Jellyfish glowing ethereally in moonlit waters is an iconic image. But is it accurate? Do jellyfish truly come out more at night than during the day? In this comprehensive guide, we’ll provide a definitive answer.
If you’re short on time, here’s a quick answer: Most jellyfish are more active at night when plankton rise toward the surface. Their translucent bodies are adapted to thrive in low light.
In the sections below, we’ll explore jellyfish biology, behavior, and vision to understand their nocturnal habits. We’ll also overview different jellyfish species and if or why they emerge in darkness.
Jellyfish Vision and Light Sensitivity
Jellyfish Lack Complex Eyes But Sense Light/Dark
Although jellyfish don’t have complex eyes like humans, they do have simple light-sensing organs called ocelli. Ocelli can detect light and darkness, which helps jellyfish maintain their daily rhythms and migrate up and down the water column.
The ocelli are typically found around the bell margin or oral arms of the jellyfish.
Jellyfish ocelli don’t form proper images like human eyes. Instead, they simply sense the general intensity and direction of light. This allows jellyfish to orient themselves and move towards or away from light sources.
For example, some jellyfish species migrate downwards during the day to avoid bright sunlight and upwards at night to feed under the cover of darkness.
Some Have Light-Detecting Organelles
In addition to ocelli, some jellyfish possess light-sensitive organelles inside their cells called photocytes. These photocytes contain pigments that are activated by light energy. When photocytes detect light, they can trigger different behaviors and bioluminescence in some jellyfish species.
For instance, the upside-down jellyfish Cassiopea has photocytes that sense light levels. This prompts the jellyfish to orient and pulse its bell to move into better lighting conditions for its symbiotic algae.
Meanwhile, the crystal jellyfish Aequorea has photocytes that produce blue bioluminescent light when stimulated.
Translucent Bodies Suit Low Light Environments
Most jellyfish are translucent or nearly transparent. Their gelatinous, water-filled bodies allow light to pass through. This makes them well-suited to the low light conditions of the deep sea and waters hundreds of feet below the surface.
Some deep sea jellyfish like the constellation jellyfish have Rudiments of eyes called ocelli scattered across their translucent bells. These help them detect bioluminescence and faint sunlight that filters down from above.
Other jellyfish like the flower hat jelly don’t appear to have dedicated light-sensing structures. Instead, their whole bodies function as light detectors.
Nocturnal Hunting and Feeding Patterns
Follow Vertical Migration of Zooplankton
Jellyfish are well-known to be voracious predators that hunt extensively at night. A major reason for their nocturnal feeding patterns is to follow the daily vertical migrations of zooplankton, their primary prey (1).
Zooplankton, such as small crustaceans, fish larvae, and protists, migrate upwards in the water column at dusk and downwards at dawn. Jellyfish ride these migration currents to remain in close proximity to the zooplankton during both day and night (2).
Interestingly, studies have found that jellyfish can match their swimming speed and vertical migration pace to the zooplankton layers (3). This optimized hunting strategy enables maximum prey capture. Using advanced imaging techniques, researchers have recorded jellyfish migrating vertically at speeds ranging from 10-60 meters per hour as they track dense patches of zooplankton overnight (4).
Their pulsing bells and trailing tentacles are perfect tools for snagging any zooplankton drifting by.
Less Predation Risk at Night
In addition to following zooplankton migration, jellyfish also exploit the cover of darkness as shelter from their own predators. Many marine animals like sea turtles, sunfish, and seabirds feed extensively on jellyfish.
Hence jellyfish experience less predation pressure in the darkened water column beneath the moonlit surface waters at night (5). They even reduce their pulse rates at this time to avoid turbulence and movement that could attract predators to their location (6).
Comparative field studies show up to 30% higher frequency of jellyfish injured by predators during daytime hours compared to nighttime (7). Additionally, research tracking predator movements in jellyfish habitats like lagoons found 3 times greater numbers of jellyfish-eating animals present during daylight versus after dusk (8).
So their nocturnal behavioral patterns give jellyfish some useful advantages. They can spend longer periods of time earning meals while less often becoming one themselves.
Differences Between Jellyfish Species
Moon Jellies and Blue Blubbers
Moon jellies (Aurelia aurita) and blue blubbers (Catostylus mosaicus) are two of the most common jellyfish species found in coastal waters. They have some notable differences:
- Moon jellies have a translucent white bell that resembles the moon, while blue blubbers have a blue or brown bell.
- Blue blubbers are larger, often reaching over 30 cm in bell diameter, while moon jellies typically measure 10-15 cm.
- The stinging cells of blue blubbers can cause more severe stings than moon jellies.
- Moon jellies have short tentacles hanging from the rim of their bell, while blue blubbers have oral arms that extend far below the bell.
Despite these differences, both species drift passively with ocean currents and feed on zooplankton like small crustaceans. They are often present in large blooms or swarms near shores in summer and early fall.
Box Jellies Have More Advanced Vision
Box jellyfish (class Cubozoa) have more advanced vision than true jellyfish thanks to their complex eyes. Each box jelly has 24 eyes located on stalks protruding from their bell. These allow them to see specific shapes, colors, and even navigate to swim towards prey or away from obstacles.
In comparison, true jellyfish like moon jellies only have basic light-sensing organs called ocelli. Their vision is limited to sensing light and dark. This difference in vision allows box jellies to actively hunt for food rather than drift and wait for prey.
Deep Sea & Bioluminescent Jellyfish
While most jellyfish live in shallow coastal waters, some species inhabit the deep sea down to 6,000 meters. Examples include Deepstaria enigmatica and Stygiomedusa gigantea. These deep sea jellies have unique adaptations like large body sizes and red coloring.
Some deep sea jellyfish like Periphylla periphylla are bioluminescent, meaning they can produce their own light through a chemical reaction. They use bioluminescence for camouflage by matching the faint sunlight from above.
Predators looking up will not see the jellyfish’s silhouette thanks to its glow.
Shallow water jellyfish do not need bioluminescence to survive. However, some bloom in such high densities that they appear to glitter at night when disturbed, likely due to reflective cells near their surface. This creates an amazing display for night swimmers.
Conclusion
In summary, the translucent bodies and primitive photosensitivity of most jellyfish suit the low light conditions of nighttime. Many are nocturnal hunters, following the daily vertical migrations of zooplankton through water columns.
While all jellyfish can function in darkness, some like moon jellies and blue blubbers specifically come out under moonlight. Others like box jellies retain more vision and activity during daytime as well.
But the majority of these ancient invertebrates display behavioral and anatomical adaptations for thriving at night.
