Jellyfish are ancient marine creatures that have navigated Earth’s oceans for at least 500 million years, predating dinosaurs. These simple yet intriguing animals possess unique biological features that allow them to thrive in diverse aquatic environments, from freezing Arctic waters to tropical reefs and the deep sea. Their enduring presence in marine ecosystems highlights a simplicity in their design and function.
The Immortal Jellyfish
The Turritopsis dohrnii, often called the “immortal jellyfish,” exhibits a remarkable biological ability. This small species, typically around 4.5 millimeters in size, can revert to an earlier life stage after reaching sexual maturity. When faced with environmental stress, physical damage, sickness, or old age, its medusa form can transform back into its juvenile polyp stage.
This process involves cellular transdifferentiation, where cells change from one type to another, effectively resetting the jellyfish’s biological clock. Theoretically, this cycle of reversion can repeat indefinitely, granting the jellyfish biological immortality by allowing it to bypass death from aging. However, despite this capability, Turritopsis dohrnii can still succumb to predation or disease in its medusa stage, preventing true immortality in nature.
Anatomy Without Organs
Jellyfish possess an anatomy different from most animals, lacking a brain, heart, lungs, and bones. Instead of a centralized nervous system, they have a diffuse nerve net spread throughout their body, which allows them to detect environmental changes like temperature, salinity, and vibrations. Some species also have nerve clusters called rhopalia that can sense light and help maintain balance.
Respiration occurs through a diffusion process, where oxygen from the surrounding seawater passes directly through their thin epidermis. Their bell-shaped body, composed mostly of water and a gelatinous substance called mesoglea, pulsates for propulsion. This organ-free structure enables them to survive in diverse oceanic conditions.
Living Lights of the Ocean
Many jellyfish species illuminate the ocean depths, a phenomenon known as bioluminescence. This light is produced through a chemical reaction within their bodies, typically involving a molecule called luciferin and an enzyme called luciferase, in the presence of oxygen. In some jellyfish, these components are part of a photoprotein that releases light when triggered.
While the light produced is often blue or green, the color can vary depending on the specific chemicals involved. Bioluminescence serves various purposes for jellyfish, including defense against predators by startling or confusing them, or to attract prey. This ability makes them living light shows.