Jellyfish, often referred to as sea jellies or simply jellies, are predominantly marine cnidarians distinguished by their gelatinous, bell-shaped medusa form. The bell is composed mainly of mesoglea, a translucent, jelly-like substance that is more than 95% water. Their bodies exhibit radial symmetry, more particularly tetramerous symmetry, in which body parts are arranged in multiples of four.
These cnidarians belong to the subphylum Medusozoa, encompassing all species that exist primarily in the medusa stage rather than as sessile polyps. Around 2,000 species of jellyfish are found across all oceans. They move by rhythmically contracting and relaxing their bell, gliding through water with remarkable energy efficiency.
Many jellyfish species are bioluminescent, producing light through specialized proteins that help them deter predators, attract prey, or communicate in the dark ocean depths. Notably, the Green Fluorescent Protein (GFP), the compound that revolutionized biological research, was first discovered in the jellyfish Aequorea victoria.
These animals range from about 0.04 in (1 mm) to around 6.5 ft (2 m) in bell height and diameter. The smallest jellyfish, belonging to the genera Staurocladia and Eleutheria, have bell diameters of around 0.019 in (0.5 mm).
One of the largest jellyfish, which also happens to be one of the longest animals in the world, is the lion’s mane jellyfish (Cyanea capillata), which is capable of attaining a bell diameter of over 6 ft 7 in (2 m) and has tentacles that may be as long as 119 ft 9 in (36.5 m). Other large species include Nomura’s jellyfish (Nemopilema nomurai), which grows over 2 m in bell diameter and weighs up to 440 lb (200 kg).[1][2]
Most jellyfish are characterized by the umbrella-shaped, hollow bell composed of a transparent jelly-like material known as mesoglea. The mesoglea is ensheathed by the epidermis on the outer side and the gastrodermis on the inner side. Most species bear four to eight long tentacles around the edge of the bell. As cnidarians, the tentacles are lined with stinging cells (cnidocytes) that release venom to paralyze or kill prey.
The underside of the bell bears a stalk-like structure, the manubrium, which hangs down from the roof and bears the mouth (which also functions as the anus). Typically, four oral arms are connected to the manubrium, which are used for handling and transporting food to the mouth.
The mouth leads to the gastrovascular cavity, which is further subdivided by four thick septa into a central stomach and four gastric pockets.
They lack specialized organ systems. Since their epidermis is thin, oxygen and nutrients diffuse directly through the surface cells.
Although they lack a brain or a centralized nervous system, they do possess a loose network of nerves, the nerve net, under the epidermis, as well as a circular nerve ring that runs around the edge of the bell.
The edge of the bell is divided into rounded lobes called lappets, and the spaces between them contain simple sensory structures known as rhopalia. In many jellyfish species, these rhopalia bear simple eyes (pigment spot ocelli) that detect changes in light and darkness. However, box jellyfish (class Cubozoa) are exceptional, possessing 24 eyes arranged in four rhopalia, each containing six eyes of four types (two complex lens eyes and four simpler ones). Together, these eyes give box jellyfish a 360-degree view of their surroundings.
All species of jellyfish are classified into 4 subclasses.
Members of Scyphozoa, Cubozoa, and Staurozoa are medusa-dominant, while those of Hydrozoa show diversity in form. They may exist entirely as polyps (as in Hydra), entirely as medusae, or alternate between the two stages. The medusae of Hydrozoa possess a thin internal diaphragm called the velum inside the bell. Among these classes, only Staurozoa are stalked and sessile.
Since jellyfish lack hard body parts, their fossils are scarce. Of the limited evidence available, the oldest unambiguous fossils, belonging to the genus Burgessomedusa, date back to the Middle Cambrian Period.[3]
These cnidarians are found in all oceans worldwide, from the tropics to the poles. Most species are marine, occupying shallow coastal waters to even the deep sea. However, a few species, such as the freshwater jellyfish (Craspedacusta sowerbii), are found in freshwater environments, including rivers, lakes, quarries, and impoundments.[4]
Some jellyfish populations are also found in saltwater lakes. For example, in Jellyfish Lake of Palau, swarms of around 1.5 million golden jellyfish (Mastigias) once lived in a landlocked marine lake.[5]
Most jellyfish are a part of the plankton, living much above the ocean floor, though a few, such as upside-down jellyfish (Cassiopea), are benthic, lying on the bottom of shallow lagoons.
