Jellyfish are aquatic, gelatinous invertebrates belonging to the Phylum Cnidaria. This phylum includes animals that share specialized stinging cells called nematocysts, which they use to capture prey and for defense. The common name “jellyfish” covers organisms across four distinct biological classes. These classes are separated based on differences in their life cycles, body structures, and the presence or absence of certain anatomical features.
True Jellyfish (Class Scyphozoa)
Class Scyphozoa encompasses the true jellyfish, characterized by a life cycle where the free-swimming medusa stage is the most prominent form. These organisms typically possess a large, dome-shaped or bell-like body. Scyphozoans lack a velum, a muscular shelf found under the bell margin in smaller jellyfish.
The Moon Jelly (Aurelia aurita) is a widespread example, easily identified by its four horseshoe-shaped gonads visible through its translucent bell. The Lion’s Mane Jellyfish (Cyanea capillata) is among the largest known, with bells reaching up to two meters in diameter and tentacles extending over 30 meters.
True jellyfish use a nerve net scattered throughout their bodies to coordinate movement, propelling themselves by rhythmic contractions of the bell. Their stings are generally painful but are not life-threatening to humans, though severity depends on the species and the extent of contact.
Box Jellyfish (Class Cubozoa)
Box jellyfish (Class Cubozoa) possess a unique, cube-shaped bell structure. This allows them to be much more active and directional swimmers than scyphozoans. They can reach speeds of up to 1.5 meters per second by using a shelf-like structure called a velarium, which focuses the water expulsion from the bell.
Cubozoans feature a significantly more advanced sensory system, including clusters of sensory structures called rhopalia located on the corners of the bell. Each of the four rhopalia contains six eyes, including two with a cornea, lens, and retina, allowing them to form images. This complex visual capability aids their active hunting behavior.
The group contains some of the most venomous marine animals in the world, such as the Sea Wasp (Chironex fleckeri). The venom of species like the Sea Wasp and the tiny Irukandji jellyfish (Malo kingi) is extremely potent and can be fatal to humans in minutes. The venom is concentrated in thousands of stinging cells lining their tentacles, which are attached to muscular pads called pedalia at the bell’s corners.
Stalked Jellyfish (Class Staurozoa)
Stalked Jellyfish (Class Staurozoa) lack a free-swimming medusa stage in their life cycle. Instead, they spend their entire adult lives attached to a substrate, such as rocks or seaweed, via a stalk, or peduncle.
These small organisms have a trumpet or goblet-shaped body known as the calyx. The calyx is oriented upwards, with the mouth and eight clusters of short, knob-tipped tentacles facing the water column. They are primarily found in cold, shallow, near-shore waters in boreal and polar regions.
Staurozoans combine features of both the polyp and medusa body plans. Their sessile, attached lifestyle is what sets them apart. They use their specialized tentacles to capture small prey, such as crustaceans, that drift past their fixed position.
Hydrozoans and Colonial Forms (Class Hydrozoa)
Class Hydrozoa represents the most diverse group, exhibiting a wide range of forms, including solitary polyps, solitary medusae, and complex colonies. Hydrozoan medusae are typically small, ranging from 0.5 to 6 centimeters in diameter. A distinguishing anatomical feature is the presence of a velum, a muscular ring under the bell margin that helps them propel through the water.
Most hydrozoans have a life cycle that alternates between a sessile polyp stage and a free-swimming medusa stage, although some species exist only as one form.
This class includes organisms often mistaken for single jellyfish, such as the Portuguese Man O’ War (Physalia physalis). This organism is a siphonophore, a free-floating colony composed of several specialized, interdependent polyps. These polyps are specialized for distinct functions, including feeding, defense, reproduction, and flotation, with the pneumatophore, or gas-filled float, keeping the colony buoyant.