Jellyfish are ancient and diverse marine invertebrates that have inhabited the oceans for over 500 million years. Despite their seemingly simple structure, these gelatinous creatures exhibit a variety of behavioral adaptations that allow them to survive in their diverse aquatic environments. These behaviors range from how they find food to how they avoid becoming prey.
Acquiring Food
Jellyfish obtain food through two methods: passive filter feeding and active predation. Many species drift with ocean currents, using their tentacles to passively capture plankton and other small organisms. This method allows them to conserve energy while gathering food.
Active feeding involves specialized stinging cells called nematocysts, found on their tentacles. When prey, such as small fish or crustaceans, contacts these tentacles, the nematocysts discharge a coiled, venomous thread that paralyzes or kills the prey. Tentacles then maneuver the prey towards the mouth. Some species target specific prey or use unique tentacle arrangements to increase efficiency.
Evading Predators
Jellyfish employ several defenses from predators like fish, sea turtles, and other marine animals. Their most direct defense involves their stinging nematocysts, which can deter attackers with a painful or paralyzing sting. The potency of the sting varies among species, with some capable of delivering a powerful defense.
Beyond stinging, jellyfish use rapid pulsating movements of their bell to escape danger. Their bodies, composed of at least 95% water, are largely transparent, providing a natural camouflage that makes them difficult for predators to spot in the open water. Some species also use bioluminescence to startle or distract predators, or to attract larger predators to their own attackers.
Navigating Their World
Jellyfish move through the water by rhythmically contracting their bell-shaped bodies, creating pulsations that propel them forward. While they can swim, many species also rely on ocean currents for transport, a strategy linked to their drift-feeding behaviors. This allows them to cover large areas with minimal energy.
Jellyfish exhibit responses to environmental cues such as light, known as phototaxis, and gravity, called geotaxis. These sensitivities guide behaviors like daily vertical migrations, where some species ascend towards the surface at night to feed and descend to deeper waters during the day, likely to avoid visually hunting predators. Some species also detect chemical cues, which help them locate food or avoid unfavorable conditions.
Ensuring Future Generations
The reproductive behaviors of jellyfish often involve mass spawning or swarming, increasing the likelihood of successful fertilization. Many species release their gametes directly into the water for external fertilization, where eggs and sperm unite. This coordinated release maximizes the chances of reproductive success in the vast ocean.
Jellyfish exhibit an alternation of generations in their life cycle, which includes both a free-swimming medusa stage and a sessile polyp stage. After fertilization, a larva develops and settles on a surface, transforming into a polyp. These polyps can reproduce asexually by budding, creating new polyps or even directly budding off tiny medusae. Environmental cues, such as changes in temperature or salinity, can trigger a process called strobilation, where the polyp undergoes a series of constrictions to produce multiple young medusae, ensuring the continuation of the species.