That is a weird fish.
The Angler Fish
The Angler Fish lives more than a mile deep in ocean water. On her forehead the female has a "fishing rod" tipped with an lighted "artificial worm" which she dangles over her mouth to attract her next meal. This cold light displays highly advanced technology. A compound called Luciferin is oxidized with the help of an enzyme scientists named Luciferase, and this reaction produces cold light.1
It is difficult for the male and female to find each other in the darkness of the deep, so the eggs of the female float up through a mile of ocean to the surface where they form a jelly-like mass and then hatch. The young fish, male and female, grow and mature in the surface waters. At a certain point in their development, the male finds a female and bites and holds on to her abdomen. Soon the tissues of the female grow into and attach to the mouth tissues of the male, and the female drops to the bottom of the ocean carrying her parasite male with her for life.1
Unlike other fish, the Angler fish does not have a swim bladder - an air sac to provide buoyancy and to prevent sinking. Another feature of the deep sea Angler is its special body, which is designed to prevent crushing. A pressure of over 2,000 pounds per square inch is exerted on he body of the fish at one mile deep. It survives this great pressure with no problem.1
If the first Anglers were surface fish and lost their air bladders, and then sank to the bottom of the sea, they would have been crushed. Dead animals don't evolve any further.1