Jellyfish are ancient marine invertebrates, often called sea jellies, that inhabit every layer of the ocean. Existing for at least 500 million years, they predate most complex life forms. Their gelatinous bodies are typically over 95% water. Understanding their scientific grouping requires looking beyond the common name to the fundamental characteristics that define their phylum.
Defining the Phylum Cnidaria
Jellyfish belong to the Phylum Cnidaria, a diverse group of aquatic invertebrates that also includes sea anemones, corals, and hydroids. All Cnidarians are defined by specialized stinging cells called cnidocytes, which contain nematocysts. These cells are used for defense and capturing prey, injecting venom upon contact.
Cnidarians exhibit radial symmetry, meaning their body parts are arranged around a central axis. They possess a simple, diploblastic body plan composed of two main tissue layers: the outer epidermis and the inner gastrodermis. These layers are separated by the mesoglea, a non-living, jelly-like layer that is particularly thick in jellyfish.
The phylum is characterized by two distinct body forms that may appear in a species’ life cycle: the sessile, cylindrical polyp and the mobile, bell-shaped medusa. While some Cnidarians exist only as polyps, the animals commonly called jellyfish are defined by the presence of the free-swimming medusa stage, which is the sexual, reproductive stage.
The Four Classes of True Jellyfish
The animals recognized as jellyfish fall into the subphylum Medusozoa, which is divided into four main classes based on distinct morphological and life cycle characteristics: Scyphozoa, Cubozoa, Hydrozoa, and Staurozoa.
Scyphozoa
Scyphozoa is the class commonly known as the “true jellyfish.” In this group, the medusa stage dominates the life cycle and often grows to considerable sizes. These animals are exclusively marine and rely on rhythmic contractions of their bell for propulsion. The Lion’s Mane jellyfish, one of the largest species, is a well-known example.
Cubozoa
The Cubozoa, or box jellyfish, are distinguished by a bell that is square or cube-shaped in cross-section. They possess a velarium, a flap-like structure that allows for faster, more directed swimming than Scyphozoans. Cubozoans also have highly developed sensory organs called rhopalia. These rhopalia contain complex, image-forming eyes complete with lenses.
Hydrozoa
Class Hydrozoa is the most diverse group, including hydroids, small medusae, and colonial organisms. The life cycle of many Hydrozoans is polyp-dominant, and their medusae are generally smaller than those of Scyphozoa. Hydrozoan medusae often feature a shelf-like velum on the bell margin. The Portuguese Man o’ War is a notable example, which is a specialized colony of polyps and medusae working together.
Staurozoa
Staurozoa, or stalked jellyfish, represent a smaller group with a sessile lifestyle. They spend their entire lives attached to a substrate, such as seaweed or rocks. They lack a free-swimming medusa stage. Instead, they possess a stalked, upside-down body form that shares characteristics of both the polyp and medusa.
Observable Differences in Classification
The morphological and life cycle differences between these four classes result in clear distinctions visible in their natural habitats. Bell shape is a primary differentiator, shifting from the dome-shaped bell of a Scyphozoan to the angular cube of a Cubozoan. The cube shape allows box jellyfish to move with greater agility and speed.
Differences also exist in their sensory and defense mechanisms, particularly the potency of their stinging cells. Cubozoans are recognized as having the most powerful venom of all Cnidarians, linked to their highly developed eyes that enable active hunting. In contrast, the sting of most large Scyphozoans, like the Moon Jelly, typically causes only mild irritation.
Locomotion and attachment further separate the groups. Scyphozoans and Cubozoans are free-swimming, while Staurozoans are permanently anchored to the seafloor by a tubular stalk. Hydrozoans show the most variation, ranging from small, solitary polyps to large, floating colonial organisms.