One example of co-operation between species is cleaning. The cleaners, usually shrimp or Wrasse, pick off parasites and debris, so obtaining a meal. The cleaned, eels, other fish, wobbegongs benefit from the removal of irritants. Some particular locations in the ocean are known as "cleaner stations" where particularly cleaner wrasse carry out their business otherwise the fish wishing to be cleaned may signal its wish by either changing colour or by holding its mouth open.

A cod opens its mouth to invite cleaning. The vertical structures inside the mouth are the gill arches.
An Amboin Cleaner Shrimp cleans the teeth of a Green Moray.
An Hinge-bacK Shrimp cleans the teeth of another Green Moray.

A variety of species is groomed by Cleaner wrasse.

Three-barred Porcupinefish.
A Surgeonfish, which is usually black, signals its desire for cleaning by turning white.
more about colour change
Brown sweetlips, one of a group of three being cleaned in a location where the Sweetlips often go for grooming.
Three Cleaner Wrasse working on a Blue Groper.
A White-eyed Moray.
A Spotted Wobbegong being cleaned.