Sexing bichirs is a little different from most fish, where patterns, colours, body shape or size are used to easily distinguish between genders. Polypterus sexing is done almost exclusively through the differences in the anal fin; males have a thick, fleshy fin while females have a reduced, pointed fin. This diagram posted by Oddball @Oddball shows the difference clearly:
This differentiation becomes apparent in juvenile fish and is about the only way to sex them at a young age. Mature females may be slightly larger than males, especially when swollen with eggs.
Another point of interest is the difference in maturation times; males will begin breeding at 2-3 years of age while most female bichirs mature after 5+ years. Female bichirs also seem to be more prominent than males. A poll on the forum gave a rough ratio of 30% males to 70% females, this was also observed by Budgett in his studies of polypterus. The exact reasoning behind this is unknown but may be to ensure that a greater amount of female fish survive to maturity.
- While the sex of bichirs can be determined at 7-9 inches,Some species may not be sexually mature till 6 years of age