I can't find reference to purpose of this sack that keyholes have under their throat. This picture shows it mid-way down so it goes a lot deeper. The fish seem to have full control over it and I *think* all males and maybe the females have it (not sure about the females). I have 7 keyhole and based on fish size likely 2 female 4 males and 1 unknown. Also what is it called - i've not been able to find references to it. It seems like such an oddity that it would be described as a ?unique? feature of this species.
