Arsenic is a odd little element that is usually known for its toxic effect on most carbon based life. It is however often used in semiconductors along with Gallium. There is an extremophile bacterium that loves arsenic-rich environments (generally volcanic fissures, tailing pools from mining) it is called unimaginatively Arsenobacteria.
Selene is a goddess of the moon, selenium was thus called so because of her, and Tellurium which had just been discovered and named for the Earth. Clearly having similar properties meant it should be called with a similar naming convention, and since Tellurium is heavier and us in the set of elements below Selenium, the name became obvious. So, what the hell does this have to do with Tuna? Selenium is a powerful antioxidant in Tuna especially, though clearly too much is toxic just like too much of anything is!