Its the same as for all
Hypancistrus sp. (See L262 below). Do a forum search for more info.
Male:
Thickened "hairy" pectoral spine
Triangular body shape - widest across eyes
Big bulky head
Hides in same cave a lot
Larger than female
Female:
Round body shape - widest at start of dorsal fin
Smaller, more pointed head
Less cave fidelity
Its easier from above, and is pretty obvious when mature. If it is not obvious, the fish are either out of condition or immature.
