Phylum Ctenophora – Characteristics, Classification, Examples, Evolutionary significance

What is Phylum Ctenophora?

Kingdom	Animalia
Subkingdom	Eumetazoa
Phylum	Ctenophora

Phylum Ctenophora, commonly referred to as comb jellies, encompasses a group of marine invertebrates known for their distinctive, jelly-like appearance and unique locomotive mechanisms. These creatures, often mistaken for jellyfish, play a significant role in marine ecosystems across the globe. Below is an exploration of the key aspects and fascinating traits of ctenophores:

• Appearance and Structure: Ctenophores possess transparent, gelatinous bodies that are generally oval or spherical. They are characterized by biradial symmetry and are equipped with eight rows of cilia, known as comb plates, which facilitate swimming and prey capture.
• Locomotion and Feeding: Unlike jellyfish, ctenophores do not have stinging cells (nematocysts). Instead, they use colloblasts, sticky cells on their tentacles, to ensnare their prey, which mainly includes small crustaceans, fish eggs, and other marine invertebrates.
• Habitat and Size Variations: These marine animals are found in a diverse range of oceanic environments, from shallow waters to the deep sea. Some ctenophore species can reach impressive lengths of up to 1.5 meters.
• Bioluminescence: A notable feature of many ctenophore species is their ability to produce light bioluminescence, adding a mystical aspect to their presence in the ocean waters.
• Ecological Role: Serving as important predators within their ecosystems, ctenophores help maintain the balance by preying on a variety of smaller marine organisms.
• Reproduction: Ctenophores exhibit a range of reproductive strategies, including both sexual and asexual reproduction, with some species being hermaphroditic.
• Invasive Species: While they are a natural part of marine biodiversity, certain ctenophore species have become invasive in some regions, posing challenges to local ecosystems.
• Scientific Understanding: Despite their ecological importance and long history, dating back over 500 million years, ctenophores remain relatively understudied, with much about their biology and ecology still to be discovered.

Phylum Ctenophora represents one of the earliest animal groups to evolve on Earth, showcasing unique adaptations and contributing to the complexity of marine life. Their presence highlights the diversity of life forms in the ocean and underscores the importance of continued research to fully understand their role in marine ecosystems.

Definition of Phylum Ctenophora

Phylum Ctenophora consists of marine invertebrates known as comb jellies, characterized by their transparent, gelatinous bodies and biradial symmetry. They navigate the oceans using distinctive rows of ciliary plates for locomotion and employ unique adhesive cells called colloblasts for capturing prey. Unlike jellyfish, ctenophores lack stinging cells, making them unique in their feeding habits. These ancient organisms, which have been around for over 500 million years, play a crucial role in marine ecosystems as predators of small marine invertebrates.

General characteristics of Phylum Ctenophora

• Habitat and Lifestyle: Ctenophores are solitary, pelagic animals, thriving in marine environments without any polymorphic or attached stages throughout their lifecycle.
• Body Structure: Their bodies are transparent, gelatinous, and exhibit biradial symmetry along an oral-aboral axis, making them distinct in appearance and function.
• Locomotion: A notable feature is the presence of eight external, comb-like ciliary plates that facilitate movement, earning them the name ‘comb jellies’.
• Tentacles: Ctenophores possess a pair of long, solid, and retractile tentacles, aiding in various functions including feeding.
• Body Organization: These organisms show a cell-tissue grade organization, with their bodies being acoelomate and primarily diploblastic, comprising an ectoderm and an endoderm. However, the presence of a middle jelly-like mesoglea layer with scattered cells and muscle fibers also lends them a quasi-triploblastic aspect.
• Digestive System: The digestive apparatus consists of a mouth, stomodaeum, intricate gastrovascular canals, and two aboral anal pores, facilitating nutrient absorption and waste expulsion.
• Sensory and Adhesive Cells: Unlike many marine organisms, ctenophores lack nematocysts but have colloblasts or lasso cells on their tentacles, which play a crucial role in capturing prey.
• Organ Systems: These creatures do not possess skeletal, circulatory, respiratory, or excretory systems, showcasing a simplified internal structure.
• Nervous System: The nervous system is of a diffused type, with a sensory organ known as the statocyst located at the aboral end, aiding in orientation and balance.
• Reproduction: Ctenophores are hermaphroditic, with their gonads located endodermally along the walls of the digestive canals. Their development is direct, featuring a characteristic cydippid larva stage.
• Regeneration and Paedogenesis: Remarkably, ctenophores have the ability to regenerate and exhibit paedogenesis, adding to their resilience and adaptability in the marine ecosystem.

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

Class 1. Tentaculata

Class 1 Tentaculata includes marine organisms distinguished by their unique tentacular structures. These creatures exhibit diverse body forms and adaptations, ranging from simple, round shapes to elongated, ribbon-like forms. The tentacles, used primarily for feeding and sensory functions, vary significantly among different orders within this class. Below is a detailed examination of the key characteristics and orders within Tentaculata.

Key Characteristics

• Tentacles: Adults are characterized by two long aboral tentacles, which may be branched or simple, and are often retractable into pouches or sheaths for protection.
• Morphology: The body shapes vary from spherical to oval, laterally compressed, and even flattened in the oral-aboral axis, adapting to diverse marine environments.
• Digestive System: The mouth is typically narrow, leading to a small pharynx. The digestive canals usually terminate blindly, lacking anal pores in most orders.

