Aquatic ecosystems consist of two different layers with respect to light. The photic zone is where light penetrates the water, at the surface and a little below. The aphotic zone is where the water is perpetually dark because the light cannot penetrate that deep into the water. The zone depths vary due to salinity and clarity of water.

Which of the following must be true about the organisms living in the aphotic zone?
A.
They have adapted to having no light for an energy source and are able to live in dark and cold areas.
B.
They all must move to the surface of the water once a day to receive adequate light energy.
C.
They have all become fluorescent so they can all be seen by one another.
D.
Many organisms have not been able to survive in the cold, dark waters, so population is low.