Jellyfish are aquatic invertebrates representing the medusa stage of organisms within the phylum Cnidaria, which also includes corals and sea anemones. They are not fish, but free-swimming organisms characterized by bell-shaped bodies and trailing tentacles. Jellyfish are ancient life forms, existing for at least 500 million years, making them one of the oldest multi-organ animal groups on Earth. They inhabit every ocean basin, from surface waters to the deep sea, surviving in diverse marine environments globally.
Anatomy: The Simple, Water-Based Body
The structure of a jellyfish is defined by its simplicity, allowing it to exist as a buoyant, gelatinous mass. The body, known as the bell, exhibits radial symmetry, with parts arranged around a central axis. The defining feature is the mesoglea, a non-living, jelly-like substance sandwiched between two thin layers of epithelial tissue. This mesoglea is responsible for the animal’s shape and volume, giving it a water content that often exceeds 95%.
Jellyfish lack a centralized brain, heart, lungs, and skeletal system, relying on this water-based structure for support. Movement is achieved through rhythmic, pulsating contractions of the bell margin, which ejects water for propulsion. A nerve net is distributed throughout the epidermis, allowing them to sense and react to their environment. Sensory information is processed by specialized rhopalia, which contain rudimentary eyes, chemoreceptors, and statocysts for balance and orientation.
The Mechanism of the Sting
The sting is the most well-known characteristic, used to capture prey and for defense. The stinging apparatus is contained within specialized cells called cnidocytes, located primarily on the tentacles. Inside each cnidocyte is a harpoon-like organelle known as a nematocyst, which is a highly effective venom delivery system. Physical or chemical contact triggers an extremely rapid discharge, propelling a coiled, hollow tubule outward in milliseconds.
The tubule penetrates the victim’s skin, injecting a complex venom composed of proteinaceous porins, neurotoxic peptides, and other bioactive molecules. Venom strength varies dramatically across species, ranging from mild skin irritation to being potentially lethal to humans. Box jellyfish (Class Cubozoa) are notorious for possessing some of the most potent venoms. For example, the venom of Chironex fleckeri attacks the heart, nervous system, and skin, capable of causing cardiovascular collapse and death within minutes.
Sting Treatment and Effects
Venom from certain box jellyfish, such as those causing Irukandji syndrome, is neurotoxic, leading to severe pain, cramping, and hypertension. Even after a jellyfish dies or a tentacle detaches, the nematocysts can remain functional and capable of firing. Avoiding fresh water or urine on a sting is important, as the osmotic change can trigger undischarged nematocysts to fire, releasing more venom.
Unusual Life Cycles and Blooms
The typical life cycle involves an alternation between two distinct body forms: the sessile polyp and the free-swimming medusa. The adult medusa reproduces sexually, releasing eggs and sperm into the water, which develop into a planula larva. The planula settles onto a hard surface and transforms into a small, attached polyp.
The polyp stage reproduces asexually, often by cloning or by a process called strobilation. During strobilation, the polyp forms segments that resemble a stack of discs. These segments break off individually, releasing miniature, free-swimming jellyfish called ephyrae, which grow into the mature medusa form.
The Immortal Jellyfish
A few species, most famously Turritopsis dohrnii, can reverse this life cycle. When stressed or damaged, they revert from the adult medusa back to the polyp stage, earning the nickname “immortal jellyfish.”
Jellyfish Blooms
A common and increasingly significant phenomenon is the formation of jellyfish blooms, which are massive aggregations of individuals. These blooms, sometimes triggered by favorable water temperature or abundant food, can consist of millions of jellyfish. The massive population spikes impact human activities by clogging fishing nets, damaging aquaculture farms, and blocking water intake pipes at coastal power plants.