Three adaptations of sea life in the abyss?

Answer 1

Many have

sharp fang-like teeth to hold prey. Many have greatly expandable stomachs

and jaws that unhinge to allow them to swallow prey larger than themselves.

Some have photophores, pockets of bioluminescent bacteria, that they may use

as fishing lures to attract prey close to their hungry mouths.

Photophores may also function as identification tags so fish can find others

of their species in order to reproduce. Special adaptations for reproduction

must be important in an unlighted sea where fellow fish may be few and far

between.