As daylight fades, the ocean transforms, and sharks, as apex predators, exhibit distinct nocturnal behaviors. Their nocturnal habits encompass a range of activities, from how they manage periods of rest to their specialized hunting techniques and movements through the water column. Understanding these nighttime patterns offers insights into the complex lives of these animals.
Rest and Activity Patterns
Sharks do not experience sleep in the same manner as many terrestrial mammals, yet they engage in periods of reduced activity and rest. Many shark species, such as great white sharks, are obligate ram ventilators, meaning they must continuously swim to force oxygen-rich water over their gills to breathe. For these sharks, resting involves a state of “sleep swimming,” where parts of their brain may be less active while movement continues, often at a slower pace or by facing into currents to allow passive water flow over their gills.
Conversely, numerous other shark species, including nurse sharks and Port Jackson sharks, employ a method called buccal pumping. This allows them to actively pump water over their gills using cheek and neck muscles, enabling them to remain stationary while breathing. These species can rest motionless on the seafloor, in caves, or sheltered spots, sometimes exhibiting signs of reduced responsiveness. Some research suggests that certain sharks may even engage in unihemispheric slow-wave sleep, where one half of the brain rests while the other remains active, allowing for continued awareness of their surroundings.
Nocturnal Hunting Strategies
Sharks adapt their hunting strategies to the darkness by relying heavily on their highly developed sensory systems, which become important in low-light conditions. Their acute sense of smell allows them to detect minute concentrations of substances, such as blood from injured prey, from significant distances. Up to two-thirds of a shark’s brain is dedicated to processing scents, making it a primary tool for tracking prey.
Sharks also possess a lateral line system, a network of fluid-filled canals along their body that detects subtle vibrations and pressure changes in the water. This sense enables them to perceive the movements of other animals, including struggling prey, even in complete darkness. Additionally, sharks utilize electroreception through specialized organs called the Ampullae of Lorenzini, jelly-filled pores primarily located on their snout and head. These organs detect the weak electrical fields generated by the muscle contractions of living organisms, allowing sharks to locate prey hidden in sand or obscured by low visibility.
Habitat and Movement at Night
Sharks often alter their spatial behavior and movements as night falls. Many species move into shallower coastal waters during nighttime hours, following the migrations of their prey. Smaller fish and invertebrates, which constitute a significant portion of a shark’s diet, frequently become more active in these shallower areas under the cover of darkness. This movement provides sharks with an opportunistic feeding advantage.
Other species might descend to deeper areas for rest or to track prey that undertake vertical migrations in the water column. Environmental factors, such as tides and the moon phase, can also influence these nocturnal movements. For example, some great white sharks have been observed to frequent deeper waters during the day, potentially following prey, and then move closer to the surface at night. These shifts in depth and location reflect their adaptability to changing conditions and the availability of food.
Species-Specific Behaviors
Nocturnal behavior varies among different shark species, reflecting their diverse physiologies and ecological niches. Nurse sharks (Ginglymostoma cirratum) spend much of the day resting motionless on the seafloor, often in groups piled together in caves or under ledges. At night, they become active predators, using their enhanced senses to forage for bottom-dwelling crustaceans, mollusks, and small fish.
Great white sharks (Carcharodon carcharias), while often seen hunting during daylight hours, remain active at night and are capable nocturnal hunters. Observations suggest they may travel long distances under the cover of darkness, seeking new feeding grounds or potential mates. Other species, like Port Jackson and draughtsboard sharks, have been studied for their distinct resting behaviors, where they exhibit reduced responsiveness and metabolic rates during periods of inactivity at night, highlighting the diversity of nocturnal adaptations.