The phylum Cnidaria contains over 11,000 species of aquatic invertebrates, mostly found in marine environments, including familiar organisms like jellyfish, sea anemones, and corals. A defining characteristic of this group is the presence of specialized stinging cells called cnidocytes, used primarily for capturing prey and defense. Cnidarians exhibit radial symmetry and are characterized by a simple body wall composed of two layers of tissue separated by a gelatinous layer called the mesoglea. This simple body plan is expressed in two fundamental morphological types: the sessile polyp and the motile medusa.
The Sessile Form: Polyp
The polyp represents the cylindrical, stalk-like body form adapted for a sessile existence. The aboral end (opposite the mouth) is typically attached to a substrate, such as a rock or coral, by a pedal disc or holdfast structure. This fixed position allows the organism to remain in one place, like a sea anemone or a freshwater Hydra.
The body is tubular, with the oral end facing upward toward the water column. The single opening serves as both a mouth for ingesting food and an anus for expelling waste, leading into the central gastrovascular cavity. This opening is encircled by a ring of tentacles armed with stinging cnidocytes, ready to capture prey.
The polyp form is primarily responsible for asexual reproduction, allowing for rapid population growth and colony formation. Reproduction often occurs through budding, where a small knob of tissue develops on the side of the parent and grows into a new, genetically identical individual. If the new polyp detaches, it is considered solitary, but if it remains connected, it contributes to a colony, such as those seen in most corals and hydrozoans.
In colonial species, such as the hydrozoan Obelia, polyps can differentiate into specialized individuals (zooids) that perform distinct functions. Some zooids are adapted for feeding (gastrozooids), while others are adapted for defense or reproduction (gonozooids). The sessile nature of the polyp stage allows it to efficiently utilize localized food resources and form extensive benthic colonies.
The Motile Form: Medusa
The medusa represents the free-swimming, bell- or umbrella-shaped body form, commonly known as a jellyfish. This structure is essentially a polyp turned upside-down and flattened, adapting it for a pelagic lifestyle. The bell shape is composed of a significantly thicker layer of mesoglea, giving the medusa its gelatinous consistency and buoyancy.
In contrast to the polyp, the mouth is located on the concave underside of the bell, often hanging down on a structure called the manubrium. Tentacles, equipped with nematocysts, extend from the rim of the bell and trail downward. These tentacles ensnare prey as the medusa drifts or propels itself through the water column.
Medusae move by rhythmically contracting muscles around the rim of the bell, generating a jet-like propulsion. This mobility aids in dispersal and allows them to actively hunt or move to new feeding grounds. The primary function of the medusa stage is sexual reproduction, ensuring genetic mixing and species distribution.
The medusa is the sexual stage, producing gametes in specialized gonads located in the gastrodermis or mesoglea. These gametes are typically released directly into the water for external fertilization (broadcast spawning). The resulting zygote develops into a larva.
The Alternating Cnidarian Life Cycle
The two body forms are frequently linked in metagenesis, describing the alternation between the asexual polyp stage and the sexual medusa stage in many cnidarians. This complex life cycle allows the organism to exploit both the benthic and pelagic environments.
The cycle begins when the medusa releases gametes into the water. After fertilization, the resulting zygote develops into a ciliated, free-swimming planula larva. The planula eventually settles onto a hard surface, attaches, and metamorphoses into a new, asexual polyp.
The newly formed polyp then grows and reproduces asexually, often budding off more polyps to form a colony. At a certain point, the polyp stage begins to produce medusae through a specialized form of asexual budding. In classes like Scyphozoa (true jellyfish), the polyp undergoes strobilation, horizontally dividing into a stack of tiny, saucer-shaped medusae that break off and swim away.
Not all cnidarians exhibit this complete alternation; for example, sea anemones and corals (Class Anthozoa) exist exclusively in the polyp form. In these groups, the polyp itself becomes the sexually reproductive stage, releasing gametes to directly produce new planula larvae that settle and form new polyps. The full metagenesis cycle is a common feature in the life history of many hydrozoans and scyphozoans, maximizing their reproductive and dispersal potential.