Summary
Highlights
The phylum Cnidaria consists of over 10,000 species with a fossil record reaching back 580 million years. Its name, derived from the Greek 'knide' meaning nettle, refers to their unique stinging cells called cnidocytes, which contain nematocysts and are exclusive to this phylum.
Cnidocytes release venom-filled harpoons called nematocysts. These harpoons rapidly discharge (less than a millionth of a second) when triggered by a cnidocil, a chemical and mechanoreceptor. Some cnidocils are highly sensitive, while others require confirmation from surrounding cells before firing, preventing self-stinging or harm to mutualist species like clownfish. The discharge is driven by a sudden influx of calcium ions, causing a massive osmotic pressure change that everts the nematocyst, injecting venom with tiny barbs.
Over 30 types of cnidocytes exist. The three most common are penetrants, which inject venom; volvents, which use a spineless elastic thread to lasso prey; and glutinants, which secrete sticky mucus for anchoring.
Cnidarians are diploblasts, possessing two germ layers: the ectoderm (epidermis with cnidocytes and sensory cells) and the gastrodermis (inner digestive layer). The mesoglea, a water-filled layer between them, acts as a hydrostatic skeleton. They lack true organs and circulatory, digestive, or respiratory systems, relying on diffusion due to a large surface-area-to-volume ratio. They have a decentralized nerve net without cephalization and exhibit radial symmetry.
A key characteristic is polymorphism, with two main body forms: polyps (sedentary, tubular, with a mouth surrounded by tentacles and an attached aboral end) and medusae (bell-shaped, free-swimming, with a central mouth and tentacles). Some cnidarians, like jellyfish, exhibit both forms, while others, like anemones and corals, are exclusively polyps. Colonial hydrozoans show varying degrees of polymorphism, with different polyps serving specific functions.
Cnidarians are abundant in marine habitats, particularly warm waters near the equator, though some live in deep oceans, polar regions, or freshwater (e.g., Hydra). Many form symbiotic relationships, such as anemones with boxer crabs or clownfish, and corals with algae. Carnivorous cnidarians feed on small crustaceans, zooplankton, or larger fish by paralyzing them with nematocysts and digesting them in their gastrovascular cavity, which serves for digestion, gas exchange, waste excretion, and reproduction.
Cnidarians reproduce sexually through spawning, where gametes are released into the water for external fertilization and develop into a ciliated planula larva, which then settles and grows into a polyp. Asexual reproduction occurs through budding or splitting. All cnidarians can regenerate, with some being considered biologically immortal, capable of living indefinitely without aging.