Oxyrrhis marina is a species of heterotrophic dinoflagellate with flagella that is widely distributed in the world's oceans.

[1] The species is thought to have a global distribution except for the polar seas, where it is likely absent or rare, though few samples have been taken of these waters.

[3] There are specific records from waters near Europe, North America, Asia, New Zealand, the Canary Islands,[4] Hawaii, and the Azores.

It may have been slowly dispersed on the currents, carried in mats of algae, or transported by humans when shipping arose.

[8] It has been observed eating Nannochloris oculata and Micromonas pusilla, other flagellates such as Goniomonas amphinema, Pfiesteria piscicida, and Stoeckeria algicida, and some bacteria.

It has been observed spinning one of its flagella in such a way that it creates a current, pulling the item closer so it can seize it.

[10] The locomotion of the O. marina cell is helical due to the simultaneous movement of its two flagella.

[12] Its blooms when forming red tides are likely stimulated by environmental factors, such as drops in salinity or increases in prey abundance.

[5] O. marina may also affect the environment by producing dimethyl sulfide, which is released when it grazes on some prey types, such as E.

[10] Predators of O. marina include protozoa such as the ciliate Strombidinopsis jeokjo, copepods such as Acartia tonsa[10] and rotifers.

The mixotrophic flagellate Prymnesium parvum is a prey item for O. marina when the former is nutrient-replete, but can become a predator when it is nutrient-stressed[13] It has been used as food for fish larvae, including those of black porgy (Mylio macrocephalus), lemonpeel angelfish (Centropyge flavissima), and grey mullet (Mugil cephalus).

It is a model organism for the study of many aspects of protist biology, including feeding behavior,[8][9] physiology,[1] ecology,[5][7] growth,[5] trophic position,[7] evolution, genomics, and biogeography.

O. marina itself is very diverse, with many varied strains, and their biology is influenced by the environment, so it can be hard to find a representative specimen to use as a model.