The big queston: Three kilometres down, it's a female's world ...

Q. What is the strange fish that lives in the deepest depths of the ocean and has a luminous stalk protruding from its head?

A. The triplewart seadevil anglerfish (Cryptopsaras couesi) is rarely seen by humans because it lives up to 3km deep. As with most anglerfish, the female can grow to more than 1m while the male reaches a mere 6.3cm.

The female is distinguished by the weird, luminous "fishing rod" or lure protruding above her eyes and the three luminescent sacs just forward of her dorsal fin, which give the triplewart its name.

The light from the female's lure is thought to be produced by colonies of luminescent bacteria, and she eats the small fish and shrimp attracted to it. The lure also attracts prey by vibrating.

As with other species of anglerfish, the males permanently attach themselves to the side of the female for breeding, attracted by what scientists think is probably a special scent. After the male attaches itself by gripping the female's skin with its teeth, it becomes dependent on the female for nourishment and protection and feeds by hooking itself into the female's bloodstream - the male lacks a digestive tube.

The male then begins to degenerate, its large eyes grow smaller and it eventually loses them, becoming simply a source of sperm. Once the female gives birth, the male remains attached for life.

* Send your Big Question on space, science, conservationand the environment to anne_beston@nzherald.co.nz

Herald feature: Environment