If you’ve ever been stung by a jellyfish while swimming in the ocean, you’ve probably wondered what draws these gelatinous creatures to humans. Jellyfish don’t purposefully target people, but there are a few key reasons why you might feel that occasional sting.
Jellyfish are attracted to the chemicals, movement, warmth and glow that humans exhibit when in the water. They drift into swimmers accidentally rather than intentionally stinging. Understanding what attracts them can help you avoid jellyfish encounters.
Chemical Attractants
Ammonia from Swimmers
One of the main chemical attractants for jellyfish is ammonia, which is excreted from the skin and urine of swimmers. When we enter the ocean, our bodies release sweat, urine, and other fluids containing ammonia.
Even small amounts of ammonia can be detected by jellyfish from far away and lure them closer. Jellyfish are equipped with specialized sensory structures called rhopalia that can pick up chemical cues in the water. Ammonia acts as a dinner bell for hungry jellies looking for their next meal (1).
Carbon Dioxide from Breath
Carbon dioxide is another attractant secreted by beachgoers. When we exhale, we release carbon dioxide into the air. Some of that CO2 gets absorbed by the ocean. Jellyfish can sense the elevated carbon dioxide from swimmers’ breath and will swim toward the source.
In lab experiments, researchers found that adding carbon dioxide to seawater made jellyfish more likely to move toward the area (2). Carbon dioxide signals to jellies that a potential prey organism is near.
Scents from Lotions and Perfumes
The scented sunscreens, perfumes, colognes and lotions beachgoers wear can also grab the attention of jellyfish. These strong synthetic fragrances contain chemicals that dissolve into the water. Jellyfish detect these unnatural aromas and may identify them as injured prey.
One study found that leatherback sea turtles are attracted to the smell of plastic bags floating in the ocean, likely because they mistake them for jellyfish due to a similar scent (3). It’s possible that jellyfish could make this same sensory error and be drawn toward the perfumed swimmers wearing these scents.
References:
- Jellyfish Behavior and Responses to Chemical Cues
- Carbon Dioxide as a Directionally Orienting Stimulus for Jellyfish
- Plastic Bag Leachates Impair Marine Turtle Odor Sensorium and Behavior
Movement and Vibrations
Jellyfish are attracted to certain movements and vibrations in the water that signal potential prey or threats. Here’s an overview of how they detect and respond to motion:
Sensitivity to Water Currents
Jellyfish have specialized structures called statocysts that contain sensory hairs attuned to the motion of water currents. Even tiny disturbances in the water gets picked up by these structures, alerting the jellyfish.
Response to Vibrations
In addition to water currents, jellyfish can sense low-frequency vibrations through receptors on their tentacles. These allow them to hone in on struggling prey or potential mates fluttering their swimming bells.
Reactive Swimming and Stinging
Once a jellyfish detects promising vibrations, it can jet or glide towards the source by contracting its bell. Its trailing tentacles may automatically sting whatever it encounters to capture food or defend itself.
Interest in Splashing Motions
Jellyfish tend to be drawn to splashing water motions reminiscent of trapped prey. So swimmers doing kicks or strokes can inadvertently attract curious jellies looking for a possible meal. It’s not that they purposely attack humans, but may accidentally sting us out of reflex.
In areas with high jellyfish populations, officials often advise bathers to avoid excessive splashing and to shuffle their feet while wading to alert rather than attract nearby jellies.
Exceptions and Variations
It’s worth noting that not all jellies react to movements and vibrations in the same way. Here are some exceptions:
- Upside-down jellies anchor onto eelgrass instead of swimming, so they don’t pursue prey and largely ignore vibrations.
- Moon jellies pulses slowly and primarily drift with ocean currents rather than chasing food sources.
- The massive lion’s mane jellyfish moves too slowly to catch most agile prey, so it focuses more on casting out fishing lines of stinging cells.
So while many jellies do instinctively go towards motion and disturbances, behaviors can vary by species ecology and size. But broadly, their sensory structures make them quite responsive to the movements of potential prey, threats, and mates.
Heat Signatures
Jellyfish are attracted to the heat signatures given off by humans and other warm-blooded animals. As ectotherms, jellyfish rely on external heat sources to regulate their internal body temperature. The metabolic heat from humans and other mammals acts as an enticing thermal plume in the ocean, drawing jellyfish toward areas of greater warmth.
Research has shown that jellyfish can sense temperature differences as slight as 0.2°C through specialized sensory organs called rhopalia. Located around the jellyfish bell, rhopalia contain primitive eyes and thermal receptors that allow jellyfish to orient toward heat gradients.
This enables them to find warmer areas of the ocean, detect thermal plumes from other animals, and drift toward heated effluent from power plants or other sources.
So when humans swim in waters populated by jellyfish, we inadvertently send out inviting thermal signals. To a jellyfish, a swimming beachgoer or snorkeler represents a mobile hot spot, while groups of people clustered in the water create irresistible heat signatures.
This thermal attraction helps explain why jellyfish congregate around popular swimming beaches, snorkeling sites, and boat docks.
While jellyfish rely on heat to survive and navigate, human-generated thermal gradients have also contributed to population booms. Warming ocean temperatures and the heated water from coastal power plants provide beneficial habitats for jellyfish polyps to thrive.
And as climate change continues to heat and alter marine ecosystems, some scientists predict jellyfish populations will increase, heightening thermal interactions with humans.
So the warm glow we give off that makes ocean swimming so enjoyable for humans acts as a siren song calling more jellyfish to share the waters. Understanding this unique thermal attraction can make us savvier ocean users, and perhaps will allow people and jellyfish to more peacefully enjoy the seas together.
Bioluminescence
One of the most fascinating aspects of jellyfish is their ability to produce bioluminescence. Bioluminescence refers to the production and emission of light by a living organism. Some jellyfish species contain special light-emitting organs called photocytes that allow them to glow in vibrant colors and patterns in the dark ocean depths.
There are a few theories as to why certain jellyfish evolved the ability to be bioluminescent. One possibility is that they use their glow as a self-defense mechanism. The sudden flash of bright light can startle predators and deter them from attacking.
The light may also illuminate the jellyfish’s tentacles, making their stinging capabilities more visible as a warning sign to potential predators. Some jellyfish can even eject bioluminescent mucus to create a smokescreen effect and evade predators.
Another theory is that bioluminescence aids jellyfish in communication and mating. Each jellyfish species has a unique glowing pattern that may help jellyfish identify their own kind in the darkness of the oceans. The flashing light patterns could serve as signals to attract mates.
After all, it’s hard to find a date when you live in a vast, pitch-black habitat!
There is also evidence that some jellyfish use bioluminescence to lure prey toward their lethal tentacles. The glowing jelly acts like a fishing lure, attracting curious fish that then get entangled in the jellyfish’s stingers.
Some jellies may even flash their lights in patterns that mimic the movements of smaller prey animals, essentially tricking their next meal into coming closer.
While the exact purpose is still debated, one thing is clear – bioluminescence provides jellyfish with a powerful advantage as masters of the dark ocean depths. Their mystical glow contributes to the otherworldly aura that makes these marine creatures so biologically fascinating.
Conclusion
While jellyfish don’t intentionally target humans, we do emit signals that catch their attention. Understanding what attracts them can help you take precautions. Avoid perfumes and lotions before swimming, refrain from thrashing around excessively, and watch out for bloom conditions.
With some mindfulness about jellyfish attraction factors, your odds of encounters may diminish.
