Phylum Ctenophora
** Examples and Characteristics
Overview
Phylum Ctenophora are known as sea walnuts or comb jellies and are marine organisms that resemble Cnidaria.
Like Cnidaria, Ctenophora has been shown to be one of the oldest phylum living today. For this reason, they are sometimes described as living fossils.
Currently, Ctenophora is estimated to consist of between 100 and 150 species. The majority of known species are characterized by a transparent, bilaterally symmetric
Phylum Ctenophora are known as sea walnuts or comb jellies and are marine organisms that resemble Cnidaria.
Like Cnidaria, Ctenophora has been shown to be one of the oldest phylum living today. For this reason, they are sometimes described as living fossils.
Currently, Ctenophora is estimated to consist of between 100 and 150 species. The majority of known species are characterized by a transparent, bilaterally symmetrical morphology and can be found in marine environments throughout the world.
* Phylum Ctenophora belongs to the kingdom Animalia and the division Metazoa. It's further divided into two main classes namely, Tentaculata and Nuda.
Some of the species that belong to the phylum Ctenophora include:
- Ctenophora apicata
- Ctenophora festiva
- Ctenophora amabilis
- Ctenophora guttata
- Ctenophora nikkoensis
Examples of Phylum Ctenophora and their Characteristics
As mentioned, the phylum Ctenophora is divided into two main classes, Tentaculata and Nuda. Their respective members display a number of structural differences and can be found in various habitats in the marine environment.
Tentaculata
The class Tentaculata consists of eight Orders whose members are characterized by retractile tentacles.
These include:
Cydippida - Members of the order Cydippida are thought to be some of the most primitive species of the phylum Ctenophora. Also known as Sea Gooseberries, Cydippidea have a rounded body but may also be ovoid (the body might be slightly compressed).
As members of the order Cydippidea, they have retractile tentacles (2). The two tentacles (branched) are located on the opposite ends of the body.
Some examples of Cydippidea species include:
- Dryodora
- Hormiphora
- Callianira
Platyctenida - Platyctenida are benthic Ctenophora and thus found on the floor of the ocean. Like Cydippida, they also have a pair of tentacles with sheaths. However, they lack ctenes (fused cilia) that most Ctenophora species use for swimming.
This is attributed to the fact that they have adapted well to the benthic lifestyle where they are either fixed in one place (sessile) or creep along the floor. That said, it's worth noting that ciliary plates, which are short or sunken, might be found in larval forms.
Morphologically, they are dorsoventrally flattened and possess a system of branching peripheral vessels.
Some of the most common examples of Platyctenea include:
- Vallicula
- Lyocteis
- Ctenoplana
Cambojiida - According to some studies, members of the order Cambojiida may be a cross between two genera (bigeneric). However, some have maintained that the order is monogenetic.
Currently, the order is only known to have one species which is planktonic and found in the pelagic zone.
Cryptobiferida - Like Cambojiida, Cryptobiferida might also be mono or bigeneric.
Thalassocalycida- Found in the Gulf of Mexico and the Californian Coast, Thalassocalycida consists of a single-family and genus.
The representative species, Thalassocalyce inconstans, is characterized by a dome-like body with short tentacles. It exists as both a forager and a predator and feeds on small crustaceans among other small organisms in their surrounding.
Ganeshida - Ganeshida is a monogenetic Ctenophora that consists of two known species, G. elegans, and G. annamita. They can be found in the western central pacific in the pelagic zone. They are also hermaphroditic which means that individuals have both the male and female reproductive organs.
Some of the other unique characteristics of these organisms include small roles located around the mouth as well as an extended pharynx.
Lobata - Members of the order Lobata are characterized by large oral lobes (two contractile lobes) and four auricles. Like the other Tentaculata, they also possess tentacular sheaths.
Generally, the lateral tentacles are nonretractile and contained in a groove. Morphologically, Lobata species have a laterally compressed body.
In the marine environment, some of the species can be found at depths of over 1,000 meters where they feed on a variety of organic matter including fish eggs, rotifers, and zooplankton.
Some of the species in the order Lobata include:
- Bolinopsis
- Deiopea
Cestida (Cestidea) - Cestida species have a ribbon-like body and can grow to lengths of about 1.5 meters. The ribbon-like morphology is attributed to extreme compression in the sagittal plane/lateral Plane. They also have two tentacles that are enclosed in sheaths as well as numerous lateral tentacles within the groove.
Some of the other characteristics evident in these organisms include the fusion of the meridional and stomodaeal vessels and rudimentary ciliary plates. Cestus veneris is the representative member of this order.
Nuda
Members of the class Nuda are distinguished from Tentaculata species by the fact that they lack tentacles. The class only has a single order known as Beroida.
Beroida (family Beroidea) - Beroidea species are characterized by a conical or thimble-shaped body. They have a wide mouth that together with the gullet occupies most of the anterior portion of the body.
