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Aquarium of the Pacific - Online Learning Center - Species Print Sheet

Conservation Status:  Species of Special Concern

Aquatic SpeciesComb Jelly (Sea Walnut)

Mnemiopsis leidyi CnidariansSea jelliesInvertebrates

Comb Jelly on Blue Background | Andrew Reitsma / Aquarium of the Pacific

Species In-Depth | Print full entry

At the Aquarium

Comb jellies are on exhibit with other sea jelly species in the Northern Pacific Gallery.

Geographic Distribution

Endemic: east coasts of North and South America from the Canadian Maritime Provinces to the southern tip of South America. Invasive species: Black Sea, Azov, Aegean and Marmara Seas, western coast of Sweden, southern and northern Baltic Sea.


This species generally prefers coastal saltwater habitats in bays and estuarine locations; however, it is tolerant of a wide range of salinity, 3% to 39%; temperature, 4.0o-31.0o C (39.2o-87.8o F); and water quality conditions. It is frequently found in brackish water that is low in oxygen content and high in pollution. It can also be occasionally found in the open ocean waters long distances from land. It seems to prefer near surface waters, but will work its way into deeper water during periods of rough seas.

Physical Characteristics

The vertical cross section of the comb jelly is bell shaped with the lower margin of the oral lobes forming the rim of the bell. The mouth is positioned where the bell clapper might be. There are wart-like bumps on the walnut shaped body.

The visible internal structures are primarily the gonads and the digestive system. Externally the animal has eight longitudinal rows or bands of cilia (tiny movable hairs) that divide the body into eight symmetrical shapes and also give it the ability to slowly move through the water. There are two fine, filamentous lobes on either side of the mouth used for feeding that can be retracted into the body. There are other smaller fine accessory tentacles that aid the feeding process.

Although this animal has a translucent almost colorless body, it frequently presents a real color show. The moving cilia refract ambient light into all colors of the rainbow and bright fluorescent stripes are visible on the body. Many ctenophores are bioluminescent, including this species, and at night soft green or blue-green light may be observed.


Maximum length of the sea walnut generally ranges between 100-120 mm (3.9-4.7 in} although larger specimens have been reported from the Caspian and Black Seas. The body width is approximately half of its length.


This comb jelly is a voracious carnivore and a major predator of edible zooplankton consuming up to 10 times its weight per day. It prefers a broad-based diet of zooplankton including eggs and larval forms of various invertebrates and fishes, juvenile fish, copepods, sea jellies, and even other ctenophores. Phytoplankton is also found in the gut but is apparently incidental to the preferred diet.

It feeds by continuing pumping water into its body cavity trapping small prey on adhesive cells (colloblasts) found on the tentacles and the inside surface of the two lobes. The food is then transferred to the mouth for ingestion. It captures large prey by swimming with lobes out-stretched, then snapping them closed to trap the meal. Mnemiopsis leidyi never feels full. If food is present it will never cease feeding.


This species is a free-spawning simultaneous hermaphrodite that is capable of self fertilization. Spawning takes place during summer months and will vary with habitat conditions. Internal fertilization can occur, but most commonly eggs and sperm are broadcast into the water column where fertilization takes place. The eggs produce a fast growing larva that is fully developed in 20 hours. On hatching the larvae are 0.3-0.4 mm (0.12-0.16 in) long. Sexual maturity is rapid and some specimens may begin to produce eggs in as little as two weeks after hatching. In the Caspian Sea spawning occurs at night when 2-3,000 eggs per day are produced dependent on food availability.


These planktonic animals are subject to movement by currents and wind and wave action. The bands of cilia provide some mobility, probably relating mostly to vertical position in the water column. Primary activities relate to feeding and reproduction.


This species can tolerate a rather broad range of water temperature, salinity, and pollution. If food supply becomes limited, these comb jellies have the ability to reduce their physical size and metabolism and therefore reduce food requirements to the point where they can survive for up to three weeks on a limited intake of food.

Because they are sexually self-fertilizing it is feasible that a single, displaced specimen could start a whole new, non-native population.


This species has a number of natural predators including several species of fishes, some sea jellies and even other ctenophores, but natural population control is minimal. They have no positive commercial value but have had a tremendous economic impact on areas where they are an invasive species.

The unintentional introduction of Mnemiopsis leidyi into the Caspian Sea had a catastrophic effect on the entire ecosystem. Several years later it was introduced into the Baltic Sea where it devastated the anchovy fisheries. More recent introductions into other parts of Europe have caused severe hardships in local fisheries. Large populations of voracious comb jellies significantly reduce the volume of fish eggs and larvae and also diminished other planktonic forms that these developing animals require for food. Populations of fishes and dolphin have crashed. The resulting loss of harvestable fishes has caused a severe decline in the fisheries industry. The income loss due to this jelly since the beginning of the 1990s is estimated at over 300 million US dollars

Seventeen years after the introduction of M. leidyi into the Black Sea, another introduced comb jelly Beroe ovata, a natural enemy of the American comb jelly that preys almost exclusively on it has caused the decline of some populations of these comb jellies to a low enough level that the ecosystem seems to have recovered to some extent.

Special Notes

The ballast water of ships unintentionally introduced Mnemiopsis leidyi into the Black Sea and adjacent seas in 1982. In 1999 it appeared in the Caspian Sea, introduced in the ballast water of oil tankers; in 2006 in waters on the western coast of Sweden and the southern Baltic Sea; and in 2007, the northern Baltic Sea. The American comb jelly is now listed by the Global Invasive Species Program as among the 100 of the World’s Worst Invasive Alien Species.

Mnemiopsis is considered to be the most studied ctenophore genus in the world both because of its abundance as a native species in estuaries in heavily populated urban areas of the US and its abundance, rapid growth, and spread since its introduction into the Black Sea in the early 1980s where it spread to other areas to become a major invasive species.