Jellyfish are gelatinous aquatic animals belonging to the subphylum Medusozoa. These free-swimming invertebrates evolved around 500 million years ago and have been roaming the oceans around the world, in both warm water and cold water. They are easily identified by their trailing tentacles and umbrella-shaped bells.
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The subphylum Medusozoa consists of four major classes, including Scyphozoa (or large jellyfish), Hydrozoa (or small jellyfish), Cubozoa (or box jellyfish), and Staurozoa (or stalked jellyfish). There are more than 1200-1800 species of jellyfish divided into these four classes, out of which Scyphozoa has about 200 species, Staurozoa contains roughly 50 species, Hydrozoa includes 1000-1500 species, and Cubozoa consists of approximately 20 species. Here are some of the commonly found types of jellyfish species:
Size: Their size varies depending on the species, the smallest being the creeping jellyfish of the genera Eleutheria and Staurocladia, having bell disks with a diameter of 0.5 mm (0.02 in). The lion’s mane jellyfish is probably the largest species with tentacles extending up to a length of 36.5 m (120 ft). Another giant species is the Nomura’s jellyfish, which can have a bell diameter of 2 m (6 ft 7 in).
Weight: An adult box jellyfish can weigh up to 2 kg (4.4 lb) while the giant Nomura’s jellyfish weighs about 200 kg (440 lb).
Color: Some are clear, while others are purple, blue, yellow, and pink. There are bioluminescent jellyfish, too, meaning they can create light.
Bell: It is an umbrella-shaped hollow structure containing a mass of jelly-like tissue, called mesoglea, which functions as its hydrostatic skeleton. The mesoglea comprises water, collagen, fibrous proteins, and mobile amebocytes that can engulf bacteria and debris.
Manubrium: It is a tube-like structure located on the underside of the bell. It hangs down from the middle, along with the mouth, which opens into a gastrovascular cavity used for digestion and absorption of nutrients.
Eyes: Several species of jellyfish have light-sensitive organs called ocelli or simple eye. Others, such as the box jellyfish, have an advanced vision with each individual possessing 24 eyes.
Jellyfish are found in both seawater and freshwater throughout the world. Examples of freshwater species include cosmopolitan hydrozoan jellyfish and golden jellyfish. Some species like the lion’s mane jellyfish are capable of surviving in cold Arctic water, while others such as the tropical box jellyfish live in warm waters near the tropics.
Although most jellyfish species float close to the ocean’s surface, some species, including the upside-down jellyfish that lie on the bottom of lagoons, and stalked-jellyfish that remain attached to seaweed or rocks on the bottom.
Not much is known about the lifespans of jellyfish species that are found on the ocean floor. However, the moon jellyfish found in coastal waters have an average lifespan of 12-18 months. With proper care, captive moon jellyfish can survive for about 20 years. The flame jellyfish, found in warm temperate waters, have a life expectancy of 3-12 months.
They are typically parasitic or predatory animals, and their diet includes planktonic organisms (algae, bacteria, protozoa, and archaea), crustaceans, fish eggs, larvae, small fish, and other jellyfish. Some species, like the Aglaura hemistoma, are omnivores that feed on a mixture of microscopic plants and free-floating protozoa and crustaceans.
Adult jellyfish, in most cases, discharge sperms and eggs in the surrounding water where the fertilized eggs mature into larvae. In some species, the sperms travel into the mouth of the female jellyfish and fertilize the eggs within the body. These fertilized eggs develop into planula larvae, which become cylindrical vase-shaped polyps. These inconspicuous polyps pass through the immature ephyra stage and finally transform into matured medusae.
The planula is a small, free-swimming, flattened, and ciliated larval form that develops into a polyp, containing a small stalk with a mouth having upward-facing tentacles. These polyps resemble corals and sea anemones.
1. Do jellyfish have brains?
No, jellyfish do not have a central nervous system or brain. However, they possess a basic set of nerves at the lower end of their tentacles. The nerves help detect temperature, salinity, and touch.
2. What predators may eat jellyfish?
Swordfish, tunas, sharks, sea turtles, and some salmon species usually prey upon jellyfish. Large species of jellyfish may also feed on the smaller ones.
3. Do jellyfish have eyes?
No, a jellyfish does not have any eyes or ears.
4. Can you eat jellyfish?
Some jellyfish species, including cannonball jellyfish, Nomura’s jellyfish, and jelly blubber, are suitable for consumption. These jellyfish are harvested and consumed as seafood in several Asian countries, such as China, Korea, Japan, Thailand, Vietnam, Malaysia, the Philippines, and Indonesia.
5. How to treat a jellyfish sting?
First, get out of water. Then, rinse the affected area with vinegar for about 30 seconds to stop the itching. You may use an oral antihistamine or mild hydrocortisone cream to relieve discomfort and swelling.
6. What is a group of jellyfish called?
A group of jellyfish is called a ‘swarm’ or ‘bloom’.
7. Do jellyfish have hearts?
No, they do not have a heart.
8. Do all jellyfish sting?
Although all jellyfish sting, not all of them are venomous. The box jellyfish and the Portuguese man o’ war are the two most poisonous jellyfish species on earth today.
