Here are two different perspectives on one of the reef creatures that I can't help but find a bit disgusting:  the Isopod.  There are 12 species in 2 genera of these things that have been identified in the Caribbean.  They are arthropids, in Class Crustacea, Order Isopoda, Family Cymothoidae.  Despite appearances, Isopods are not parasitic.  Here is what Humann & DeLoach has to say about them:

"Instead of dining on the tissue of hosts, single individuals or mated pairs attach to the head region of reef fishes with several pairs of hook-like legs and benignly scavenge specks of floating food from the water....Once associated with a fish, they lose their ability to swim and remain where they settle for life.  To increase their odds of finding mates, males have the ability to transform into females.  When a male settles on a host with a female already in place, it mates with the larger female.  Later, after the female dies, the males change sex and await the arrival of a young male.  If a mate is not present at settlement, a male accelerates growth and changes into a female."

I have only seen them attached to Soldierfish, like the one pictured above, but I have seen photos of them on other species.