The illicium is a modified first dorsal fin spine that arches forward over the anglerfish's mouth. At its tip sits the esca, a chambered bulb whose interior is packed with bioluminescent bacteria belonging to lineages related to the genus Photobacterium. The fish supplies nutrients and a protected space; the bacteria supply the light.
Because the bacteria are not passed from mother to offspring in the egg, each generation of anglerfish must take up its symbionts from seawater. Genome studies of escal bacteria have found the hallmarks of a symbiotic lifestyle, including loss of genes that free-living relatives retain, while the bacteria still remain capable of a life outside the host.
What the lure actually does to prey is less settled than the popular image suggests. Direct observation of deep-sea anglerfishes hunting is extremely rare, and most accounts of prey being drawn to the light are inferred from anatomy and from gut contents rather than filmed behaviour.