Most of the species are small in size, not exceeding 30 centimeters, and possess a row of unbranched papillae. It consists of over 20 species that are distributed worldwide.
Some of the species in this order:
- Beroe abyssicola
- Beroe cucumis
- Beroe gracilis
- Neis cordigera
General Characteristics of phylum Ctenophora
Morphology
Like true jellyfish, the body of phylum Ctenophora (ctenophores) is mostly water (about 95 percent water). As result, they are mostly transparent (or translucent) and very delicate. Because of the delicate nature of their bodies, ctenophores are very rarely fossilized.
Though most species are a few centimeters in size (lengthwise), they can range anywhere from one millimeter to over 1.5 meters in length. For instance, Cestum veneris, also known as Venus girdle, is one of the largest ctenophore and can grow to over 1.5 meters in length and about 2 inches wide.
The body is divided into two main parts that include the head and stalk.
Head (Collosphere) - The head of a Ctenophore is the hemispherical part (with a bulb or kidney-like shape). Cells of the collosphere (head) consist of the typical organelles including the Golgi network, endoplasmic reticulum, and mitochondria among others.
They have been shown to contain fine-grain material. Some of the other cellular components include granules, spiral filament, and a spheroidal body (the spheroidal body is suggested to play an important role in nucleation).
Some of the other features located in the head region include the mouth and comb row.
Stalk - The stalk section of the body is conical in shape. Cells of this region have an elongated nucleus with evenly dispersed chromatin.
Like the cells found in the head region, these cells also contain typical organelles such as mitochondria, vesicles, microtubules, and endoplasmic reticulum. The tentacles, which vary in length from one species to another, originate from the tentacular sheath attached to the head region.
While ctenophores resemble true jellyfish, there are several differences between the two worth noting. For instance, unlike jellyfish, ctenophores do not produce polyp. Moreover, they do not display dimorphism and colonial organization.
Rather than muscles, they use cilia for locomotion. Ctenophores are also unique in that they have a complete gut that is open on both sides. This means that they have a mouth region as well as an anal opening (anal pore).
As mentioned, they possess two tentacles originating from sheaths attached to the head region. These tentacles are branched and can be retracted into the sheaths.
Some of the other morphological traits of ctenophores include:
- Diploblastic
- Exhibit boradial symmetry
- Have a gastrovascular cavity
- Nervous system is in the form of a nerve net
- Possess adhesive structures known as colloblasts (these are located on the tentacles)
* As diploblastic organisms, ctenophores have two layers of cells between the mesoglea (jelly-like middle layer which gives rise to the distinct muscular tissue).
The following is a diagrammatic representation of a Ctenophore from the phylum Ctenophora:
* The word ctenophores means comb bearers. They are named so because they have eight symmetrical comb rows (tracks) of ctenes on their body surface.
* Ctenophores are the largest animals capable of using cilia for locomotion (the fused cilia found in these organisms are known as ctenes).
* Ctenophores appear to flash a variety of colors when moving about. This is made possible by the cilia which act as prisms and break down light into individual colors. As well, some of the species have been shown to be capable of making their own light through a process known as bioluminescence.
Based on a number of studies, this involves making use of proteins known as opsins to flash blue-green light as a defensive mechanism.
Distribution and Ecology of Ctenophores
As mentioned, phylum Ctenophora are exclusively marine organisms. Most of the studies have been restricted to given geographic areas, however these species can be found in marine environments throughout the globe.
Some of the species can be found in abundance in given habitats as compared to others. For instance, whereas Pukia is common in Australia, Leucothea filmersankeyi is common the Bass Strait waters of Tasmania and parts of southeastern Australia.
* Some of the locations that have seen significant studies on these organisms include the Gulf of Naples, Northwestern Atlantic waters, Eastern tropical Pacific, as well as the Northern Pacific.
* In marine environments, phylum Ctenophora can be found in various habitats including coastal areas and open waters.
As most of the species are planktonic, they can be found at various depths from the surface to about 3,000 meters deep. However, a few species can also be found in the epibenthic and benthic zones with varying conditions from warm to cold.
Most of the species found on the cold, deep floor are often associated with a number of other organisms such as sponges and echinoderms.
Locomotion
Ctenophores are the largest animals that use cilia for locomotion. A unique feature among these organisms is the presence of eight comb rows located on the outer surface of the body.
In the majority of species, these comb rows run from the areas around the mouth region to the area around the statocyst (the aboral pole). Fused cilia (ctenes or comb plates) run across each of the rows. Therefore, each row might consist of thousands of cilia of varying lengths (about 2 millimeters on average).
The beating action of these structures (ctenes) allows the organism to move in any direction. Here, however, it's worth noting that the propulsion stroke is generally away from the mouth. As a result, the beating action of ctenes moves ctenophotes in whatever direction the mouth is pointing.
In some species, osmotic pressure is suggested to play an important role in regulating buoyancy.