Orders within Class 1 Tentaculata

1. Order Cydippida
   • Body Form: Members of this order have a simple, round, or oval body structure.
   • Tentacles: Characterized by two long, branched tentacles that are retractable into pouches or sheaths.
   • Examples: Notable species include Mertensia, Pleurobrachia, and Hormiphora.
2. Order Lobata
   • Body Shape: These organisms possess an oval, laterally compressed body.
   • Tentacles and Lobes: Adults feature two large oral lobes and four slender flap-like auricles around the mouth, with tentacles being reduced and without sheaths.
   • Examples: Mnemiopsis and Bolinopsis are representative species.
3. Order Cestida
   • Morphology: Cestida members have an elongated, compressed, or flat, ribbon-like body.
   • Tentacles: They exhibit two main tentacles in sheaths and many small lateral tentacles along the oral edge.
   • Examples: The order includes species like Cestum and Velamen.
4. Order Platyctenea
   • Body Compression: The body is greatly compressed or flattened in the oral-aboral axis.
   • Locomotion: Adapted for creeping, these organisms have reduced comb plates in adults.
   • Examples: Species such as Ctenoplana and Coeloplana belong to this order.
5. Order Thalassocalycida
   • Habitat: Found in surface waters down to depths of 2,765 meters in the Atlantic Ocean and the Mediterranean Sea.
   • Body and Feeding: The body resembles a bell of Medusa and may reach up to 15 cm in diameter, using it to capture prey like zooplankton.
   • Locomotion: This species has limited swimming ability compared to other comb jellies.
   • Examples: Thalassocalyce inconstans is a known species within this order.

Class 2. Nudu

Class 2 Nudu encompasses a group of marine organisms known for their distinctive body structure and feeding habits. This class is characterized by large, conical bodies that are compressed laterally. Unlike many other marine invertebrates, Nudu species lack tentacles and oral lobes, which sets them apart in terms of anatomy and feeding mechanisms. Here are the key features and orders within the Class 2 Nudu:

Key Features

• Body Structure: Nudu species possess a large, conical body shape that is notably compressed laterally, giving them a unique appearance among marine organisms.
• Absence of Tentacles and Oral Lobes: These organisms do not have tentacles or oral lobes, which are common in many other marine invertebrates.
• Mouth and Pharynx: They are equipped with a wide mouth and a large pharynx, accommodating their voracious feeding habits.

Order within Class 2 Nudu

Order 1: Beroida

• Tentacles and Oral Lobes: Members of the Beroida order also lack tentacles and oral lobes, consistent with the general characteristics of Class 2 Nudu.
• Body Shape and Size: Beroida species exhibit a large, conical body that is compressed laterally, similar to the overall class description.
• Feeding: These organisms have a large mouth and a voluminous stomach, supporting their role as voracious feeders within their marine ecosystems.
• Examples: The genus Beroe is a well-known example within this order, showcasing the typical features of the Beroida order.

Class 2 Nudu, particularly the Order Beroida, highlights the diversity of marine life and the various adaptations organisms have evolved for feeding and survival. The absence of tentacles and oral lobes in Nudu species, combined with their large size and conical body shape, make them a distinct group within the marine invertebrate community. Their role as voracious feeders adds to the ecological dynamics of their marine environments, illustrating the complexity of food webs and predator-prey relationships in the ocean.

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.

Evolutionary significance of ctenophora

The evolutionary significance of ctenophores, commonly known as comb jellies, is profound and multifaceted, revealing crucial insights into the development of early animal life and the intricate relationships among marine organisms. Ctenophores are of particular interest to researchers due to their unique morphological and physiological traits, alongside their ancient lineage. Below is a synthesis of information that elucidates their evolutionary importance.

• Ancient Origins: Fossils believed to represent ctenophores date back to the early Cambrian period, approximately 515 million years ago. These specimens, often found in exceptional fossil beds known as lagerstätten, indicate the presence of ctenophore-like organisms long before the emergence of more complex life forms. This early appearance suggests that ctenophores have played a significant role in the evolutionary history of marine ecosystems.
• Soft Body Preservation: The gelatinous bodies of ctenophores render them rarely fossilized, making their evolutionary history challenging to reconstruct. However, the exceptional conditions in lagerstätten have yielded some critical specimens. Until the mid-1990s, only two notable fossils of ctenophores were known, both dating to the early Devonian period. Since then, additional fossils from the Burgess Shale and similar deposits have revealed early ctenophore characteristics, such as multiple comb rows and internal structures not present in modern species.
• Morphological Diversity: Early ctenophores exhibited significant morphological diversity, with some fossils displaying between 24 and 80 comb rows compared to the eight typical of extant species. This diversity suggests that early ctenophores may have possessed a range of adaptations that allowed them to exploit different ecological niches in their environments.
• Evolutionary Relationships: The discovery of Ediacaran fossils like Eoandromeda, which exhibits eightfold symmetry and spiral arms resembling the comb rows of modern ctenophores, supports the hypothesis that ctenophores are closely related to the origins of Bilateria, a major clade of animals. This connection suggests that ctenophores may represent a critical evolutionary link in understanding the emergence of complex multicellular life.
• Sessile Ancestors: Evidence suggests that ctenophores may have evolved from sessile organisms that transitioned to a swimming lifestyle. The frond-like fossil Stromatoveris, dated to approximately 515 million years ago, exhibits features akin to modern ctenophores, such as rows of cilia. This implies that early ctenophores could have adapted cilia originally used for filter feeding into a propulsion system for swimming.
• Complex Structures: The extinct class Scleroctenophora, known from Cambrian fossils, possessed a complex internal skeleton and soft-bodied flaps, indicating adaptations for swimming and feeding. This complexity underscores the evolutionary innovations present in early ctenophores and their role in shaping the ecological dynamics of ancient marine environments.
• Ecological Impact: The unique feeding strategies and swimming mechanisms of ctenophores contribute to their ecological roles in contemporary marine systems. Their existence since the Cambrian suggests they have influenced marine biodiversity and nutrient cycling, highlighting their significance in the evolutionary narrative.

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

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