Among large animals, the open waters are the home of vertebrate fish especially, though some invertebrates like the giant squid and some mammals like the mighty whale live there too. The free swimmers move lightly through a world in which swiftness, keen senses, and protective col-oration are the keys to survival. Kings of speed are the blue marlin, capable of spurts up to 50 miles per hour; the sailfish; the dolphin; and the dolphin's favorite prey, the flying fish, which taxis at 35 miles per hour and soars rather than flies through the air. Not much slower are tuna and oceanic bonito.
Most surface-dwelling fish have been tinted by nature to blend with their surroundings. Dominant colors are green, blue, and violet. Sea creatures in the sunlit upper levels are often bluish above and silver underneath. From about 600 to 1,500 feet, a twilight grayish zone, the fish are correspondingly light-hued like silvery Sternoptyx, Diretmus, and Opisthaproctus. Below, in the zone of utter blackness, animals display the brown and black shades of Melanocetus and Photostomias, though a few, such as the scarlet deep-sea prawns, wear bright colors—which cannot be seen anyway!
Even in the depths, few fish are blind. Between 800 and 1,500 feet, many have enlarged eyes, sensitive to very dim light. Innumerable sea creatures glow, some with lights they can turn on and off at will; and in the hunt for food, the ability of a fish to see this glow is doubtless a real advantage.
Most deep-sea fish are equipped also with enormous mouths and long, needle-sharp teeth. One, Chiasmodon, has a stretchable stomach so that it can swallow prey larger than itself.
The greatest battles of the deep are those waged a quarter of a mile down by the sperm whale and its familiar prey, the giant squid.