* Some of the species move by means of creeping along the ocean floor. One of the best examples is the species Coeloplana bocki found near the Munseon Island in Korea. This is achieved through the flapping or waving of the auricles of the lobes.
Feeding/Nutrition
The majority of etenophores are predators and use their tentacles to capture prey (smaller crustaceans, various pelagic organisms, and invertebrate larvae in their surrounding). The two tentacles consist of sticky cells (colloblasts) which make it easier to capture prey.
Different species use different strategies to capture prey. For instance, though members of the genus Pleurobrachia (Sea gooseberries) are capable of swimming (using cilia), they often use the sit-and-wait strategy when it comes to hunting prey.
Essentially, this involves extending the branched tentacles and waiting with their mouth upright. If the prey swims into the net-like tentacles, it is captured (with the help of sticky cells) and pulled towards the mouth as the tentacles retract into the sheath.
Using fine tintilla (branched tentacles), smaller and more fragile species like Bathyctena have been shown to be capable of hunting and immobilizing smaller and relatively larger prey.
Some ctenophores (lobates, e.g. Mnemiopsis) use lobes for feeding. In these species, lobes are located near the oral end of the body. During feeding, studies have shown these organisms to generate feeding currents using the auricles. This mechanism makes it possible to accumulate prey towards the sticky lobes so that they can ingest.
* Some ctenophores lack tentacles and feed by engulfing prey. Using teeth-like macrocilia , they can rapidly capture and engulf or bite off pieces of the prey to consume it.
This method involves advancing their lips over the surface of the prey in order to engulf or bite off smaller pieces. These include members of the genus Beroe lie Beroe cucumis and Beroe forskalii.
* The detection of different types of prey has been shown to influence the elevation of colloblasts located on tentillum. If cases where ctenophores detect small prey, a single tentillum is discharged at speeds of between 40 and 60 m/s. At this speed and with elevated colloblasts, the prey is easily captured.
In the presence of a larger prey, however (e.g. copepods), they have to use more than one tentilla to increase the chances of capture.
* Some ctenophores, especially among creeping species, exist as parasites of other organisms.
Nervous and Sensory System
The nervous system in ctenophores exists in the form of a nerve net.
In this form, it looks like a ring around the mouth. It's especially more dense near several structures including the pharynx, tentacles, and comb rows among others. This network of nerves allows them to sense changes in the environment and even detect food or other predators in their surroundings.
The nerve net also contains a number of specialized structures that serve various functions. For instance, statocysts located on this network allow the organisms to sense when they are positioned upside down and unbalanced. Structures known as ocelli allow them to detect light intensity.
Defensive Mechanisms
As mentioned, the majority of ctenophores are capable of producing light using proteins located in some of their tissue (bioluminescence). These flashes have been shown to help deter some predators.
Aside from the flashing lights, some species (e.g. Pleurobrachia) can secrete a layer of slime from cells located in their tentacles. This makes it possible to escape from the grasp of the predators. Lastly, they can detect and rapidly move away from the predator.
Reproduction
The majority of ctenophores are hermaphrodites. As such, they have both the male and female reproductive organs. Both the eggs and other gametes are released into the environment (water) and drift until fertilization takes place. Generally, eggs are released and continue floating until they are fertilized.
* Some species are said to be capable of self-fertilization.
* Larvae of the species Mertensia ovum are capable of reproduction.
By releasing eggs into the surrounding, individuals increase the chances of fertilization. Following fertilization, the embryo gives rise to a larva that looks like a juvenile ctenophore. With sufficient nutrients and favorable conditions, the juvenile develops into an adult and the cycle continues.
Though they generally have a simple lifecycle, especially when compared to jellyfish, ctenophores are capable of regenerating damaged tissue, cloning, and splitting off to give rise to new individuals (especially among platyctenids).
* Regeneration among ctenophore species occurs through a process known as proliferation-dependent non-blastemal-like regeneration. Using this method, most of the species (with the exception of beroids) have been shown to be capable of replacing almost half of their body structure.
* Members of the genus Ocryopsis are not hermaphrodites.
* Gonads of ctenophores are located under the comb rows within the internal canal. During reproduction, gametes are released into the surrounding through pores found in the epidermis.
See also: Acoelomata
What does Phylum mean in Biology?
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References
Allaby, M. (2020). A Dictionary of Zoology.
Bavestrello, G. and Sarà, m. Porifera, Cnidaria, and Ctenophora. Biological Science Fundamentals and Systematics – Vol. III.
Gershwin, L., Lewis, M., Gowlett-Holmes, K., and Kloser, R. 2014. The Ctenophores. In: Pelagic Invertebrates of South-Eastern Australia: A field reference guide. Version 1.1. CSIRO Marine and Atmospheric Research: Hobart.
Links
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/tentaculata
https://animaldiversity.org/accounts/Ctenophora/classification/
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