For many marine fish, it is impossible to determine the sex of a given reef species simply by looking at it. Many species of saltwater fish have differently colored sexes, dichromatic, or different shaped sexes, dimorphic, or in the case of many wrasses and clownfish there is some form of hermaphroditism going on.
Similar challenges to breeding freshwater fish can arise when the sex is not known, as was the case when Panta Rhei of Hannover Germany acquired six of the ultra rare zebra pike cichlid, Crenicichla zebrina. The zebra pike cichlid is not only rare but exquisitely beautiful commanding 1000€ ($1360) a piece, so when it came time to offer up a sexed pair to one of their customers, Panta Rhei took their fish to the veterinarian to undertake one of the first known medical examinations for sexing an aquarium fish.
When the valuable zebra pike cichlids arrived at the veterinarian’s office, they were sedated one by one and their internal organs examined using ultrasound. Females of the Crenicichla zebrina showed the development of ovaries while in males they were absent. In short order and through a straightforward process, Panta Rhei was able to definitely sex their pricey fish without having to wait for them to spawn, or relying on uncertainties.
This type of ultrasound sexing technique could also be applied to valuable marine fish being considered for broodstock in the marine aquarium hobby, large marine angelfish and tangs come to mind as prime candidates for this type of procedure.