Jellyfish are both beautiful and strange. Jellyfish are the oldest multi-organ animal, having survived all five great extinction events on Earth, as evidenced by their presence 500 million years ago. They preyed on dinosaurs for millions of years and are older than dinosaurs. Because jellyfish lack a brain, heart, and complex neurological system, they do not experience pain in the same manner that people do. They are not true fish; scientists call them “gelatinous zooplankton.”
Because of their unpleasant stings, many are terrified to swim among jellyfish. One box jellyfish’s venom may kill a human in less than five minutes, making it one of the most deadly venoms in the world. To inject venom into the victim, nematocysts, or coiled harpoons, exert almost 550 times the pressure of Mike Tyson’s hardest punch. Jellyfish have a limited life span, however one species is known to cheat death. Some jellyfish species illuminate in the dark, enhancing their attractiveness along with their long drifting tentacles.
So, despite their fragile bodies, how do jellyfish survive five great extinction events on Earth?
What causes jellyfish to shine and how do they do it?
How do jellyfish manage to avoid death?
Scyphozoa is the scientific name for jellyfish (phylum cnidaria). Jellyfishes are invertebrates and planktonic marine creatures. The phylum cnidaria has around 10,000 species of jellyfish. There are around 300,000 different types of jellyfish. Jellyfish have an umbrella-shaped bell with four to eight tentacles hanging from it, although some species have hundreds. Jellyfishes range in size from less than half an inch to about 16 inches. The tiniest jellyfish is 1mm broad, while the lion’s mane (cyane capillata) is around 3 feet wide. Most jellyfish weigh between 0.7 and 14 ounces.
The majority of jellyfish dwell in saltwater, however certain hydromedusae survive in freshwater. The worldwide hydrozoan jellyfish, craspedacusta sowerbii, is the greatest example of a freshwater jellyfish. It has a diameter of less than one inch, is colourless, and has no stings. Some populations can be found in coastal saltwater environments, such as Palau. Jellyfish have three layers in their bodies: an outside layer called the epidermis, a middle layer formed of a thick, elastic, jelly-like material called mesoglea, and an interior layer called gastrodermis. The mesoglea, which is 95% water, collagen, and other fibrous proteins, comprises the hydrostatic skeleton of jellyfish. Manubrium, a stalk-like structure descending down from the centre of the bell, is present.
The mouth also serves as an anus. The manubrium is related to the four oral arms. The mouth opens into the gastrovascular cavity, where digestion and nutrition absorption occur. Their bodies include a basic neurological system or nerve net, which allows them to smell, perceive light, and respond to stimuli. Despite the lack of a genuine brain, cnidarians have a “ring” nerve system that is involved in motor and sensory activities. The neural network controls muscle contraction and movement.
Stings are made up of nematocytes and are found on tentacles. Jellyfish stings can also be seen around the lips and stomach. They are devoid of a heart, a brain, and bones. The majority of jellyfish lack specific osmoregulation, respiration, and circulatory systems. Because they lack a respiratory apparatus, oxygen diffuses through the epidermis. Although they have minimal control over their locomotion, certain species can swim actively.
The rhopalia are made up of primitive sensory organs that let them to perceive light, odour, direction, and aquatic vibration. A jellyfish’s nerve net detects inputs and transmits impulses. Neurons in the rhopalial ganglia govern direction and swimming pace. The rhopalia of some jellyfish species have ocelli, which are light-sensitive structures that distinguish light from darkness. Ocelli include coloured cells. Box jellyfish have more advanced vision than other kinds.
Box jelly contains 24 eyes, arranged into six groups of six eyes each, two of which perceive colour and four of which process parallel information that they may observe 360-degrees around their surroundings.
The visual system of jellyfish ranges from photoreceptor cells to sophisticated image-forming eyes.
Jellyfish have a complicated life cycle with both sexual and asexual stages. Jellyfish congregate and spawn during dawn or dusk. Some jellyfish may lay up to 45,000 eggs in one stand. The sperms that were released fertilised the eggs that were nearby. Sperm swims into the female mouth and fertilises the egg in her body in certain species.
Fertilized eggs float in water and develop into small oval planktonic planula larvae lined on the exterior with cilia. It swims down to the substrate or floor, attaches, and transforms into a polyp, which can be sessile or free-floating, solitary or colony. Polyps can be present for years. On its upper side, the polyp features an attaching disc and a mouth surrounded by tentacles.
