Word of the manatee spread this week after a video surfaced showing the creature swimming in the waters of Florida's Homosassa River. Sure enough, somebody had scraped T-R-U-M-P into the algae growing on its skin.

Wildlife officials say the manatee doesn't appear to have been harmed, but that doesn't mean people aren't pissed. Manatees are a protected species in the area, and it's against the law to harass them ... punishable up to a year in prison and a $50,000 fine.

Now, the U.S. Fish and Wildlife Service is heading up an investigation into the matter, soliciting any info that might lead to whoever did this ... with a $5,000 reward being offered as well by the Center for Biological Diversity.

The Center's director, Jaclyn Lopez, says it's sickening that somebody would do this to a helpless creature of the sea, adding ... the person is "capable of doing grave violence and needs to be apprehended immediately."