Many people wonder if jellyfish truly contain “jelly” given their name and appearance. While these marine creatures look gelatinous, the substance giving them their form is not the same as edible jelly. Their unique composition allows them to thrive in oceans worldwide, revealing a complex biological reality.
The Truth About Their “Jelly”
The “jelly” that forms the bulk of a jellyfish’s body is primarily a non-living, gelatinous material called mesoglea. This substance is sandwiched between two thin layers of cells: the epidermis and gastrodermis. Mesoglea is largely composed of water, making up 95% to 98% of the animal’s total mass.
Beyond water, mesoglea contains fibrous proteins like collagen and heparan sulfate proteoglycans, which contribute to its gelatinous texture and provide structural support. It functions as a hydrostatic skeleton, helping the jellyfish maintain its umbrella-like shape and providing buoyancy. This elastic property also aids in locomotion by allowing the body to return to its original form after muscle contractions.
More Than Just Water: Their Unique Body Plan
Despite their simple appearance, jellyfish possess a functional body plan that allows them to survive without many organs found in other animals. Their main body part is the bell, also known as the medusa, which is typically umbrella-shaped. Trailing from the underside of the bell are tentacles and oral arms, both equipped with stinging cells to capture prey and for defense.
Jellyfish lack a centralized brain, heart, bones, blood, and lungs. Instead of a brain, they have a diffuse nerve net distributed throughout their body, which allows them to sense their environment. This nerve net helps them detect light, balance, water currents, and chemical changes. Movement is achieved through rhythmic contractions of muscle-like fibers in their bell, expelling water to create a form of jet propulsion.
Why “Jellyfish” Isn’t Quite Right (But Sticks)
The common name “jellyfish” is misleading because these animals are not fish. True fish are vertebrates, possessing a backbone, gills, and fins. Jellyfish, however, are invertebrates, lacking a backbone and belonging to the phylum Cnidaria, a group that also includes corals and sea anemones.
The term “jellyfish” likely originated from early observations, where anything in the ocean was broadly categorized as a “fish.” Their gelatinous appearance contributed to the “jelly” part of the name. Despite its biological inaccuracy, the name persists due to long-standing use. Many scientists and aquariums now prefer “sea jellies” or “jellies” to better reflect their unique biology.