Zooplankton are microscopic animal drifters that form a foundational component of aquatic food webs. They serve as a primary food source for countless species, transferring energy from the phytoplankton they consume up to higher trophic levels. Inhabiting the open water of oceans and lakes, these tiny organisms face constant survival challenges. Their environment requires methods to stay afloat, avoid predators, and persist through difficult conditions.
Adaptations for Buoyancy Control
A primary challenge for zooplankton is that their bodies are denser than water, creating a risk of sinking away from the sunlit surface where their food, phytoplankton, thrives. To counteract this, many species developed strategies to control their buoyancy. One method is storing lipids—fats and oils—which are less dense than water. For example, copepods accumulate oil droplets that provide energy reserves and help them remain neutrally buoyant.
Another strategy involves increasing drag to slow the rate of sinking. Zooplankton achieve this by evolving body shapes and appendages that maximize their surface area. Many species possess flattened or elongated bodies, while others have feathery appendages or long spines that increase friction against the water. These traits act like parachutes, allowing them to resist gravity and remain in the food-rich upper layers with minimal energy expenditure.
Physical Defenses Against Predators
Zooplankton have evolved physical defenses to avoid being eaten. One of the most effective is transparency, which serves as camouflage in the open water. By being nearly invisible, many zooplankton, such as jellies and arrow worms, make it difficult for visual predators like fish to detect them.
Many zooplankton also have structural defenses. Cladocerans, such as Daphnia, are protected by a hard, shell-like carapace. Other species, including various crustacean larvae, have sharp spines or tough bristles. These features can increase their effective size, making them difficult for smaller predators to swallow. Some zooplankton can even grow longer spines or larger shields when predators release chemical signals, a phenomenon known as cyclomorphosis.
Behavioral Adaptations for Survival
Zooplankton exhibit behaviors to enhance their survival, such as Diel Vertical Migration (DVM). This daily, synchronized movement is considered the largest migration on Earth by total biomass. The pattern involves ascending to food-rich surface waters at dusk to feed and descending into deep, dark waters at dawn to hide. This behavior is a trade-off between feeding and safety.
The primary driver of DVM is predator avoidance. By retreating to the dark depths, zooplankton become invisible to visual predators hunting near the surface during the day. This migration balances the energy cost of travel and reduced feeding time against a lower probability of being eaten. The timing and depth of migration can vary based on factors like season, moonlight, and the predators present.
Zooplankton have other behavioral adaptations. Copepods have a powerful escape response, reacting to disturbances like the pressure wave from an approaching predator in milliseconds. They can execute powerful jumps, reaching speeds of over 500 body lengths per second to distance themselves from an attack. This fast reaction is often the difference between life and death.
Reproductive and Dormancy Strategies
For long-term survival through harsh conditions, zooplankton use reproductive and dormancy strategies like diapause. This state of suspended development allows them to outlast extreme temperatures, food scarcity, or drought by producing durable “resting eggs.” These eggs are embryos encased in a protective shell.
These resting eggs are resilient. Produced by species like Daphnia and brine shrimp (Artemia), they can settle into sediment and remain viable for decades, or even centuries in some freshwater cases. The eggs are resistant to freezing, drying out, and low-oxygen conditions. When favorable conditions return, they hatch and re-establish the population, creating a “seed bank” in the sediment that ensures the species’ continuity.