What happens to the "deep scattering layers" in the water of the sea during night?

The correct answer highlights the behavior of deep scattering layers, which are aggregations of marine organisms (particularly zooplankton and small fish) that play a significant role in the ocean's ecology. During the night, these organisms migrate towards the surface waters from the deeper layers of the ocean. This nightly migration is commonly referred to as "vertical migration."

The main reason they exhibit this behavior is to feed on phytoplankton, which are more abundant in the upper layers of the ocean due to sunlight promoting their growth. By rising to the surface at night, these organisms can take advantage of the increased food availability while utilizing the cover of darkness to evade predators that are less active in the upper water layers during that time.

The other choices do not accurately reflect the behavior of these organisms during nighttime. For instance, they do not sink to the bottom or form a broad band in the middle depths; their primary movement is upward towards the surface. Additionally, they do not disappear completely, as that would contradict the established patterns of nocturnal behavior observed in marine ecosystems.