resembling cydippids.
• Common are regeneration and bioluminescence.

Body layers of Phylum Ctenophora

• The bodies of ctenophores, like those of cnidarians (jellyfish, sea anemones, etc.), consist of a rather thick, jelly-like mesoglea sandwiched between two epithelia, layers of cells joined by inter-cell connections and a fibrous basement membrane that they secrete.
• Ctenophores’ epithelia feature two layers of cells instead of one, and some cells in the upper layer have several cilia per cell.
• The outer layer of the epidermis (outer skin) is made up of sensory cells, cells that generate mucus to protect the body, and interstitial cells that can convert into other cell types.
• In specialised regions of the body, the outer layer comprises colloblasts, which are located along the surface of tentacles and are used to capture prey, as well as cells with many big cilia, which are utilised for motility. The inner layer of the epidermis comprises a neural network and muscle-like myoepithelial cells.
• The internal cavity consists of a mouth that is typically closed by muscles, a pharynx (the “throat”), a larger space in the middle that functions as the stomach, and a network of internal canals. Two of these four branches finish in anal pores.
• The epithelium, gastrodermis, lines the inside surface of the hollow. Both cilia and well-developed muscles are present in the mouth and pharynx. In other portions of the canal system, the gastrodermis differs on the sides closest to and farthest from the organ it serves.
• The closer side contains tall nutritive cells that store nutrients in vacuoles (internal compartments), germ cells that create eggs or sperm, and photocytes that generate bioluminescence.
• The side farthest from the organ is covered with ciliated cells that circulate water through the canals and are punctuated by ciliary rosettes, apertures encircled by double whorls of cilia that link to the mesoglea.

Feeding, excretion and respiration of Phylum Ctenophora

• When prey is swallowed, it is liquefied in the pharynx by a combination of enzymes and muscle contractions.
• The resulting slurry is carried through the canal system by the cilia and digested by the nutritive cells. The ciliary rosettes in the canals may aid in nutrient transfer to the mesogleal muscles.
• Some particles may be expelled through the anal pores, however the majority of undesired material is regurgitated through the mouth.
• Nothing is known about how ctenophores eliminate cell-produced waste products. The ciliary rosettes in the gastrodermis may assist in the removal of wastes from the mesoglea as well as in the regulation of the animal’s buoyancy by pumping water into or out of the mesoglea.

Locomotion of Phylum Ctenophora

• The outside surface typically features eight swimming-plate rows, also known as swimming-comb rows. The rows are oriented to run from near the mouth (the “oral pole”) to the opposite end (the “aboral pole”) and are more or less evenly spaced around the body, although spacing patterns vary by species and in most species the comb rows extend only a portion of the distance from the aboral pole to the mouth.
• The “combs” (also known as “ctenes” or “comb plates”) run across each row and are composed of thousands of cilia that are up to 2 millimetres in length (0.08 in). In contrast to typical cilia and flagella, which have a filament structure ordered in a 9 + 2 pattern, these cilia have a filament structure arranged in a 9 + 3 pattern, with the extra compact filament likely serving a supportive function.
• They typically beat with the propulsion stroke away from the mouth, although they can also beat in the opposite direction. In contrast to jellyfish, ctenophores typically swim in the direction in which their mouths are feeding.
• One species can accelerate to six times its regular speed when attempting to evade predators; other species reverse direction as part of their escape behaviour by reversing the power stroke of the comb plate cilia.
• Uncertainty surrounds the mechanism by which ctenophores regulate their buoyancy, however tests have revealed that some species rely on osmotic pressure to adjust to water of varying densities.
• Typically, their bodily fluids are as concentrated as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump it into the mesoglea to expand its volume and decrease its density, so preventing them from sinking. If the rosettes transition from brackish to full-strength seawater, they may pump water out of the mesoglea to reduce its volume and raise its density.

