Cnidarians, which include jellyfish, hydroids, sea anemones, corals, and small marine parasites, are aquatic invertebrates within the phylum Cnidaria. They are characterized by specialized stinging cells or cnidocysts used to capture prey, and a decentralized nervous system distinguishes them from other invertebrate groups.
These invertebrates inhabit both marine and freshwater bodies and exhibit two fundamental body forms (polymorphism): the stationary polyp and the mobile medusa. Each form displays radial symmetry and is characterized by tentacles encircling their mouths.
Currently, over 11,000 species of cnidarians fall under four main classes: Anthozoa (sea anemones, corals, sea pens), Scyphozoa (jellyfish), Cubozoa (box jellies), and Hydrozoa (hydrozoans).
Cnidarians are diploblastic organisms, with their body cells organized in two distinct germinal layers, the ectoderm and endoderm. These cells are bound by intercellular connections and rest on carpet-like basement membranes.
They vary greatly in size, from the tiny hydra Psammohydra nanna, which is less than half a millimeter long, to the massive lion’s mane jellyfish, which can have a diameter of over 6 feet 7 inches (2 meters) and tentacles stretching up to 246 feet (75 meters).
Most cnidarians exhibit polymorphism and exist in one of the two radially symmetrical body forms: free-swimming, bell-shaped medusae or sessile, tube-like polyps. Hydrozoans can switch between the two forms, depending on their requirement.
These forms do not have a distinct head; hence, polarity is determined by the location of the mouth. The end of the body nearest to the mouth is the oral end, while that farthest from it is known as the aboral end. They do not have a separate anus for excretion.
A set of tentacles dotted with cnidocytes (stinging cells) is found along the fringes of these forms. In medusae, an additional ring of tentacles surrounds the mouth.
These specialized cells contain large secretory organelles called cnidocysts, which are useful in stinging enemies. There are three types of cnidocysts found in cnidarians:
Components
The cnidocytes are composed of several components:
The medusoid forms of cnidarians are primarily supported by the mesoglea, a thick and elastic layer of extracellular matrix. In organisms like Hydra and sea anemones, the water within their digestive cavities serves as a hydrostatic skeleton.
In contrast, some colonial polyps have a skeleton made from a chitinous epidermis, often reinforced with externally attached sand grains and shell fragments. Stony corals, belonging to the order Scleractinia and members of the class Octocorallia, possess a hard, calcareous exoskeleton composed of calcium carbonate. These exoskeletons are also present in the hydrozoan families Milleporidae, Stylasteridae, and Hydractiniidae, which are part of the order Anthoathecata.
The ectodermal and endodermal layers rest on a fibrous basement membrane and are separated by a jelly-like mesoglea. The upper layer, or ectoderm, is composed of the following cell types:
In addition to these cells, the gastrodermis of the stomach includes glandular cells that release digestive enzymes. The mesoglea may also contain amoeba-like cells, similar to those found in sponges, although the number of layers formed by these cells differs between the two groups.
Cnidarians do not have a distinct circulatory system; instead, nutrients absorbed by their cells are distributed through the mesoglea.
Since these invertebrates lack specialized respiratory organs, gas exchange occurs directly between their cellular layers and the surrounding water. Some cnidarians associated with photosynthetic symbionts produce large quantities of antioxidants to neutralize the excess oxygen the symbionts produce.
Although cnidarians lack a centralized nervous system, their bodies are innervated by nerve nets connected to musculature and sensory structures known as rhopalia. These structures detect external stimuli, such as light, pressure, and chemical changes, and generate corresponding signals.
In medusae, the rhopalia are arranged around the margin of the bell-like aboral surface and control the motor nerve net, which innervates the swimming muscles. Scyphozoans have a diffuse nerve net functioning as a parallel nervous system, with ganglia acting as local coordination centers and linking sensory and motor neurons.
In hydrozoans, chemical synapses between nerve cells are connected by gap junctions. These synapses often contain neurotransmitters such as glutamate, gamma-aminobutyric acid (GABA), and acetylcholine.
Cnidarians and comb jellies, originally grouped together in the phylum Coelenterata, share a common feature: using a single cavity for feeding, respiration, and excretion. However, recent research has revealed significant differences between these two groups, leading to the reclassification of cnidarians into their own distinct phylum, Cnidaria.
In 2005, scientists Katja Seipel and Volker Schmid suggested that members of Cnidaria and Ctenophora (both diploblasts) are descendants of triploblasts. Further molecular research has shown that cnidarians, including sea anemones and corals, contain genes typically found in bacteria, protists, plants, and fungi.
According to the World Register of Marine Species, modern cnidarians are classified into four main classes: Anthozoa (sea anemones, corals, sea pens), Scyphozoa (jellyfish), Cubozoa (box jellies), and Hydrozoa (hydrozoans). The parasitic classes Myxozoa and Polypodiozoa are now considered separate due to their highly derived nature.
Additionally, small sessile cnidarians known as stauromedusae were traditionally grouped under Scyphozoa but have recently been reclassified into a separate class, Staurozoa, based on new research findings.
Within Cnidaria, the class Anthozoa is considered the sister group of the rest, suggesting the earliest cnidarians were sessile with no medusa stage. However, how the other groups acquired the medusa remains unclear.
