This fish has colorless blood.

While diehard sports fans may claim to bleed blue, orange, or whatever color adorns their favorite team, the actual blood found in humans and other vertebrates tends to be red. But the Antarctic icefish is an exception to that rule, as they’re the only known adult vertebrates to lack hemoglobin — a red-colored protein essential for transporting oxygen — in their bloodstreams. Because of that hemoglobin deficiency, an icefish’s blood is completely transparent.

When this physical oddity was first discovered in the 1950s, it was believed to be an adaptation to the frigid Antarctic water. But upon further research, it was determined to in fact be a potentially fatal mutation that icefish were lucky to survive. Despite lacking hemoglobin, icefish are still able to absorb sufficient oxygen into their system thanks to the fact that colder Antarctic waters possess higher levels of dissolved oxygen than warmer waters.