jellyfish: Quick Facts

Jellyfish are planktonic marine invertebrates belonging to the class Scyphozoa (phylum Cnidaria), with about 200 described species. The term is also used for other cnidarians with a medusoid body form, like hydrozoans, and even unrelated organisms such as comb jellies and salps.

Most jellyfish are free-swimming and found in all oceans, typically ranging from 1 to 16 inches in diameter, though some can be much larger. Their bodies are composed of nearly 99 percent water, with a thick gelatinous layer called mesoglea between two cellular layers. They capture prey, such as copepods and fish larvae, using tentacles armed with stinging cells called nematocysts. Their life cycle usually involves a sessile polyp stage that buds off young medusae, which then grow into adults.

Jellyfish inhabit all of the world's oceans, from surface waters to the deep sea. While most jellyfish are marine, a few species, like those in the genus Craspedacusta, can be found in freshwater environments. The "immortal jellyfish" (Turritopsis dohrnii), though native to the Mediterranean Sea, has been introduced to marine habitats globally. They are found in various regions, including coastal areas of North America and the waters around Japan.

Jellyfish move through the water using a combination of passive drifting and active propulsion. They possess a nerve net, a decentralized nervous system that coordinates their movements. By contracting and relaxing their bell-shaped bodies, jellyfish can expel water, creating a jet-propulsion effect that allows them to move. This process is aided by a rudimentary internal circulatory system that passes water through canals within their gelatinous bells. While they can actively swim, jellyfish also drift with ocean currents, making them planktonic, or "sea-flea". Their movement is also influenced by sensory organs like rhopalia, which help them detect environmental cues for navigation.

Jellyfish are carnivores that capture their food using stinging cells called nematocysts located on their tentacles. While they don't actively pursue prey, they ensnare organisms that come into contact with their tentacles.

Their diet primarily consists of small animals such as copepods, fish larvae, and zooplankton. Some jellyfish also suspension feed, extracting minute animals and algae from the water. Once prey is captured, tentacles move it to the mouth for consumption. The lion's mane jellyfish, for example, uses its numerous tentacles to immobilize prey with neurotoxins before specialized oral arms transport the food to its mouth.

Jellyfish sting and protect themselves using specialized stinging cells called nematocysts. These microscopic capsules, found on their tentacles, contain a coiled, barbed thread. When stimulated, the thread rapidly everts, injecting venom into prey or potential predators. This venom can paralyze or kill, serving as both a defense mechanism and a tool for capturing food. Some species, like the box jellyfish, possess potent venom that can be extremely painful and even deadly to humans. Jellyfish do not have brains but use a nerve net to coordinate movements and respond to stimuli, aiding in navigation and hunting.

Jellyfish play a dual role in marine ecosystems as both predators and prey. They capture small animals like copepods, fish larvae, and other small organisms using their tentacles, which are equipped with stinging cells called nematocysts. Some jellyfish also function as suspension feeders, extracting phytoplankton and minute animals from the water.

However, jellyfish are also a food source for various marine animals, including fish, sea turtles, sea anemones, penguins, and other jellyfish. This positions them as an important link in the ocean food web.