Final answer:
The daily vertical movement of small marine animals between the photic zone and lower depths is known as Diel vertical migration, a crucial component of the oceanic food web.
Step-by-step explanation:
The daily movement of small marine animals between the photic zone and lower depths is called Diel vertical migration. This process involves zooplankton and other small marine animals that migrate to the photic zone at night to feed on phytoplankton and descend to the depths during the day to avoid predators. This migration is considered the largest biomass migration on Earth and is critical for the oceanic food web, linking the surface and deeper waters.