bwaaahahhahahaa... yeah, right...
Sixgill sharks are bottom-feeders. It's sorta like saying there are giant man-eating STURGEON. Uh... ain't gonna happen. :-)
Well, there are giant sturgeon in AD&D :) In real-life, they like to lurk on the bottom along with sixgills, waiting for Ruthys to come along. . .and sometimes they even leap into the boat in search of Ruthy snacks:
Florida Sturgeon "Attacks" Over For Now
Last summer, at least four people were injured when the fish with prehistoric roots leapt out of the water and crashed into them.