As cnidarians, jellyfish are carnivores with a primary diet comprising copepods and other microscopic crustaceans, comb jellies, as well as fish (and their larvae). Some species, such as the Lion’s mane jellyfish (Cyanea capillata), feed on juveniles of other jellyfish, while others, like Aglaura hemistoma, are omnivorous, feeding on a mixture of zooplankton and phytoplankton such as dinoflagellates.[6]
The spotted jelly (Mastigias papua) houses photosynthetic algae (zooxanthellae) within its body, deriving some of its nutrition from photosynthetic products.
Jellyfish catch their prey using the stinging cells on their tentacles. When a prey touches the tentacles, these cells fire and inject venom that paralyzes or kills it. The jellyfish then uses its oral arms to direct the food toward its mouth.
The food passes into the gastrovascular cavity, where digestive enzymes break it down. Finally, the nutrients are absorbed by the cells lining the cavity.
They are among the most energy-efficient swimmers, moving by expanding and contracting their bell-shaped bodies to push water behind (pulsation). During contraction, muscles squeeze the bell, expelling water and creating a vortex ring that propels the jellyfish forward. When the bell relaxes, the stored energy in the mesoglea automatically restores it to its expanded shape. As the bell expands, a second vortex ring forms inside, spinning faster and sucking water back into the bell. This inward flow pushes against the center of the body, creating a secondary, passive forward boost.
Some species, such as the cannonball jellyfish (Stomolophus meleagris), form associations with a variety of marine animals, including both fish and crustaceans. This species is known to host up to ten fish species beneath its bell and tentacles, along with juvenile longnose spider crabs (Libinia dubia) taking refuge within the bell. In both cases, the animals benefit by gaining food and protection, while the jellyfish experiences little to no harm (commensalism).
Under certain environmental conditions, such as warm temperatures, high salinity, and nutrient-rich agricultural runoffs, jellyfish populations tend to grow rapidly, leading to dense aggregations in the ocean. They then consume large quantities of zooplankton, fish eggs, and fish larvae, thereby reducing the food available for aquatic animals. Moreover, when these jellyfish die, they release large amounts of organic matter, such as ammonium, phosphate, and carbon compounds. The microbial breakdown of these compounds consumes too much oxygen, leading to low-oxygen (hypoxic) conditions in the habitat.
While some species, such as the moon jellyfish (Aurelia aurita), have an average lifespan of 8 to 12 months, others may live even longer. For example, Turritopsis dohrnii, often called the ‘immortal jellyfish’, is capable of transdifferentiation (in experimental conditions of the laboratory), which allows the jellyfish to revert from a sexually mature medusa to an immature polyp stage. Thus, hypothetically, this species can potentially survive indefinitely.[7]
Most jellyfish are dioecious, meaning each individual is either a male or a female. Both males and females release their gametes into the surrounding water, with all individuals spawning at about the same time of day. The eggs fertilize externally, giving rise to ciliated planula larvae (planulae) typically within 6 to 24 hours.
After complete development, the planula settles on a solid surface, such as a rock or shell, and transforms into a polyp. The polyp is usually sessile, attached to the substrate by a stalk, and has tentacles surrounding its mouth. It may exist as a solitary individual or form a colony.
After a while (which may extend to years for some species), the polyp stops feeding and starts to reproduce asexually by budding, creating more polyps. A drop in temperature or hormonal changes may trigger transverse fission, a process in which the polyp divides horizontally into a stack of thin, disc-like segments called strobilae. Each segment at the top of the stack eventually detaches as a free-swimming ephyra. The ephyra, which is the precursor of the adult medusa, swims away from the polyp and starts developing individually. In 3 to 6 weeks, it attains the characteristic bell shape, develops reproductive organs, and transforms into an adult medusa.
Owing to the toxins they release through their cnidocysts, jellyfish have few predators. They are preyed upon by fish, such as sharks, tuna, and swordfish, as well as sea turtles and even penguins.[8] Some sea anemones have also been observed feeding on jellyfish if they drift into their range. When washed ashore, jellyfish may be consumed by terrestrial mammals, such as foxes, as well as birds.
Jellyfish serve as intermediate hosts for several parasites, including some endoparasitic helminths and digenean trematodes (particularly of the family Lepocreadiidae).[9]
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