Underwater activities in Akumal, Mexico
Many fish which are not observed during daylight hours can be observed when snorkeling after nightfall. This is usually because they are night-feeders or resting during the day.
The octopus is a perfect example of this phenomenon. Octopuses only come out to feed at nighttime, and change color when a light is focused on them. Virtually all of the life seen at night are safe for snorkelers, so night snorkeling is a safe activity also.
Plant life and some types of fish are also luminescent after dark. This creates greenish lights shining either continuously or flashing at night – quite a view for one who is only accustomed to snorkeling during the day!