Cnidarians, the phylum that includes jellyfish, sea anemones, and corals, represents one of the oldest groups of animals on Earth. These aquatic invertebrates, which exist as free-swimming medusae or stationary polyps, have evolved a remarkable diversity of feeding strategies. Their diet is incredibly varied, encompassing everything from microscopic organisms captured through potent stinging cells to a near-total reliance on energy produced internally through a symbiotic relationship with algae. This range of nutritional methods allows them to thrive in virtually every marine environment.
The Signature Hunting Tool: Nematocysts
Predation in cnidarians is defined by a unique weapon known as the nematocyst, housed within specialized cells called cnidocytes. Each cnidocyte contains this capsule-like organelle, which holds a tightly coiled, venom-coated, harpoon-like tubule. The exterior of the cell features a hair-like trigger, the cnidocil, which acts as a mechanoreceptor and chemoreceptor.
When a potential prey item makes contact with the cnidocil or releases certain chemicals into the water, the nematocyst is explosively discharged. This rapid ejection is driven by high internal osmotic pressure, forcefully launching the thread and penetrating the target organism. The explosive discharge can reach extreme accelerations, injecting a neurotoxin to quickly paralyze or subdue the prey. Since each cnidocyte is a single-use cell, the cnidarian must expend significant energy constantly replacing these miniature, highly effective biological weapons.
Predatory Diets and Prey Capture
Following successful paralysis by the nematocysts, the cnidarian transfers the captured food to its digestive cavity. In large, free-swimming jellyfish (Scyphozoa), the tentacles or bell margin secure the prey before passing it to specialized, frilled appendages called oral arms. These oral arms maneuver the food into the central mouth opening, located on the underside of the bell. Sessile forms, like sea anemones, use their tentacles to push the prey directly into the mouth.
Cnidarians are primarily carnivores, and the size of their prey is determined by the species and its habitat. Hydras (Hydrozoa) mainly consume tiny organisms like water fleas and brine shrimp, while larger jellyfish target zooplankton, which includes copepods, krill, and fish larvae. Some species are opportunistic and can consume small fish, or even other jellyfish, depending on local abundance.
Once ingested, the food enters the gastrovascular cavity, a central body space with a single opening for both intake and waste expulsion. Digestion begins extracellularly as enzyme-secreting cells lining the cavity break down the prey into a nutrient-rich slurry. This partially digested material is then absorbed by other cells lining the cavity, which complete the process through intracellular digestion. The gastrovascular cavity is also responsible for circulating nutrients throughout the body.
Energy from Light: Symbiotic Feeding
A distinct feeding strategy exists in reef-building corals and many sea anemones, relying on an internal partnership with photosynthetic algae. This relationship involves hosting vast numbers of microscopic dinoflagellates, commonly known as zooxanthellae, within the tissues of the cnidarian polyp. The algae utilize the host’s metabolic waste products, such as carbon dioxide, nitrogen, and phosphorus, for their own growth.
In return for this shelter and nutrient supply, the zooxanthellae generate energy through photosynthesis and transfer a substantial portion of the resulting organic compounds, primarily sugars, to the host. This photosynthetic output provides the cnidarian with up to 90% of its total required energy. This energy source explains why reef-building corals are restricted to shallow, clear tropical waters, where sufficient sunlight can penetrate for the algae to photosynthesize effectively.
While this symbiotic exchange provides the bulk of their energy, these cnidarians still engage in active predation to supplement their diet. They use their nematocyst-armed tentacles to capture zooplankton, which provides the remaining percentage of energy and supplies necessary elements like nitrogen compounds. The most successful cnidarians in these environments employ a dual feeding mechanism, capitalizing on both internal solar power and external hunting.