Jellyfish are a diverse group of marine animals recognized for their gelatinous, bell-shaped bodies and trailing tentacles. The term “jellyfish” is a common name, not a formal scientific grouping, so its application can be inexact. To understand these creatures, scientists use a system of biological classification based on shared anatomical and genetic characteristics. This system reveals their evolutionary history and distinguishes them from other marine life.
The Phylum Cnidaria
All “true” jellyfish belong to the phylum Cnidaria, a group of over 11,000 aquatic species. The defining characteristic of this phylum is the presence of specialized cells called cnidocytes. These cells contain nematocysts, which are tiny, harpoon-like structures that can be ejected to sting, envenom, and capture prey. This feature is unique to this group and central to their predatory behavior.
Another trait of cnidarians is their body plan, which exhibits radial symmetry, meaning their bodies are arranged around a central axis like the spokes of a wheel. Their bodies are composed of a non-living, jelly-like substance called mesoglea situated between two layers of tissue. This phylum is not limited to jellyfish; it also includes animals such as corals, sea anemones, and hydroids.
Class Scyphozoa (True Jellyfish)
The class Scyphozoa is what most people would recognize as a “true jellyfish.” This group includes over 200 exclusively marine species distinguished by a life cycle dominated by the medusa stage. The medusa is the free-swimming, bell-shaped form responsible for sexual reproduction. The polyp stage is a sedentary phase where the animal is attached to a surface and is typically small and less conspicuous.
The anatomy of a scyphozoan includes the bell, a gelatinous body that contracts to propel the animal through the water. Hanging from the bell’s underside are long tentacles armed with stinging cnidocytes for defense and capturing food. Also on the underside is a central mouth surrounded by thicker appendages called oral arms, which transport prey to the mouth.
Well-known examples of Scyphozoa include the moon jellyfish (Aurelia aurita) and the lion’s mane jellyfish (Cyanea capillata). The lion’s mane is the largest known jellyfish species, with a bell that can reach over two meters in diameter and tentacles that can extend even longer.
Class Cubozoa (Box Jellyfish)
Distinct from true jellyfish are the Cubozoa, or box jellyfish. Their most apparent feature is their bell, which has a square or box-like shape when viewed from above, setting them apart from the rounded bell of scyphozoans. From each of the four corners of this body hangs a tentacle or a cluster of tentacles. There are approximately 50 species of Cubozoa.
Box jellyfish possess a more sophisticated nervous system and a more complex set of eyes than other jellyfish. They have 24 eyes, some of which are lens eyes, allowing them to form images and actively hunt for prey rather than simply drifting. This sensory capability makes them more agile and effective predators.
Cubozoans are also known for their venom, which is among the most potent in the animal kingdom. The Australian box jellyfish (Chironex fleckeri) is a prominent example, capable of delivering a sting that is extremely harmful to humans. These jellyfish are active swimmers, with some species recorded moving at speeds of around five centimeters per second.
Other Jelly-like Cnidarians
Within the phylum Cnidaria, other classes exist that have a jellyfish-like appearance. The class Hydrozoa is a diverse group of about 1,000 to 1,500 species that can produce medusae. Many hydrozoans are colonial organisms, living as a single entity composed of specialized individuals. A famous example is the Portuguese man o’ war, which appears to be a single jellyfish but is a colony of polyps.
Another group is the class Staurozoa, or stalked jellyfish. These cnidarians have a trumpet-shaped body and are attached to surfaces like rocks or seaweed by a stalk. Unlike their free-swimming relatives, staurozoans are sessile, meaning they do not move through the water column. These small jellyfish, with about 50 known species, live their lives attached to the marine environment.
Distinguishing Comb Jellies
A common source of confusion involves comb jellies. Despite their gelatinous appearance, comb jellies are not jellyfish and do not belong to the phylum Cnidaria. They are classified in an entirely separate phylum called Ctenophora. This distinction is based on several biological differences.
The primary difference lies in their method of propulsion. Ctenophores move through the water using rows of cilia, which are hair-like structures that beat in unison. These rows are often called “combs,” and their movement can refract light, creating a shimmering, rainbow-like effect. This is different from the pulsating bell contractions used by true jellyfish.
Furthermore, ctenophores lack the stinging nematocysts that characterize cnidarians. Instead of stinging their prey, they capture food using sticky cells called colloblasts. These cells are located on their tentacles and release an adhesive substance to ensnare small prey. This separate mechanism for prey capture indicates a distinct evolutionary path from jellyfish.