Instructions
Note the size of the head. Species of lionfish vary in overall body size from 7 to 55 centimeters, but male lionfish in each species feature consistently larger heads than females in the same species. If you are comparing two lionfish of the same species and age, the fish with the larger, boxier head is likely the male.
Compare the length of the pectoral fins with the length of the posterior fin. In male lionfish, the pectoral fins, located behind the gills on each side of the body, extend beyond the posterior, or tail, fin. Female lionfish feature pectoral fins that do not reach the tips of the posterior fin.
Count the stripes on the pectoral fin. In most species of lionfish, males have at least six and up to 10 stripes on their pectoral fin, while females rarely have more than six stripes.
Observe the lionfish during courtship rituals. The males in some species of lionfish, like brachypterus, zebra and Pterios volitans, tend to alter their overall coloring to several shades darker than normal during courtship. Females of the same species also lighten their coloring during courtship.
How to Identify a Male Lionfish From a Female
Few aquarium fish have such a dramatic presence as the lionfish. Lionfish, also known as tigerfish, scorpionfish and dragonfish, are a poisonous saltwater fish valued as tank fish because of their distinctive fins and striped coloring. Determining the gender of lionfish is notoriously difficult, making tank breeding particularly challenging. Sexing lionfish is important if you are considering breeding because lionfish are aggressive, and male lionfish are likely to fight if placed in the same tank. Despite the range of species classified as lionfish, a few consistent differences between males and females of each offer a starting point for determining sex.