Jellyfish, often referred to as sea jellies, are captivating marine invertebrates that drift through the world’s oceans. Despite their common name, these animals are not fish, as they lack the backbone and other characteristics of true fish. They belong to the ancient phylum Cnidaria, a group that also includes corals and sea anemones. Jellyfish have an evolutionary history spanning at least 500 million years, making them one of Earth’s oldest multi-organ animal groups.
These gelatinous organisms are found across diverse aquatic environments, inhabiting everything from shallow coastal waters to the deepest parts of the ocean. They thrive in both warm tropical seas and cold Arctic waters, with some species even adapted to freshwater lakes.
Basic Jellyfish Biology
The fundamental structure of most jellyfish features a bell-shaped body, often called the medusa. This bell, largely composed of a gelatinous substance known as mesoglea, provides buoyancy and gives jellyfish their characteristic transparent appearance. Trailing beneath the bell are tentacles and oral arms, essential for feeding and defense.
Jellyfish exhibit radial symmetry, with body parts arranged around a central axis, enabling them to respond to stimuli from any direction. They lack a centralized brain, blood, or heart; instead, their nervous system is a simple, uncentralized nerve net distributed throughout their body. This network allows them to sense light, chemicals, and physical contact, coordinating their movements. Propulsion occurs through rhythmic contractions and relaxations of the bell, which pulsates to move the animal through water.
A defining feature of all cnidarians is their specialized stinging cells, called nematocysts or cnidocytes. These microscopic structures are primarily found on the tentacles and oral arms. Each nematocyst contains a coiled, venom-filled thread that is explosively discharged upon contact with prey or a perceived threat. This rapid firing mechanism allows jellyfish to effectively capture food and deter predators.
Class Scyphozoa: The True Jellyfish
Class Scyphozoa encompasses the “true jellyfish,” an exclusively marine group of approximately 200 species. These animals are primarily recognized by their prominent medusa stage, which is typically larger and more developed than their polyp form. Most scyphozoans feature a bell-shaped body, often possessing four oral arms surrounding the mouth on the underside, alongside numerous tentacles fringing the bell margin.
The life cycle of true jellyfish involves an alternation of generations. Fertilized eggs develop into tiny, ciliated planula larvae, which settle and transform into small, sessile polyps called scyphistomae. These polyps can reproduce asexually. Under suitable conditions, the scyphistoma undergoes strobilation, releasing immature medusae known as ephyrae, which then mature into free-swimming adults.
A common example is the Moon Jellyfish (Aurelia aurita), identified by four distinct horseshoe-shaped gonads visible through its translucent, saucer-shaped bell. These jellyfish typically measure 5 to 40 centimeters in diameter and are abundant in coastal and brackish waters, consuming zooplankton. The Lion’s Mane Jellyfish (Cyanea capillata) is among the largest jellyfish species. Its bell can exceed 1 meter, and its extensive tentacles can stretch over 30 meters, earning its name. This predator inhabits cold, boreal waters, preying on small fish and crustaceans.