One of the best known mutualist relationships (a relationship that is beneficial for both parties involved) is that of the cleaner wrasse and the larger fish or rays on the reef.
The cleaner wrasse, in this image the smaller fish (a Bluestreak cleaner wrasse), spends its time at a ‘cleaning station’ on the reef. This area is frequented by the larger fish who stop by every now and then for a ‘clean’. The small wrasse will pick parasites and dead tissue from the skin, gills and even mouth of the larger fish (in this case a brightly coloured sweetlip). The wrasse gets an easy meal and the other fish benefits from being free of any harmful parasites which might otherwise be impossible to shake.
Article Source: Catlin Seaview Survey