Sexual dimorphism in Idiacanthus atlanticus is among the most extreme of any fish. The female is a long-bodied predator with fangs, a barbel bearing a light organ and photophores down each flank. The male is a short, dark, toothless animal without a barbel and without a working digestive tract.
The division reflects the economics of the deep sea. Food is scarce and mates are scattered, so a male that invests everything in finding a female rather than in feeding machinery can be smaller, cheaper and produced in greater numbers. The same logic, taken further, produces the parasitic males of ceratioid anglerfishes.
The dragonfish's other notable trait is red light. Several barbeled dragonfishes in the related genus Malacosteus produce far-red bioluminescence that most deep-sea animals cannot see, which has been described as a private channel. Whether Idiacanthus shares that capability is not something to assume from the family alone.