The polyp has an advanced digestive system and feeds the majority of the time. In a process known as strobilation, polyp tentacles are reabsorbed. The body begins to narrow, and a transverse constriction forms. These constrictions are widened and separate the immature medusa section known as ephyra. Each ephyra separates from the parent polyp and swims and eats autonomously. This ephyra develops into a mature medusa, a jellyfish that may restart its life cycle.
Fission is a method of reproduction used by certain hydromedusae. Strobila that reproduces asexually can survive for many years and generate a large number of young medusae each year. Jellyfish are either male or female, with the odd hermaphrodite. Jellyfish travel through water by radially expanding and contracting their bell-shaped bodies, pushing water behind them. Muscles assisted in body contraction. Jellyfishes eat planktonic creatures, crustaceans, eggs, larvae, and tiny fishes and are carnivorous or parasitic.
They consume food and expel undigested food through their mouths. Jellyfish have tentacles up to 10 feet long with numerous cells called cnidoblasts that hold nematocysts. When the jellyfish comes into contact with a foreign object, the pressure inside the nematocysts forces the threads to uncoil. The stinging venom is released by the jellyfish’s tentacle. Jellyfish sting to defend themselves against predators and prey. Many predators consume jellyfish, including sharks, sea anemones, tunas, sea turtles, and penguins.
The cannonball jellyfish has a symbiotic connection with ten different fish species as well as a long nose spider crab that lives within the bell and shares the jellyfish’s food and chews its tissues. When floating in the ocean, some jellyfish are so little that they are undetectable. One jellyfish species (Turritopsis nutricula) can avoid death and become “immortal.”
When threatened, this species can undergo cellular transdifferentiation. It may be found in warm seas such as the Caribbean and the Mediterranean. Scientists are conducting extensive investigation into this capacity.
Some jellyfish are edible and are consumed in countries such as Japan and Korea. The jellyfish was also turned into sweets by the Japanese. Sugar and starch syrup are used to make the sweet and salty caramel.
They are also used in salads, fried into crispy noodles, and eaten as sushi with soy sauce. Jellyfish are an excellent source of nutrition. They are low in calories and abundant in antioxidants, which protect cells from damage. Swimming up in reaction to somatosensory stimulation, swimming down in response to low salinity, diving in response to turbulence, avoiding rock barriers, and swimming horizontally are all examples of their ecological behaviour. These aren’t just reflexes.
Jellyfish with transparent bodies are less noticeable to prospective predators. Their translucent or flexible body is another adaption. Jellyfish have genetic modifications that allow them to maintain molecular salt content in their bodies. This modification allows them to move vertically and horizontally to capture food while remaining unaffected by changes in ocean salinity.
The most attractive and noticeable aspect of jellyfish is the emission of multicoloured lights from the body. This is known as jellyfish bioluminescence. Bioluminescence is the production of light by a chemical reaction in the bodies of living creatures. When luciferin combines with oxygen in the presence of the luciferase enzyme, energy is released, and GFP uses this energy to create green light.
Sometimes luciferin and luciferase combine with oxygen to form a single molecule known as a photoprotein. An organism constantly introduces new luciferin into its system. Some jellyfish get it from their food, while others make it. In the presence of calcium ions, luciferin creates scintillating blue flickers. The Atolla jellyfish lives in the deep water, where there is very little light. In reaction to predator defence, it produces blue-green light. This bioluminescence is a warning sign for jellyfish, indicating that some species are extremely hazardous.
Scientists have exploited the light of GFP to observe cancer cell proliferation, trace the progression of Alzheimer’s disease, and illuminate a variety of other biological processes. If you cut a jellyfish in half, the fragments can recover and become two new jellies. If a jellyfish starves or becomes unwell during its juvenile period, the polyp will not regenerate and will die.
The body of a jellyfish is 98% water. As a result, it mixes in with the surrounding water. They lack a brain, blood, lungs, and heart. These qualities enable jellyfish to live the longest period of time of any multi-organ creature. Because of all of the traits and adaptations of jellyfish, people are compelled to declare them the most beautiful and intriguing characters in the maritime world.
Reviewed by:
Dr. Muhammad Khalid Mukhtar (Ph.D.)
University of Sargodha, Sargodha