It is found in the blood of bluefin tuna,[1][2] and other sea dwelling animals like turtles, mackerel, beluga,[3] and giant petrels.

[4] It is an antioxidant,[5] combining with reactive oxygen species and boosting the action of GPx1.

[6] Selenoneine is produced by microorganisms using enzymes that form a selenium-carbon bond.

[4] A related derivative, Se-methylselenoneine is found in lesser quantities in mackerel, sardine and tuna.

[11] In air selenoneine is easily oxidised to a dimer containing a diselenide bond (Se-Se).