Warty Comb Jelly, Mnemiopsis leidyi

The Warty Comb Jelly (Mnemiopsis leidyi), also known as the Sea Walnut, is a species of tentaculate ctenophore originally native to the western Atlantic coastal waters. Three species of Mnemiopsis have been named, but are now generally categorized as different ecological forms of the species leidyi. This species tolerates a wide range of salinity (2 to 38 psu), temperature (36 to 90 degrees F), and water quality.

This creature was introduced in the Black Sea in the 1980s, where only one other species of comb jelly (small sea gooseberry) occurred. The introduction has been viewed as an accidental carriage via the ballast water by merchant ships. It was first recorded in the Black Sea in 1982. The species reached its highest level in 1989, with about 400 specimens per square meter of water. The population slowly dropped thereafter, mostly likely due to depleted food stocks.

Since this species eats eggs and larvae of pelagic fish, it was blamed for the dramatic decline in fish populations there, notably the commercially important anchovy Engraulis encrasicholus, in which they both competed for the same food sources. Experts then introduced another comb jelly (Beroe ovata) aimed to control the Warty Comb. Some success was made and it now appears a fairly stable predator-prey dynamic has been reached in the Black Sea.

The Warty Comb was introduced in the Caspian Sea in 1999, where it affected the entire food chain of the lake by consuming nearly 75 percent of zooplankton found there. Since taken a foothold in the Caspian Sea, the species has spread throughout the Mediterranean basin and the northwestern Atlantic.

This species was first recorded in the North Sea in 2006, and then subsequently found in the Baltic Sea, namely the Kiel Fjord and The Belts. A year after being discovered there, the Baltic population was found to have spread to the Gotland Basin and the Bay of Puck. The impact of this species as an invasive predator has significantly stressed the Baltic ecosystem, yet it remains unknown to what extent.

The species over-winters in the deep waters where temperature does not drop below 39 degrees Fahrenheit, and the fact that the Baltic is heavily stratified, with the waters above and below the halocline mixing little, is believed to aid its survival.

The route dispersal of the Baltic populations is unknown. Experts speculate that it either occurred naturally by drifting individuals, or with ballast water of ships, either from its natural range or from the Black Sea.

This species has a lobed body that is oval-shaped and transparent, with four rows of ciliated combs that run along the body vertically and glow blue-green when disturbed. It has several feeding tentacles. Unlike cnidarians, Mnemiopsis doesn’t sting. It’s body contains 97 percent water. This small creature has a maximum body length of roughly 3 to 5 inches and a diameter of 1 inch.

This carnivore consumes zooplankton including crustaceans, other comb jellies, and eggs and larvae fish. It is also sometimes known to eat smaller individuals of its own kind. Certain bird and fish species are known to feed on the Warty Comb, as well as other species of jellyfish.

This species has the capacity for self-fertilization as it is hermaphroditic. The gonads contain both the ovary and spermatophore cells for reproduction. The animal carries 150 eggs along each meridional canal. Eggs and sperm are released into the water column where external fertilization takes place. Spawning commences in the late evening to about 2 a.m. at the latest. The spawning eggs develop a thick outer layer within 1 minute of touching the seawater. As many as 10,000 eggs can be produced from large specimens in areas with good prey abundance.

Researchers have recently sequenced the DNA of the Warty Comb, making it one of the smallest animals ever to have this mapping achieved.

Image Caption: Sea walnut (Mnemiopsis leidyi) at the New England Aquarium, Boston MA. Credit: Steven G. Johnson/Wikipedia(CC BY-SA 3.0)