Polykrikos (from Greek “poly” - many, and “krikos” – ring or circle) is one of the genera of family Polykrikaceae that includes athecate pseudocolony-forming dinoflagellates.

Polykrikos are characterized by a sophisticated ballistic apparatus,[2] named the nematocyst-taeniocyst complex, which allows species to prey on a variety of organisms.

In 1873 Butschili re-examined the specimen and concluded that the cell was an unusual ciliate, and Bergh later, in 1881, clarified Polykrikos dinoflagellate affinities.

All Polykrikos species have: 1) a slightly curved longitudinal furrow, sulcus, extending to posterior end of the organism 2) a loop-shaped acrobase, which is an anterior extension from the sulcus 3) a transverse furrow, cingulum, with the displacement 4) taeniocyst-nematocyst complexes 5) two or four times less the number of nuclei than of zooids, and 6) ability to disassemble into pseudocolonies with fewer zooids and only one nucleus.

Within the group there is some variation in which organelles are presented, but trichocysts, nematocysts, taeniocysts, mucocysts and plastids have been observed from different members within the taxon.

Furthermore, Gavelis et al.[3] deeply examined NTC morphology and ballistic mechanism that were shown to be fundamentally different from cnidarians, demonstrating nematocysts have evolved independently in single-celled dinoflagellates.

Ballistics in cnidarians nematocysts is driven by synthesis of osmotic propellant poly gamma glutamate synthase, PgsAA, while in Polykrikos it is thought to occur due induced pressure as a result of capsular fibre contraction in the capsule wall.

[9] In Polykrikos, well-defined fibrous ribbons are involved in nuclear-flagellar connections, and anchoring to flagellar apparatus might serve in orientation of the nucleus in relation to flagella during processes of movement, mitosis and cell division.

[11] For the genus of Polykrikos, detailed data is available on reproduction of a type species (holotype) P. kofoidii, whose life cycle resembles general dinoflagellate cycle as vegetative cells form gametes that fuse to form a diploid (2n) zygote that could encyst, but pseudocolonial nature adds a number of peculiarities to the Polykrikos development.

[7] There is also variation in feeding ecology as some species have plastids and can use photosynthesis to obtain nutrients but often happen to be mixotrophs (P. barnegatensis, P. lebouriae, P tanit, P. hartmannii).

[13][8] Polykrikos hartmannii is a phototrophic dinoflagellate and has been reported in waters of Canada,[14] USA, Mexico, China, India, Japan, Korea,.

[13] Predation by heterotrophic Polykrikos became a great topic of interest as some of the organisms graze on dinoflagellates that cause toxic blooms.

[4] G. catenatum is one of the species causing paralytic shellfish poisoning (PSP) and is found in waters of Australia, Japan, Mexico and Spain.

Polykrikos are known to modulate populations of dinoflagellates like Alexandrium tamarense,[3][4] and G. catenatum,[17] which are among prevalent agents of toxic algal blooms.

However, some Polykrikos pose a health risk to certain fishes, while the bloom-regulating ones are often preyed on by marine invertebrates, like amphipods, which would return the toxins back into the food web.