One of the visual clues for Longfin damselfish is the "thin bright blue edge on anal fin" as per the 4th edition and yet here is what I believe to be an unmistakable Cocoa damselfish exhibiting the same characteristic. Any thoughts on whether the "blue edge" can even be used to ID?

Could it be the lighting?

I concur that this looks like a cocoa.  I wonder if the blue edge of the anal (and in this pic, caudal and dorsal as well) fin is an iridescence that many damsels have if they catch the light the right way.   I always have trouble with adult damselfish anyway.

Adult Damsels

Thanks for the input. 

So if the light catches it "the right way" then as a visual clue it is unreliable in my view.

Someone else said that it is ONLY the anal on the Longfin that is blue...I think adult damsels are nigh impossible to ID with 100% accuracy, especially the Dusky/Longfin.


The length and shape (rounded or more pointed) of the dorsal and anal fins seems a more reliable feature to distinguish dusky and longfin but requires a close look and can still be often tricky. The blue line seems to be only sometimes present on the longfin or maybe not always visible due to light conditions.

Many damsels can have blue

Many damsels can have blue edges to the fins, esp. the anal. I would say if the edge is obvious, then think cocoa if you are in the north of the Caribbean/FL/GOM, and think longfin in the Caribbean. Yes, the shape of the fins helps in longfin adults. Note that no-one has tested their certainty of charcters in adult damselfishes- so it is just an opinion how well we can ID them to species. 


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