Take a look at this Broomtail wrasse (Cheilinus lunulatus) from the Red Sea and compare it with the Dusky parrotfish (Scarus niger), below.  Do you see any similarities?

Both have a very bright and very visible yellow marking just behind the eye and above the pectoral fin.  What function does this marking serve?

Both are in roughly the same size class (fifty centimeters for the largest wrasses and forty for the parrotfish). The wrasse has a more restricted range than the parrotfish (Red Sea to Arabian Gulf), while the parrotfish is far more widespread and reaches the Great Barrier Reef.  However Cheilinus trilobatus replaces it in the rest of the Indo-pacific, but it is without the yellow splodge.  Why does C. lunulatus have one and its closest relative not?

Beyond these superficial similarities the fish lead very different lives; the wrasse feeds on crustaceans and urchins using its powerful crushing jaws, with the parrotfish generally grazing on benthic algae.

My question is:  what does the yellow marking serve?

Very often, bright markings like this can be obvious eye spots (ocelli) – markings that have evolved to resemble eyes, which cause predators to strike at less important parts of the anatomy.  These are usually on the tail, on the top of a dorsal, or somewhere where a wound caused by a failed attack or a passing bite, wouldn’t be fatal.  Obviously some fish have ocelli on their flanks, which are assumed to confuse predators into thinking they are attacking a much larger creature, however, that’s another conversation.  I find it hard to imagine that these markings would be eye spots though, not on such an important part of the body.  Interestingly, when the wrasse is a juvenile it has an ocelli on its caudal peduncle.  No other parrotfish from the region has a marking remotely similar.

If there is some mimicry here, who is mimicking whom?   Has the marking evolved in one species through sexual selection? Is it just a coincidence and am I overreaching? I’d love to know, and I’d welcome any insights anyone may have.