here's one which is kinda gross.. the Greenland or Sleeper Shark is a huge shark which lives in cold waters...
most of them have a copepod which grows in the eyes of the shark, rendering it blind...
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note the worm-like thing coming out of its eye..
there are theories which have not been proven that the copepod, which is pinkish-white, help the shark by attracting prey which the shark senses with electro-senses and smell..