In many species of anglerfish, the mating process involves a extraordinary adaptation known as sexual parasitism, where the much smaller male becomes permanently attached (fused) to the larger female. This process is known as sexual dimorphism. 

Male anglerfish are much smaller than females and lack the bioluminescent lure that characterizes females. Instead, they have well-developed olfactory organs to help them locate females. When a male finds a female, he bites onto her body using specialized teeth, and enzymes in his mouth dissolve the skin of the female, allowing him to fuse with her bloodstream. 

Once attached, the male becomes dependent on the female for sustenance, receiving nutrients and oxygen from her bloodstream. In return, the male releases sperm to fertilize the female's eggs when she spawns. 

This extreme form of sexual parasitism ensures that when a female anglerfish is ready to reproduce, she has a mate readily available. This mating strategy is particularly well-known in deep-sea anglerfish species, where individuals are sparsely distributed in the vast ocean depths, making encounters between males and females rare. 

The male's adaptation to fuse with the female ensures successful reproduction when mating opportunities arise.