The earliest cnidarian fossils date back to around 580 million years ago. They resemble tube-like colonies and mineral structures similar to modern-day corals, suggesting that cnidarian and bilaterian lineages had already diverged by then. From the Ediacaran Period, the only existing fossils include the Canadian polyp Haootia and the British polyp Auroralumina, the latter being the earliest known predatory animal.
Research on more recent rock formations has uncovered mineralized fossils of cnidarians. For instance, a few fossils resembling corals have been excavated from rocks of the Cambrian Period. These corals are believed to have diversified in the Early Ordovician Period but soon went extinct during the Permian–Triassic extinction event, around 252 million years ago. Following the Cretaceous–Paleogene extinction event approximately 66 million years ago, which saw the extinction of the dominant reef-building bivalves, stony corals have dominated the reefs.
These invertebrates are carnivorous, primarily feeding on zooplankton. Additionally, they obtain nutrients from endosymbiotic bacteria and algae, like dinoflagellates. Some cnidarians are parasitic, living off the flesh of larger fish and jellyfish.
While most medusae are passively carried by water currents, they can also use jet propulsion. This involves contracting the circular muscles beneath the concave oral surface (subumbrellar surface) to expel water from their bodies, pushing them forward. The flexible mesoglea returns to its original position during the recovery stroke, preparing the cnidarian for the next action stroke.
In contrast, some hydras and sea anemones creep, crawl, or somersault over rocks and slowly move along streams or ocean beds. Additionally, some cnidarians wriggle their bases to shift from one location to another.
Cnidarians feed through various ways, such as predation, filter-feeding, parasitism, or symbiosis.
Most free-swimming cnidarians use cnidocysts to capture prey, while sessile polyps rely on other organisms to contact their tentacles and provide food particles. Active hunters use their tentacles to move captured prey into their mouths, where muscular action aids in swallowing the food.
Once inside the digestive tract, gastric glands secrete enzymes that liquefy the prey. This nutrient-rich slurry circulates through connecting tunnels, allowing gastrodermal cells to absorb the nutrients, which takes a few hours.
Nutrients are circulated either by water currents generated by cilia or through muscular movement. These nutrients are absorbed through diffusion or by special mobile cells in the mesoglea of medusae. Accumulated nitrogenous waste, primarily ammonia, is expelled from the mouth with the help of external and internal water currents.
Most cnidarians possess a remarkable capacity for regenerating lost body parts. While this ability is limited in medusae, polyps can regenerate entire sections of their bodies, even from separate cells. This regenerative capability allows these invertebrates to recover from injuries caused by predators and also facilitates asexual reproduction.
Cnidarians release their eggs into the water through spawning, influenced by various environmental factors such as changes in water temperature, moon phases, and sunrises or sunsets. Sometimes, they spawn simultaneously in the same location, producing massive ova and sperm to overwhelm predators and prevent them from consuming too many gametes. This phenomenon can be observed in the Great Barrier Reef, where around 110 coral species and several non-cnidarian invertebrates release many gametes, causing the water to become cloudy.
After fertilization by sperm, the egg cells divide to form a hollow sphere known as a blastula. This sphere then undergoes gastrulation, forming a depression at one end. In cnidarians, this depression occurs at the animal pole, the end away from the yolk. Following gastrulation, a ciliated, cigar-shaped planula larva emerges. These larvae swim freely and eventually attach to a substrate if the species has a polyp stage in its life cycle.
Larvae of the class Anthozoa have large yolks and typically feed on plankton, often forming associations with endosymbiotic algae. In contrast, larvae of Scyphozoa and Hydrozoa possess very little yolk and are usually not associated with them.
Cnidarian larvae often undergo a complex life cycle, including polyp and medusa stages. In jellyfish and box jellies, sexual reproduction forms a zygote. The zygote then develops into a larva called a planula, which settles on a suitable substrate and develops into a polyp. Over time, the polyp absorbs its tentacles and undergoes strobilation, splitting horizontally into disc-like juvenile medusae. These juveniles swim away and gradually mature while the polyps regenerate and continue to strobilate periodically. The adult medusae possess gonads in their gastroderm and produce gametes during the breeding season. This alternation between the asexual polyp and sexual medusa stages is known as the ‘alternation of asexual and sexual phases’ or metagenesis.
Some oceanic scyphozoans bypass the polyp stage entirely, while some hydras skip the medusa stage. In anthozoans, all polyps are responsible for producing gametes and sexual reproduction, distinguishing them from other cnidarian groups.
In this form of reproduction, new individuals (clones) emerge either from tissues budding off the parent or through the lengthwise or crosswise division of the parent’s body.
Hydrozoan polyps reproduce solely by budding, whereas their medusae divide along the middle of their bodies. In contrast, scyphozoan polyps can both bud and split down the middle. Anthozoans typically reproduce by splitting horizontally above their bases.
Cnidarians are preyed upon by sea slugs, flatworms, comb jellies, and starfish. Butterfly and parrot fish feed on corals, while marine turtles usually consume jellyfish.
Coral reefs are the foundation for some of the most productive aquatic ecosystems. Corals form symbiotic relationships with endosymbiotic algae, which are primary producers capable of efficiently converting inorganic substances into organic forms that other organisms use as their food.
Corals live in mutualistic relationships with plants in mangroves and seagrass meadows. They attach to the roots of these plants, providing them stability against strong water currents and preventing soil erosion. In return, mangroves and seagrass filter out silt and pollutants from the water, creating a safe and clean environment for corals to thrive.
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