Losing sight to a parasite

Excerpt from "Greenland Sharks", Text and Photos by Nick Caloyianis
National Geographic Vol.194, No.3, September 1998

...Turning a blind eye to hitchhikers, almost all greenland sharks host tiny crustaceans called copepods that attach themselves to the shark's corneas, severely damaging their eyesight.

Magnified under an electron microscope (bottom left), the three-inch invertebrate exhibits two clawlike appendages that grasp the cornea. The anchoring process creates a scar, and additional lesions occur as the copepod's body scrapes back and forth across the cornea while the parasite feeds on surface-layer cells. Before long, the abraded cornea fogs over.

Dangling like a worm, a copepod trailing an egg sac clings to a Greenland shark's eye. The fish's pupil can be seen behind the clouded cornea. "The shark's eye still moves about," says Caloyianis. "The lens itself at least seems to stay functional."
(...)
In any case, because they spend most of their time in darkness, sometimes at ocean depths reaching 7200 feet, these sharks seem to have little use for eyes. Scientists speculate that the copepod may actually aid the shark in feeding. Fish may be attracted to the parasite, which moves like a fishing lure when the shark is swimming

Although the shark's vision is severely compromised, it retains a keen sense of smell to seek out prey.