Jellyfish are ancient marine invertebrates, present for over 500 million years, making them one of the oldest multi-organ animal groups. Despite their name, these gelatinous creatures are not fish but belong to the phylum Cnidaria, characterized by their radial symmetry and stinging cells. As predators, jellyfish play a role in marine food webs.
What Jellyfish Consume
Jellyfish are carnivores, primarily consuming plankton. This includes microscopic animals (zooplankton) like copepods, larval crustaceans, and tiny shrimp or krill. They also feed on fish eggs and small fish larvae.
Some species may also consume small plants such as phytoplankton and algae, particularly when they are young. Larger jellyfish can expand their diet to include bigger prey, such as small fish, crabs, and even other jellyfish, demonstrating their opportunistic feeding nature. This diet can influence jellyfish color, as seen in moon jellyfish that may appear dark purple or bright pink depending on their food source.
How Jellyfish Capture Food
Jellyfish use specialized stinging cells (nematocysts) on their tentacles and oral arms to capture prey. When a food item makes contact, these cells rapidly deploy a barbed thread containing venom, paralyzing the prey. This mechanism efficiently immobilizes organisms that might otherwise escape.
Once stunned, oral arms guide food towards the central mouth on the underside of the bell. Jellyfish swallow food whole; digestion occurs within a central cavity functioning as both mouth and anus.
Some jellyfish actively hunt by moving their bell to displace water and bring prey within reach. Many species are passive feeders, drifting with ocean currents with tentacles extended like nets, waiting for prey.
Variations in Jellyfish Diets
Dietary preferences among jellyfish species vary significantly, influenced by factors such as size, life stage, and habitat. Smaller jellyfish typically consume microscopic organisms like phytoplankton, microzooplankton, and eggs. As they grow larger, their diet shifts to include bigger prey like shrimp and fish, reflecting their increased capacity for capture. This means larger jellyfish often occupy a higher position in the food web.
Life stage also plays a role, with baby jellyfish (ephyrae) primarily feeding on smaller food particles or eggs and larvae of other animals. Habitat influences diet; coastal jellyfish may access diverse prey, including fish larvae and crustaceans, while open ocean species rely more heavily on plankton. Prey availability is a determining factor, as jellyfish are opportunistic feeders, consuming whatever suitable food is abundant.