The stealthy sea walnut sucks to succeed
It’s the open ocean. A baby fish, less than a centimetre long, floats through the water, completely oblivious of the danger it is in. It’s caught in a current, but one so smooth that the fish cannot detect it. Its only clue to what’s happening comes too late, as it’s suddenly sucked into a ring of tentacles and swallowed by one of the ocean’s stealthiest predators – the sea walnut.
The sea walnut is a simple transparent blob, just a few inches long. It looks like a jellyfish but in fact, it belongs to a different but related group called the comb jellies, or ctenophores.
It’s not exactly a fearsome predator. It cannot see and it can only sense the movement of prey just slightly beyond its body. It has no obvious weapons and it’s not capable of great speed. As a result, it cannot chase its prey. Instead, it uses beating hairs called cilia to create imperceptible water currents, which bring its prey straight into its mouth. Its weapons are surprise and a ruthless efficiency.
Despite its simplicity, the sea walnut’s ninja-like stealth makes it immensely successful. For its size, the animal can capture prey at the same rate as fish, and that’s just when it stays still. If it moves, it can double what it consumes.
This ability makes the sea walnut a major player in the oceans. It also makes it a notorious invader. In 1980s, sea walnuts were accidentally introduced into the Black Sea in the ballast of ships. Since then, the animals have made their way into the Caspian, Mediterranean and North Seas. Where it turns up, it eats fish larvae and plankton, causing local food webs to collapse. In the Black Sea, the anchovy, an economically important species, had gone into decline and dolphins have started to vanish. In the Caspian Sea, the native seal populations have fallen too.
Sean Colin from Roger Williams University thinks that the secret to the sea walnut’s success lies in its effective and unfussy hunting habits. Colin hand-picked sea walnuts from his local dock and placed them in water seeded with small particles. The particles were lit with a laser and filmed with a camera, to understand how the sea walnut warped the flow of water around it.
Colin found that it creates a current that sucks everything in the surrounding water into its tentacle-ringed mouth. The current it creates flows smoothly at just 2 millimetres per second. Only when it passes over the animal’s mouth does it accelerate and warp, spiralling into a corkscrew motion that goes past the tentacles. The mouth closes, and virtually everything caught up in this feeding current is eaten. The sea walnut creates its own conveyor belt of food, a streaming buffet made up of a varied selection of dishes.
To get away with this, the sea walnut needs stealth. Floating plankton won’t pose much of a challenge, but some morsels like copepods (small crustaceans) are active swimmers that can detect the movements of incoming predators. If they sense danger, it takes them just two milliseconds to swim away at tremendous speed. But Colin found that the water disturbance caused by the sea walnut’s current is far less than what the copepods can detect.
Feeding currents aren’t unusual. Shellfish and other animals use them but they do so in a blunt way. Their currents are turbulent and chaotic, used to draw in small plankton that can’t swim away. It’s a brute force approach, and one that’s very different to the stealthy strategy of the sea walnut.
Reference: PNAS: http://dx.doi.org/10.1073/pnas.1003170107
Image by Steven Johnson; video by DTUAqua
More on stealth:
If the citation link isn’t working, read why here
tweetmeme_source = ‘DiscoverMag’;
tweetmeme_service = ‘bit.ly’;
Go Further
Animals
- Octopuses have a lot of secrets. Can you guess 8 of them?
- Animals
- Feature
Octopuses have a lot of secrets. Can you guess 8 of them? - This biologist and her rescue dog help protect bears in the AndesThis biologist and her rescue dog help protect bears in the Andes
- An octopus invited this writer into her tank—and her secret worldAn octopus invited this writer into her tank—and her secret world
- Peace-loving bonobos are more aggressive than we thoughtPeace-loving bonobos are more aggressive than we thought
Environment
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
- Food systems: supporting the triangle of food security, Video Story
- Paid Content
Food systems: supporting the triangle of food security - Will we ever solve the mystery of the Mima mounds?Will we ever solve the mystery of the Mima mounds?
- Are synthetic diamonds really better for the planet?Are synthetic diamonds really better for the planet?
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
History & Culture
- Strange clues in a Maya temple reveal a fiery political dramaStrange clues in a Maya temple reveal a fiery political drama
- How technology is revealing secrets in these ancient scrollsHow technology is revealing secrets in these ancient scrolls
- Pilgrimages aren’t just spiritual anymore. They’re a workout.Pilgrimages aren’t just spiritual anymore. They’re a workout.
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- This ancient cure was just revived in a lab. Does it work?This ancient cure was just revived in a lab. Does it work?
- See how ancient Indigenous artists left their markSee how ancient Indigenous artists left their mark
Science
- Jupiter’s volcanic moon Io has been erupting for billions of yearsJupiter’s volcanic moon Io has been erupting for billions of years
- This 80-foot-long sea monster was the killer whale of its timeThis 80-foot-long sea monster was the killer whale of its time
- Every 80 years, this star appears in the sky—and it’s almost timeEvery 80 years, this star appears in the sky—and it’s almost time
- How do you create your own ‘Blue Zone’? Here are 6 tipsHow do you create your own ‘Blue Zone’? Here are 6 tips
- Why outdoor adventure is important for women as they ageWhy outdoor adventure is important for women as they age
Travel
- This royal city lies in the shadow of Kuala LumpurThis royal city lies in the shadow of Kuala Lumpur
- This author tells the story of crypto-trading Mongolian nomadsThis author tells the story of crypto-trading Mongolian nomads
- Slow-roasted meats and fluffy dumplings in the Czech capitalSlow-roasted meats and fluffy dumplings in the Czech capital