Nervous system and senses of Phylum Ctenophora

• Instead of a brain or central nervous system, ctenophores have a nerve net (similar to a spider web) that creates a ring around the mouth and is densest in components such as the comb rows, pharynx, tentacles (if present), and the sensory complex farthest from the mouth.
• Fossils indicate that Cambrian species had a more complicated neurological system, with lengthy nerves connecting to a mouth ring. The only known ctenophore with lengthy nerves is the Cydippida species Euplokamis. Their nerve cells develop from the same progenitor cells as their collagenoblasts.
• The largest sensory organ is the aboral organ (at the opposite end from the mouth). Its primary component is a statocyst, a balance sensor composed of a statolith, a small calcium carbonate grain supported by four bundles of cilia known as “balancers” that sense its direction.
• The statocyst is protected by a dome of transparent, motionless cilia. A ctenophore does not necessarily attempt to maintain the statolith balanced on all balancers. Instead, the animal’s behaviour is dictated by its “mood,” or the general state of its neural system.
• For instance, when a ctenophore with trailing tentacles takes prey, it will frequently twist the mouth towards the prey by reversing some comb rows.
• Study supports the idea that the ciliated larvae of cnidarians and bilaterians share a shared ancestry. The apical organ of larvae is important in the development of the nervous system.
• Due to the fact that the aboral organ of comb jellies is not similar to the apical organ of other animals, the genesis of their nervous system has a distinct embryonic origin.
• The nerve cells and neurological system of ctenophores differ from those of other animals in their biochemistry. For example, they lack the genes and enzymes required to produce neurotransmitters like as serotonin, dopamine, nitric oxide, octopamine, and noradrenaline, which are found in all other animals with a nervous system.
• Moreover, the receptor genes for each of these neurotransmitters are absent. It has been discovered that they employ L-glutamate as a neurotransmitter and that they have a greater diversity of ionotropic glutamate receptors and genes for glutamate production and transport than other metazoans.
• The genomic content of the genes of the nervous system is the smallest of any known mammal, and may represent the genetic needs for a functional neural system.
• Hence, if ctenophores are the sister group to all other metazoans, sponges and placozoans may have lost their nervous systems or they may have evolved several times among metazoans. Moreover, monofunctional catalase (CAT), one of the three primary families of antioxidant enzymes that target hydrogen peroxide (H2O2), an essential signalling molecule for synaptic and neuronal activity, is lacking, most likely as a result of gene loss.

Reproduction and Development

• Adults of most creatures can recover damaged or destroyed parts, but platyctenids are the only ones known to reproduce by cloning, by breaking off portions of their flat bodies that grow into new individuals. The last common ancestor (LCA) of ctenophores was hermaphroditic. Simultaneous hermaphrodites produce sperm and eggs simultaneously, whereas sequential hermaphrodites produce sperm and eggs at different periods.
• It is hypothesised that Ocyropsis maculata and Ocyropsis crystallina in the genus Ocyropsis, as well as Bathocyroe fosteri in the genus Bathocyroe, have formed distinct sexes (dioecy). Sperm and eggs are ejected through pores in the epidermis. The gonads are located beneath the comb rows in the internal canal network. Typically, eggs are fertilised externally, however platyctenids fertilise their eggs inside and store them in brood chambers prior to hatching. On rare cases, self-fertilization was reported in Mnemiopsis species, and perhaps the majority of hermaphrodite species are regarded to be self-fertile.
• The fertilised eggs appear to develop without a distinct larval form. While most juveniles are planktonic, as they age, the majority of species resemble small adult cydippids, gradually forming their adult body forms. In contrast, juveniles of the genus Beroe have large mouths and lack both tentacles and tentacle sheaths, similar to adults. Several platyctenid families, such as the flat, bottom-dwelling platyctenids, have juveniles that resemble real larvae. They reside amid certain plankton and so occupy a more diversified ecological niche than their relatives, maturing only after undergoing a more extreme metamorphosis and falling to the seafloor.
• Certain kinds of adult ctenophores produce eggs and sperm for nearly as long as they have sufficient nourishment. Juvenile ctenophores are able to produce modest numbers of eggs and sperm while they are significantly smaller than adults, whereas adults produce sperm or eggs whenever they have sufficient nourishment. As they run out of nourishment, they halt the creation of eggs and sperm and diminish in size. They restore their natural size and resume reproduction when the food supply rises. These traits allow ctenophores to rapidly increase their populations. Members of the Lobata and Cydippida use dissogeny, which involves two sexually mature stages: larva, followed by juveniles, and then adults.
• As larvae, they are likely to release gametes on a regular basis. Once their reproductive larval cycle is concluded, they would not produce any more gametes until after metamorphosis. In the middle Baltic Sea, Mertensia ovum populations are becoming paedogenetic, consisting mostly of sexually mature larvae less than 1.6 mm in length.

