There are nocturnal animals in the sea, just as there are on land. Many marine animals hide all day and only come out at night to hunt, feed, or mate.
The colonial anemone pictured here (Nemanthus annamensis) is one such nocturnal marine animal -- part of the underwater 'night shift' that begins shortly after sunset and lasts until dawn. The only way to see these nocturnal marine creatures is to dive at night.
The anemones in these colonies open as soon as it gets dark. During the night they look like pretty flowers, with their tentacles extended to feed. When morning comes they fold their tentacles inward and each one of the 'flowers' turns into a tightly shut fist. While they are closed during the day they look like nondescript lumps, their nighttime beauty obscured.
The photo was taken during a night dive in the Red Sea, near Sharm el Sheikh, Egypt. If you click on the photo it will enlarge.