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I am a member of a local fish club, we have a lot of different aquarium fish and would like to trade them amongst us. We all have a few fish that don't form pairs and would like to see if they pair up with someone else's. But as you can imagine keeping track of all the fish would be extremely hard and pointless if they were the same gender, so someone suggested we should buy a microscope and sex them via tissue or blood sample. But none of us has any experience doing that or if that is even possible, so we need some expert advice on the matter.

I wish to know if you can tell the gender of a fish from a tissue or blood sample and what equipment do you need (how much magnification in a microscope)? We work with discus and other cichlids, and countless marine aquarium fish.

For the sake of being specific I restrict my question to Discus fish, however, I would like to know if any general principles of tissue-sample based identification exist, for different species of fish.

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  • $\begingroup$ I know nothing about ichtiology but if you just google discus fish sexing you get tons of hits (including videos). $\endgroup$
    – Remi.b
    Feb 8, 2016 at 19:10
  • $\begingroup$ But it's not restricted to discus fish, they are just the most common fish with sexing difficulties that came to mind. As for the google results, they are useless copy-paste guesswork. $\endgroup$
    – Tone
    Feb 8, 2016 at 19:26
  • $\begingroup$ I might misunderstand your point. Discus fish are maybe just one example but it is impossible to describe sexing generally for all fish. Is it? Or maybe you are asking for a list of methods generally used for sexing without willing to discuss sexing for any specific species? However, question like the magnification will obviously vary between 1X to 500X I suppose. $\endgroup$
    – Remi.b
    Feb 8, 2016 at 19:43
  • $\begingroup$ Well if you have a tissue sample you may be able to do genetic testing (PCR and gel electrophoresis) but it depends on species - sex determination systems are variable in nature. Asking what magnification scope you would need makes it sound like you want it to be a visual inspection method. $\endgroup$
    – rg255
    Feb 8, 2016 at 19:56
  • $\begingroup$ Depending on the number of fish, sending tissue to sequencing might be cheaper than buying a microscope. But it only works if the species is 100% GSD (genetic sex determination) and this depend on the species (won't work for clownfish for example). Also karyotyping might be a solution for GSD that have sex chromosomes which also depend on the species. The feasibility of a non-lethal visual inspection (with or without a microscope) depends on the species. So really....**it depends on the species** $\endgroup$
    – Remi.b
    Feb 8, 2016 at 20:49

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As already pointed out by Remi, you can identify the sex from a tissue sample only if there is a genetic (such as chromosomal) sex determination system (by using techniques such as PCR or in-situ hybridization). However, different species of fish employ different mechanisms of sex determination:

Focus on or attachment to genetic systems involving X and Y chromosomes must quickly be shed in studies of sex determination in fish due to the diversity of mechanisms utilized, and, by doing so, the researcher can explore alternative regulatory mechanisms that hold important clues for understanding sex determination in general. The biology and ecology of fish is sufficiently diverse to provide unique examples of sex-determination mechanisms, yet they possess many of the same processes and pathways that are used in other vertebrate systems.

Devlin, Robert H., and Yoshitaka Nagahama. "Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences." Aquaculture 208.3 (2002): 191-364.

Though Devlin and Nagahama comment (above) that fishes also employ the same signalling pathways as other vertebrates for sex determination, study of these pathways to determine the sex would be extremely difficult as you may be aware that the cell-signalling and the gene regulatory networks are highly complex.

Fishes are highly sexually dimorphic (including Discus) and the sex can be easily identified (with some practice) by visual inspection. Microscopic identification would be both money and time consuming. Morphology based identification can be automated (when the machine is trained sufficiently). You can have a look at this article1 which is about using image analysis to score salmons.

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    Merz, Joseph E., and William R. Merz. "Morphological features used to identify Chinook salmon sex during fish passage." The Southwestern Naturalist 49.2 (2004): 197-202.

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