Distribution

• Ctenophores can be found in a variety of marine habitats, ranging from arctic to tropical waters, near coasts and in the centre of the ocean, but from the ocean floor to its depths. The genera Pleurobrachia, Beroe, and Mnemiopsis are among the most investigated since these planktonic coastal species are by far the most likely to be discovered near the ocean. There were no ctenophores found in freshwater.
• Mnemiopsis leidyi, a marine ctenophore, was accidently introduced into a lake in Egypt in 2013 via the transport of fish (mullet) fry; it was the first record from a real lake, whereas other species are known from brackish water in estuaries and coastal lagoons.
• During the summer months, ctenophores are common and difficult to detect in some coastal regions, while they are uncommon and difficult to identify in others.
• Ctenophores help regulate populations of microscopic zooplanktonic species, such as copepods, in bays where these organisms are numerous and would otherwise wash away phytoplankton, an essential component of marine food chains.

Prey and Predators

• The vast majority of ctenophores are carnivorous; there are no vegetarian species, and hence only one species is partially parasitic. They will consume 10 times their body mass per day if food is plentiful.
• Some surface-water species feed on zooplankton (planktonic animals) ranging in size from microscopic molluscs and fish larvae to small adult crustaceans such as amphipods, copepods, and even krill, whereas Beroe feeds largely on other ctenophores.
• Members of the genus Haeckelia consume jellyfish and incorporate the nematocysts (stinging cells) of their food into their own tentacles.
• Ctenophores were compared to spiders in terms of their diverse prey capture strategies: some Ctenophores hang motionless in the water using their tentacles as “webs,” others are ambush predators such as Salticidae jumping spiders, and others dangle a sticky droplet at the end of a fine string like bolas spiders.
• This variety explains why a phylum with so few species contains so many diverse body types. Lampea juveniles attach themselves like parasites to salps that are too large for them to swallow, while the two-tentacled “cydippid” Lampea relies completely on salps, which are members of the family of sea-squirts that generate larger chain-like floating colonies.
• Members of the lobate genus Bolinopsis and the cydippid genus Pleurobrachia frequently attain high population densities at the same site and time because they specialise in different types of prey.
• Long tentacles of Pleurobrachia capture relatively strong swimmers, such as adult copepods, whereas Bolinopsis consumes smaller, weaker swimmers, such as mollusk and rotifer and crustacean larvae.

Phylum Ctenophora Classification

The phylum Ctenophora has approximately 100 known species organised into two groups.

Class 1. Tentaculata

• Adults have two lengthy oral tentacles.
• In some larvae, tentacles develop into oral lobes in adulthood.
• The mouth and pharynx are narrow.

Order 1. Cydippida

• Simple, round, and oval body shape.
• The digestive canals end blindly; there are no anal apertures.
• Tentacles are lengthy and branching.
• Tentacles have the ability to retract into pouches or sheaths.
• Examples: Mertensia, Pleurobrachia, Hormiphora

Order 2. Lobata

• Body is round and compressed laterally.
• Adults having two huge oral lobes and four thin auricles surrounding the mouth.
• Tentacles with a pouch or sheath in the larva.
• Adults have shortened and sheathless tentacles.
• Oral ends of gastrovascular canals are joined by a ring.
• Examples: Mnemiopsis, Bolinopsis

Order 3. Cestida

• The body is long, compressed, and ribbon-like.
• Two decreased primary tentacles are contained in the sheath.
• Many tiny tentacles line the oral margin.
• Combs plates in four rows, but simple.
• Examples: Cestum, Velamen

Order 4. Platyctenea

• Extreme compression/flattening of the body along the oral-aboral axis.
• Two tentacles with a sheath that are well-developed.
• Reduced comb plates in adults.
• Designed for crawling.
• Examples: Ctenoplana, Coeloplana

Order 5. Thalassocalycida

• In the Atlantic and Mediterranean oceans, they are found in surface waters down to 2,765 Ms.
• The body is up to 15 cm in diameter and has a Medusa bell form.
• The mouth slit is supported by a central peduncle fashioned like a cone.
• A pair of tiny tentacles hang from the peduncle’s side.
• The body of Com jelly is translucent and colourless. Typically, a unique sight to behold.
• They hold the bell open in order to capture their prey, which is Zooplankton.
• Apparently hermaphrodite.
• Compared to other comb jellies, this species has a limited ability to swim.
• Thalassocalyce inconstans are examples.

