Sexing Polypterus ornatipinnis. Sexing is pretty easy when the Ornates are larger (over 10 in). I have attached two pictures from AQUALOG: Polypterus; Flosselhechte - bichirs, by Frank Schafer (2004).
The first image is of a male. Take a look at the anal fin. The anal fin is large and "thick." The first spine of the anal fin is at least 1/2 the total length of the fin. The large fin is believed to assist in the act of copulation. The male uses this fin to cover the femail cloaca (like a dish) so that as the eggs emerge, they are fertilized.
View attachment 876241
The anal fin of the female is much smaller.
View attachment 876240
Hope this helps.
References:
- AQUALOG: Polypterus; Flosselhechte - bichirs, by Frank Schafer (2004)
- Jurassic Fishes, by Haruto Kodera, TFH Pub 1994 (originally published in Japanese by Marine Pub Co under title "Ancient Fishes" (1992)
- Ichthyol. Explor. Freshwatrs, Vol 9, No 4. pp 325-334, Dec 1998 (On the reproduction and early devleopment of Erpetoichthys calabaricus, Polypterus senegalus, and Polypterus ornatipinnis, by Ralf Britz and Peter Bartsch.