Sea anemones are marine invertebrates, often mistaken for colorful underwater plants. These animals belong to the phylum Cnidaria, making them relatives of jellyfish and corals. Found globally from shallow coastal waters to depths over 3,000 meters, they typically anchor themselves to surfaces like rocks or coral reefs. While generally stationary, sea anemones are active predators within their diverse marine habitats. Their cylindrical bodies are topped with an oral disc surrounded by tentacles, which are fundamental to their method of obtaining sustenance.
Primary Prey in the Wild
Sea anemones are carnivores, feeding on small marine organisms. Their diet commonly includes small fish, such as gobies or blennies, that venture too close to their tentacles. Crustaceans, such as small crabs, shrimp, and amphipods, also form a significant portion of their diet.
Smaller species often consume plankton, including zooplankton and other microscopic organisms. This extends to larger plankton like krill. The specific types of prey an anemone consumes are heavily influenced by its size and the particular species, with larger anemones capable of capturing more substantial animals.
For instance, some of the biggest anemone species, which can exceed one meter in diameter, are known to engulf larger prey items such as dislodged mollusks, small starfish, or even jellyfish. Prey availability in their habitat also influences their diet, leading to variations among populations.
Sea anemones found in tide pools might feed on small invertebrates trapped by the receding tide, while those in deeper waters might rely more on passing fish or larger crustaceans. Their diverse predatory habits underscore their role as important consumers within marine food webs.
The Hunting Mechanism
Sea anemones capture food using numerous tentacles equipped with stinging cells called cnidocytes. Within these cnidocytes are microscopic, harpoon-like structures known as nematocysts, each containing a coiled, hollow tubule. When prey makes contact with a tentacle, these nematocysts are rapidly discharged, firing their barbed tubules and injecting a potent neurotoxin that swiftly paralyzes or immobilizes the victim.
This potent venom allows the anemone to subdue prey much larger than itself, rendering it helpless before it can escape. Once the prey is immobilized, the tentacles collectively maneuver the catch towards the anemone’s central mouth, located on its oral disc. The mouth can stretch considerably to accommodate the size of the ingested food, enabling them to consume diverse prey items.
For smaller particles like detritus or plankton, some anemones use sticky mucus on their tentacles and oral disc to trap them. These tiny food items adhere to the mucus and are then slowly transported to the mouth by ciliary action or subtle tentacle movement. The captured food enters the gastrovascular cavity, which functions as both a stomach for enzymatic digestion and a circulatory system for nutrient distribution.
Beyond Traditional Hunting: Other Food Sources
Beyond active hunting, sea anemones obtain sustenance through other mechanisms. A significant food source for many species comes from a symbiotic relationship with photosynthetic algae, primarily zooxanthellae, that live within their tissues. These algae convert sunlight into sugars, like glucose and glycerol, through photosynthesis, which the anemone then absorbs and utilizes as a direct food source. This mutualistic relationship is so effective that some anemones can derive a substantial portion of their energy requirements from these internal algae, particularly in clear, sunlit waters.
Some sea anemone species also engage in filter feeding or absorb dissolved nutrients directly from the water. They can take in dissolved organic matter (DOM) present in the surrounding marine environment. Certain species can also filter particulate organic matter (POM) from the water column, supplementing their diet. The Venus flytrap sea anemone, for example, is known to orient itself to face currents, acting as a suspension feeder to capture passing particles.
In aquariums, sea anemone diets are managed for health and growth. Captive anemones are typically fed a variety of readily available protein sources. This can include finely chopped seafood, such as shrimp, fish, or krill, as well as specialized pellets designed for marine invertebrates.