Jellyfish, often referred to as sea jellies, are invertebrates commonly found drifting throughout the world’s oceans, from coastal shallows to the deep sea. Composed of over 95% water, these organisms lack a centralized brain but are effective predators in the water column. Their simple structure—an umbrella-shaped bell and trailing appendages—is specialized for passively capturing food that drifts into its path. Their feeding strategy relies on anatomy and venom to secure the necessary nutrients for survival.
The Core Components of the Jellyfish Diet
The majority of a jellyfish’s diet consists of zooplankton, tiny, drifting animals that form a significant part of the ocean’s food web. This includes small crustaceans such as copepods and various larval forms of other marine life. For smaller or more passive species, this microscopic prey is the primary source of nutrition, captured as the jellyfish moves through the water. Jellyfish are largely opportunistic feeders, meaning their diet is dictated by what is most readily available in their immediate environment. In addition to zooplankton, many species also consume small fish eggs and the larvae of larger fish, which are planktonic and easily intercepted.
The Mechanics of Capturing Prey
Prey capture begins with the tentacles, long filaments that trail from the bell and are armed with specialized stinging structures called nematocysts. Each nematocyst contains a tightly coiled, microscopic harpoon tipped with venom. When a small organism brushes against the tentacle, the pressure triggers the nematocyst to rapidly fire, injecting venom that instantly paralyzes or kills the prey.
Once the food is subdued, the tentacles retract and pass the meal to the shorter, often frilly, oral arms located beneath the bell near the mouth. These oral arms manipulate the prey and push it into the central mouth opening. The food enters the gastrovascular cavity, a simple sac-like structure that serves as both the stomach and intestine. Inside the gastrovascular cavity, digestive enzymes break down the captured food, and the resulting nutrients are then distributed throughout the jellyfish’s body. Any waste materials that remain after digestion are expelled back out through the same opening.
Dietary Differences Across Species
The size and potency of a jellyfish species directly influence the type of prey it can successfully capture and consume. Smaller species, such as the common Moon Jelly (Aurelia aurita), are passive filter feeders, relying on their sticky tentacles to filter out minute particles of plankton. Their feeding method involves moving water across their bell and collecting the tiny organisms that get trapped in the mucous layer.
In contrast, larger species, like the Sea Nettles or Box Jellies (Cubozoa), are active hunters capable of incapacitating substantial prey. These hunters possess potent venom that allows them to sting and consume small fish and shrimps. The Box Jelly, for example, has venom strong enough to affect the nervous system of much larger animals, ensuring its prey is immobilized swiftly.
Another significant variation is the inclusion of other jellyfish in the diet, a form of cannibalism observed across several species. Certain large predators, such as the Lion’s Mane jellyfish, will prey on smaller jellyfish when food sources are scarce.