Unfortunately, it is not possible for just anyone to do it. The gonadal examination of Pygocentrus piraya is an often cited article by hobbyists who believe they can sex piranhas using this method. However, it is a complex procedure and the fishes used in that report were in the range of 6 cm to 34 cm SL (juvenile to adult). The gonads were examined by histological sections by embedding them in paraffin blocks and slicing, and couter staining with eosin/hematociciline. This is the standard method at looking at egg development. Using the Medaka model above it is also not possible to see ovaries on piranha fry fish as well. Gonads are under-developed in fry fish, therefore not possible to determine sex of piranhas. Some hobbyists have reported seeing granules or globule like objects similar to the Medaka model. Those are more than likely fat beads. Larvae and/or fry that consume brine shrimp naupalii will also give the illusion of eggs in that region, but it is undigested food (see photo upper left).

Presently, the only piranha species known to be sexually dimorphic (external sexual characteristics) is Pygopristis denticulata, the anal fin is bi-lobed.

REFERENCES

• BLACKSMITH, R.M.A.; BAZZOLI, N.; RIZZO, E.; SATO, Y. - reproductive Aspects of piranha, Pygocentrus piraya (Teleostei, Characiformes), native species of the basin of the River Sâo Francisco. Arq. Bras. Med. Vet. Zootec. 48 (supl. 1): 71-76, 1996.

• CROSS, A.M.G.; SATO, Y.; RIZZO, E.; SAINTS, G.B.; BAZZOLI, N. sexual Maturação of piranha Pygocentrus piraya (Cuvier, 1820) (Pisces, Characidae) in the dam of Three Marias, Minas Gerais. Bios , 4 (4): 17-22, 1996.

• Cuvier, G., Memoires du Museum National d'Histoire Naturelle, Publishing date; 1819, Sur les poissons du sous-genre «Hydrocyn, sur deux nouvelles espèces de «Chalceus», sur trois nouvelles espèces de «Serrasalmes», et sur l'«Argentina glossodonta» de Forskahl, quiest l'«Albula gonorhynchus» de Bloch. Cuvier, G. . 1819 Mem. Mus. Natl. Hist. Nat.. 5:351-379.

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UPDATED: 07/09/2009