Mnemiopsis leidyi, the warty comb jelly or sea walnut, is a species of tentaculate ctenophore (comb jelly). It is native to western Atlantic coastal waters, but has become established as an invasive species in European and western Asian regions. Three species have been named in the genus Mnemiopsis, but they are now believed to be different ecological forms of a single species M. leidyi by most zoologists.

Mnemiopsis have an oval-shaped and transparent lobed body, with eight rows of ciliated combs that run along the body vertically and glow blue-green when disturbed. They have several feeding tentacles. Unlike cnidarians, Mnemiopsis does not sting. Their body comprises 97% water. They have a maximum body length of roughly 7–12 centimetres (3–5 in) and a diameter of 2.5 centimetres (1 in).

It is euryoecious, tolerating a wide range of salinity (2 to 38 psu), temperature (2–32 °C or 36–90 °F), and water quality.

Mnemiopsis is a carnivore that consumes zooplankton including crustaceans, other comb jellies, and eggs and larvae of fish. Many of its predators are vertebrates, including birds and fish. Others are members of gelatinous zooplankton such as Beroe ctenophores and various Scyphozoa (jellyfish).

The comb jelly has the capacity for self-fertilization, as they are hermaphroditic. They have gonads that contain the ovary and spermatophore bunches in their gastrodermis. It carries 150 eggs along each meridional canal. Eggs and sperm are released into the water column where fertilization takes place. The spawning commences at late evening or at 1:00 or 2:00 a.m. The spawning eggs develop a thick outer layer within a minute of encountering seawater. As many as 10,000 eggs are produced from large specimens in areas with abundant prey. Egg production can start when the animals reach about 15 mm in length. Egg production increases with ctenophore size, and it is unclear when senescence occurs. Indeed, much like the so-called immortal jellyfish, Mnemiopsis leidyi can undergo reverse development, namely reverting into a previous life cycle stage.

It has a transient anus, which means that it appears only during defecation. There is no permanent connection between the gut and the rear of the body. Instead, as waste accumulates, part of the gut starts to balloon out until it touches the outer layer, or epidermis. The gut then fuses with the epidermis, forming an anal opening. Once excretion is complete, the process is reversed and the anus vanishes. The animals defecate at regular intervals: once an hour in the 5-centimetre-long adults, and once every 10 minutes or so in the larvae.

The species moves so slowly that it is referred to as "sea walnut".

In 2024 it was described to possess the ability be biological immortal by undergoing reverse development after the onset of sexual reproduction from mature lobate to early cydippid, similar to Turritopsis dohrnii and other jellyfish with this ability from the Cnidaria phylum and the only one known from the Cteniphora phylum.