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Sea Walnut

Last updated date: 23rd May 2024
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What is Sea Walnut?

A Sea Walnut is a ctenophore organism (a stingless jellyfish-like animal having comb-like structures). Mnemiopsis leidyi, this warty comb jellyfish or sea walnut, is a species of tentaculata ctenophora (comb jelly). Inhabitant to the western Atlantic coastal waters and the east coast of South America, but has become entrenched as an invasive species in European and western Asian areas. However, since 1982 it has been spotted in the Black Sea and thereafter the Caspian Sea. In both the seas, the population has burgeoned since they do not contain any natural predators in the region, and thus this resulted in the collapse of various fisheries in the region for as much as they feed on the zooplankton that the commercial fish essentially consume.

Sea Walnut Scientific Name

Mnemiopsis leidyi (Sea Walnut) is the Latin term of sea walnut’s scientific name. This ctenophore is an inhabitant of the east coast of North and South America.

Sea Walnut Scientific Description


















Comb Jellies


Up to 10 cm

Sea Walnut Anatomy

  • Clear, gel-like skin texture

  • Walnut-shaped body

  • flanking mouth

  • Eight comb rows

  • Two lobes hanging down

  • Two very minimized tentacles.

Sea Walnut Conservation Status

Sea walnut invasive species are not threatened. It is however invasive in some seas.

Sea Walnut Physical Description

The bodies of comb jellies are quite tender and can be adversely collapsed when eliminated from water (during storms, comb jellies sink to depths to avoid harmful currents); hence, when seizing a comb jelly, it is ideal to gently scoop the organism into a jar with environs water. Since even small whirl currents can injure the specimen, enormous care is needed.

The Sea walnut is about 100-120mm in length, and the width of the body is estimated at half of its length. Their body is split up into eight symmetrical parts having longitudinal bands of cilia. They are bio-luminescent and their bodies are clear.

The diet of comb jelly comprises zooplankton along with the eggs and larva of juvenile fish, sea jellies, copepods, and even other ctenophores. They are first and foremost consumers and are also the filter feeders that feed by pumping water into its body and capturing small prey on the inside surface of the two lobes on tentacles.

Sea Walnut Size & Shape

As the name (common) suggests, sea walnuts are shaped like the meat of a walnut. As adults, they are deficient in tentacles and can grow to several inches in length. Also referred to as a comb jelly, since they have rows of cilia that somewhat look like the teeth of a comb. These cilia easily deflect beams of light and induce pulsing rainbows to travel along their body.

Sea Walnut Color

Sea walnuts are white or transparent. They can produce light when flustered, and can commonly be found flashing brightly in boat wakes at night.

Sea Walnut Habitat & Abundance

Within reach-surface waters in the Western Atlantic. The comb jelly or sea walnut is quite common across most of Chesapeake Bay, and, while the population fastens seasonally, exists year-round. This ctenophore (a stingless jellyfish-like animal) is inhabited to the east coast of North and South America. In 1982, it was found in the Black Sea, where it was carried off by ballast water. Following this, it spread to the Caspian Sea. In both places, it multiplied and developed huge populations. The sea walnuts subscribed to the slumping of local fisheries since they feed on zooplankton that the commercial fish also consume. M. leidy has also been spotted in the Baltic, Mediterranean, and North Seas.

The Secret behind Success of Sea Walnut

It is believed that the secret to the success of sea walnut’s lies in their unfussy and effective hunting habits. There has been a research study conducted by Colin where he hand-picked sea walnuts from his domestic dock and had put them in water seeded with tiny particles. The particles were lit with a laser and pictured with a camera, for the purpose to understand how the sea walnut twisted the flow of water around it.

It was discovered that it produces a current that sucks everything in the encompassing water into its tentacle-ringed mouth. The current it produces flows smoothly at just 2 millimetres per second. Only when it passes through the animal’s mouth does it amplify and warp, spiralling into a corkscrew motion, which goes past the tentacles. The mouth shuts, and virtually everything gets trapped in this feeding current. The sea walnut produces its own conveyor belt of food, a streaming buffet composed of a different selection of dishes.

To escape this, the sea walnut requires stealth. Floating plankton won’t display much of a challenge, but some morsels like small crustaceans (copepods) are active swimmers which can trace the movements of incoming predators. If they sense danger, it only takes 2 milliseconds to swim away at the highest possible speed. However, Colin discovered that the water interruption caused by the sea walnut's current is much less than what the copepods can trace.

Feeding currents are not uncommon. Shellfish and other animals use them but they do so in a plain-spoken manner. Their currents are chaotic and explosive, used to attract small plankton that can’t swim away. A brute force approach is actually applied, and one that’s very distinctive to the stealthy strategy of the sea walnut.

Sea Walnut Amazing and Quick Facts

  • The sea walnut is a simple, sheer, marine pellet, only a few inches long.

  • A sea walnut looks like a jellyfish but in fact, it comes from a different but related group known as the comb jellies or ctenophores.

  • The sea walnut invasive species is not exactly a fearsome predator.

  • Ocean’s stealthiest predators, the sea walnut are a major player in the oceans. This ability also makes it a notorious invader.

  • This invasive species cannot see but can only sense the movement of prey slightly beyond its body.

  • It has no evident weapons and it does not even have the potential of great speed.

  • As an outcome of its biological-physical characteristics, it is unable to chase its prey. Rather, it employs beating hairs referred to as cilia to create undetectable water currents that bring its prey straight into its mouth.

  • The weapons of a sea walnut are surprising and have a harsh efficiency.

  • Irrespective of its simplicity, the sea walnut’s ninja-like stealth makes it ruthlessly successful.

  • For a sea walnut’s size, the animal can seize its prey at the same rate as fish, and that’s just when it stays still. If it moves, it could double what it consumes.

