Cnidarians are a phylum of aquatic invertebrates with over 11,000 known species, 99 percent living in marine environments. These animals lack a centralized brain and complex organ systems, yet they exhibit coordinated movement and sophisticated predatory behavior. Characterized by a sac-like body plan, cnidarians are abundant in oceans worldwide, from shallow tropical reefs to the deep sea.
Defining Characteristics of Cnidarians
Cnidarians exhibit radial symmetry, meaning their parts are arranged around a central axis, allowing them to detect and respond to stimuli from any direction. Their body wall is diploblastic, composed of two distinct tissue layers: the outer epidermis and the inner gastrodermis, separated by a gelatinous mesoglea.
They possess a single body opening that functions as both a mouth and an anus, leading into the central digestive compartment known as the gastrovascular cavity. This cavity handles digestion and circulates nutrients throughout the body. Coordination of movement and feeding reflexes is managed by a decentralized nerve net, a simple network of nerve cells distributed across the body, allowing for localized responses without a brain.
The Power of the Cnidocyte
The defining feature of all cnidarians is the cnidocyte, a specialized stinging cell used for defense and paralyzing prey. Within the cnidocyte is a harpoon-like organelle called a nematocyst, which contains a coiled, venom-filled tubule.
Discharge is triggered by mechanical contact with a hair-like projection called the cnidocil and chemosensory cues from potential prey. The firing mechanism is an explosive event driven by a rapid increase in osmotic pressure inside the cell capsule. This high pressure forcefully ejects the tubule, making it one of the fastest biological processes known. Since each nematocyst can only fire once, cnidarians must continuously replace these cells.
Major Groups and Body Forms
Cnidarians exhibit two primary body forms, the sessile polyp and the motile medusa. Their life cycles often involve an alternation between the two. The polyp form is cylindrical, typically attached to a substrate with the mouth and tentacles facing upward, exemplified by sea anemones and corals. The medusa form is bell- or umbrella-shaped, free-swimming, with the mouth and tentacles hanging downward.
The phylum is divided into several major classes, each characterized by the dominance of one body form.
Class Anthozoa
This class includes all corals and sea anemones. Anthozoans exist exclusively as polyps and have completely lost the medusa stage in their life cycle.
Class Scyphozoa
Known as the true jellyfish, this class is defined by the conspicuous dominance of the medusa stage. These bell-shaped organisms are effective predators in the open ocean, relying on rhythmic contractions of their bell for propulsion.
Class Hydrozoa
This diverse grouping encompasses both solitary and colonial species that may alternate between polyp and medusa stages. Some, like Hydra, exist only as polyps. Examples include the colonial Portuguese Man o’ War, which is a colony of specialized polyps and medusae working together.
Class Cubozoa
Cubozoans, or box jellyfish, are distinguished by their cube-shaped medusa and the development of image-forming eyes. They are highly effective and often dangerously venomous hunters.
Complex Life Cycles
Many cnidarians employ alternation of generations, switching between a sexually reproducing medusa stage and an asexually reproducing polyp stage. The sessile polyp reproduces asexually through budding or fission, often generating colonies or producing small medusae. The free-swimming medusa is the sexual stage, releasing gametes into the water for external fertilization.
Following fertilization, the resulting zygote develops into a ciliated, free-swimming larva called a planula. This larva serves as the dispersal stage, traveling through the water column before settling onto a suitable substrate. Once settled, the larva undergoes metamorphosis to develop into a new polyp, completing the cycle. Not all classes follow this full sequence; anthozoans, for instance, reproduce sexually directly from the polyp and bypass the medusa stage.