Jellyfish, members of the phylum Cnidaria, have drifted through the seas for at least 500 million years. These marine creatures are not true fish, but are characterized by their umbrella-shaped bell and trailing tentacles. Their simple, gelatinous bodies allow them to thrive in nearly every ocean environment, from coastal waters to the deep sea. The unique biology of these organisms reveals several surprising facts about animal life.
They Lack Brains, Hearts, and Bones
The body of a jellyfish consists of approximately 95% water. This high water content gives them their translucent, gelatinous appearance and explains why they collapse into a shapeless blob when washed ashore. Lacking a centralized nervous system, they do not possess a brain, heart, or lungs.
Instead of a brain, they rely on a primitive network of nerves called a “nerve net” distributed throughout the epidermis. This decentralized system allows them to sense their environment, detecting light, gravity, and movement through specialized sensory organs called rhopalia located around the bell’s edge. They perform respiration through simple diffusion, absorbing oxygen directly from the surrounding water.
The Immortal Jellyfish Can Reverse Its Age
The species Turritopsis dohrnii is nicknamed “the immortal jellyfish” due to its remarkable ability to cheat death. When this tiny hydrozoan is exposed to environmental stress, physical damage, or old age, it can undergo a process known as transdifferentiation. This mechanism allows the mature medusa to transform its existing cells back into the juvenile polyp stage.
The jellyfish essentially reverts to an earlier stage of its life cycle, restarting its development and achieving biological immortality. This transformation involves converting differentiated cells into other cell types to form a new, young colony. Although the individual can still fall victim to disease or predation, the capability to reset its biological clock is unique within the animal kingdom.
Their Simple Digestive Process
Jellyfish possess an incomplete digestive system with only one opening, which functions as both the mouth and the anus for waste expulsion. Prey captured by the tentacles is moved to the underside of the bell and into the mouth. The food then enters the gastrovascular cavity, a sac-like structure that acts as both a stomach and an intestine.
The cavity secretes enzymes for extracellular digestion, followed by nutrient absorption by the cells lining the sac. Undigested waste material is then ejected back out through the same oral opening.
The Deadliest Stingers in the Sea
The Box Jellyfish, particularly species like Chironex fleckeri, is considered the most venomous marine animal, capable of delivering a potentially fatal sting to humans. Their tentacles are lined with millions of microscopic stinging capsules called nematocysts, which act like tiny, pressurized harpoons. Upon contact, these nematocysts rapidly discharge, injecting a potent venom.
The venom causes excruciating pain and acts rapidly on the cardiovascular system, often leading to cardiac arrest. Box Jellyfish are also unique among their relatives for possessing up to 24 eyes, including four highly sophisticated ones that have lenses, corneas, and retinas. This complex vision enables them to navigate and hunt actively.
Global Swarms and Ecological Importance
Jellyfish populations can experience sudden increases known as “blooms” or swarms, which can consist of millions of individuals. These blooms are often linked to human impacts, primarily overfishing, which removes the jellyfish’s predators, such as sea turtles, and their competitors, like small forage fish. Warmer ocean temperatures and nutrient-rich waters from runoff also favor jellyfish reproduction.
These large aggregations have significant ecological and economic consequences, including clogging fishing nets and the water intake pipes of power plants. Jellyfish are a fundamental part of the marine food web, serving as a food source for numerous species and providing habitat for certain small fish and invertebrates.