Sea Walnuts as an Invasive Species 

Sea Walnuts can reproduce at a pretty fast rate. These species are hermaphrodites that can self-fertilise and produce an egg on their own. In the Caspian Sea, it was recorded that sea walnuts are producing over 2000-3000 eggs every day, increasing their population rapidly. Moreover, an egg produced by a sea walnut can develop within 20 hours of being laid, which is why there has been a significant increase in the population of sea walnuts.


These species were established as invasive in the European and Asian regions due to the fast rate of reproduction. Introduced in the 1980s in the Black Sea, sea walnuts had reached the highest level of the population with 400 specimens per m³ of water. The number of sea walnuts dropped drastically due to the depletion of food stocks. Moreover, sea walnuts caused a drop in the population of pelagic fish in the Black sea, as they ate the eggs and larvae of these fishes.


Sea walnuts entered the Caspian Sea in 1999 through the Unified Deep Water System of European Russia. As soon as they came in, there was a drastic reduction in the number of sprats that caused a drop in the population levels of seals and sturgeons.


After that, sea walnuts spread across the Mediterranean basin, North Sea, and Baltic sea. They were introduced in the North Sea and Baltic Sea in 2006. These species may have entered these seas either through the natural movement of jellies or their transportation in the ballast water. However, due to the large area covered by the Baltic sea, it is still not known how sea walnuts affected this sea.

Conservation of Sea Walnuts 

A sea walnut is one of those water species that have a number of predators, including other types of ctenophores and sea jellies. But due to minimal population control, they have made a tremendous impact in many areas of the world, which also made them an invasive species in these areas.


When the population of sea walnuts was established accidentally in the Caspian sea, it caused a crippling effect on the ecosystem. These species caused a drop in the number of various fish species. The large population of sea walnuts reduced the number of fish eggs in the sea by eating them.

The comb jelly Beroe Ovata was introduced in the Black Sea, which is considered an enemy species of sea walnuts. The Beroe Ovata caused a reduction in the population of sea walnuts that helped the ecosystem to recover to some extent.


However, sea walnuts can tolerate the high salinity levels and temperature of the ocean. It means even a change in the climate is unlikely to cause a drop in the number of sea walnuts.

Why do You have to learn About the Sea Walnut?

You have to learn about the Sea Walnut, because they are one of the highly-populated species in the world. By learning about their physical description, you will be able to identify a sea walnut as soon as you see it. Moreover, you can also differentiate between jellyfishes and sea walnuts, which can be tricky because of their similar structures. You will also get to learn why these species are considered invasive in certain areas of the world. If you want to pursue zoology in future, learning about the Sea Walnut will be quite beneficial for you. You can also learn how these water species help in balancing the ecosystem.

FAQs on Sea Walnut

1. How Can We Identify a Sea Walnut?

It’s mostly in the open ocean. A baby fish, less than a centimetre long, floats across the water, completely unaware of the danger it is in. It’s trapped in a current, but one so smooth that the fish is inadequate to detect it. Its only hint to what’s happening often comes too late, as it’s suddenly sucked into a ring of tentacles and gulped down by one of the ocean’s covert predators – the sea walnut.

2. Do Sea Walnut Sting?

Ctenophores similar to the sea walnut do not sting. Rather, their tentacles have unique adhesive cells referred to as colloblasts that release a sticky, mucus-like material to trap prey. Sea walnuts do possess stinging cells, making them safe to touch. You can even swim around these ctenophore species. Also, sea walnuts have a fragile body making them vulnerable out of water. When removed from the water, their bodies can be damaged severely unless handled carefully.

3. Are Comb Jellies Similar to Jellyfish?

Comb jellies are not jellyfish, even though both put on a display of phyla characteristic of having radial symmetry and gel-like mesoglea-filled bodies. Jellyfish move through muscles that induce their bells to pulse in an acquainted manner, but comb jellies incorporate eight rows of beating cilia, which casually push them through the water. Jellyfish have stinging cells (cnidocytes), while comb jellies are dependent upon sticky collocytes to seize food. That also implies comb jellies don’t sting when handled!

Small Comb jelly movement comes to a head at half a centimetre per second. Regardless of this steady pace, Mnemiopsis leidyi is an awfully successful poacher of zooplankton, including small jellyfish, fish eggs and copepods. Motion studies have also brought to the realisation that the smooth ciliary movement of a comb jelly hardly ever interrupts the water around this predator, making it virtually invisible to its prey.

Light-reflecting off of the fluctuating cilia induces comb jellies to discharge striking scintillating rainbow colours along their rows. Comb jellies can also yield bioluminescence using calcium-activated photoprotein-yielding photocytes (light cells) which remain at the base of each of its eight cilia rows; when interrupted, sea walnuts will glow greenish-blue.

Probably by thumbing-a-ride in ballast waters, sea walnuts overrun the Black, Baltic, and Caspian Seas, where their insatiable appetites exhausted plankton levels. But, another type of comb jelly inhabiting the Western Atlantic—the pink comb jelly (Beroe ovata) — delightedly preys on sea walnuts. Pink comb jellies subsequent to sea walnuts across the ocean, lead to the stabilization of plankton population levels in overrun regions.

4. How are Sea Walnuts Discovered in Caspian, Mediterranean Seas?

In the 1980s, sea walnuts were informally introduced in the ballast of ships into the Black Sea. Since then, the organisms have entered into the Caspian, Mediterranean and North Seas. Where it turns up, it consumes plankton and fish larvae, prompting local food webs to damage. In the Black Sea, the anchovy, an economically significant species, underwent depletion and dolphins have begun to vanish. In the Caspian Sea, the inhabited seal populations have declined too.

5. What is the best platform to learn about the Sea Walnut?

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