The swordfish, not to be confused with the swordtails or marlins. These fish are pretty frickin' cool top predators, in my opinion. Tuna are generally thought of as being pretty close to the top of the food chain, especially the larger species. Swordfish eat tuna.
This species is endangered, and average-catch weights have been declining for years. Personally, I'd like to see the commercial fishery for Xiphias halted.
More cool stuff about swordfish than I care to write right now can be found here.
The only pictures I could find of living swordfish are from sport-fishing sites, like the one shown above. The first picture I had for Xiphias gladius was actually of a Sailfish. Below is a picture apparently taken during a SCUBA dive.