Best way to sex a common is to look at the papilla, if it is raised, then male, if it is inverted or more or less on the same level as the rest of the body, female.
Sexual organs. Females have a flap. Males have a point. The differences davo listed are correct, but generally most prominent when the female is gravid/male is in breeding condition.
cichlidbuddy is talking about my pleco "tank" ,the papilla is below the stomach before the tail right? Hey gave me his junior pleco and i was wondering if they whould fight during the mating/breeding season if they have one. also my pleco is a common but he has leoperd like spots on his belly i've been on here for a few days now and i see there are code numbers or name for each one how big will he get i've heard between 10" to24" he is now 8.5" and between 2.5 to 3 yrs old he is my star in my tank.
Well, there is no point breeding them if they are the same species... You have pictures of the two? P. pardalis will definately get to 14". They will just have the odd tussle etc over territory anyway... nothing specific. You will need a very big tank or a pond with some mud to get these species to breed.