Class 2. Nudu

• Large, conical, and laterally compressed body.
• Without having tentacles or oral lobes.
• A big mouth and pharynx.
• A voracious consumer.

Order 1. Beroida

• Without having tentacles or oral lobes.
• Large, conical, and laterally compressed body structure.
• Mouth is expansive.
• Voluminous Stomach.
• Examples: Beroe

Importance/Significance of Phylum Ctenophora

• While scuba diving and snorkelling, they produce breathtaking sights.
• The genetics of Ctenophora was one of its advantages. The immediate luminescence of ctenophores is utilised as a “biomarker” or “biotag.”
• Scientists use them in their research to identify activation genes by creating various glowing cats, mice, and other animals and analysing whether the genetic modifications done to these animals are beneficial.
• Certain species of comb jellies are harvested as a food source in certain places, such as portions of Asia, which may have economic benefits. In addition, comb jellies may contribute to the sustainability of fisheries by serving as a food source for commercially valuable species like as herring and anchovies.
• Some substances isolated from ctenophores have showed promise in medical and cosmetic uses, which is a possible economic benefit. In the mucus of comb jellies, for instance, researchers have identified possible antibacterial chemicals that may have uses in the development of novel antibiotics. Moreover, ctenophores create bioluminescent proteins that may be utilised in medical imaging and other scientific applications.
• It is essential to emphasise, however, that the economic significance of ctenophores is not well-established, and additional research is required to properly comprehend their potential applications and benefits. In addition, several species of comb jellies have detrimental effects on marine ecosystems, including as competition for resources and predation of other marine organisms, which might incur economic consequences.
• They control the plankton population because they reproduce rapidly and are effective predators.
• They are able to adapt to greater temperatures, giving them an advantage in fluctuating environmental circumstances.

Phylum Ctenophora Examples

• Pleurobrachia bachei – The species Pleurobrachia bachei, popularly known as the sea gooseberry, is native to the North Pacific Ocean and is recognised for its bioluminescent qualities.
• Mnemiopsis leidyi – initially endemic to the western Atlantic, this invasive species has spread throughout the globe and is notorious for its ability to outcompete native species.
• Beroe ovata – this species, which inhabits the Mediterranean Sea and other regions of the world, is renowned for its great size and stunning colours.
• Ocyropsis crystallina – Located in the waters surrounding Japan, this species is well-known for its translucent body and its ability to capture prey with sticky tentacles.
• Hormiphora plumosa – This species, Hormiphora plumosa, inhabits the waters surrounding Australia and is recognised for its delicate and ornate look.
• Euplokamis dunlapae – This species, found in the deep seas of the Pacific Ocean, is renowned for its bioluminescence and distinctive morphology.
• Coeloplana gonoctena – Coeloplana gonoctena is notable for its flattened, disc-shaped body and ability to glide through the water. This species is found in the waters surrounding Japan.
• Thalassocalyce inconstans – This species, Thalassocalyce inconstans, inhabits the deep depths of the Atlantic Ocean and is renowned for its size and spectacular beauty.
• Leucothea pulchra – This species, Leucothea pulchra, inhabits the waters surrounding Japan and is renowned for its transparent body and characteristic comb rows.
• Lyrocteis imperatoris – This species, Lyrocteis imperatoris, is found in the waters surrounding Australia and is renowned for its vibrant colours and unusual shape.

FAQ

References

• http://dhingcollegeonline.co.in/attendence/classnotes/files/1603976689.pdf
• https://www.biologydiscussion.com/invertebrate-zoology/phylum-ctenophora/phylum-ctenophora-characters-and-classification-animal-kingdom/69841
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