The animals of the open sea are conveniently divided into the active
swimmers (Nekton) and the more passive drifters (Plankton). The swimmers
include whales great and small, such birds as the storm petrel, the
fish-eating turtles and sea-snakes, such fishes as mackerel and herring,
the winged snails or sea-butterflies on which whalebone whales largely
feed, some of the active cuttles or squids, various open-sea prawns and
their relatives, some worms like the transparent arrow-worm, and such
active Protozoa as Noctiluca, whose luminescence makes the waves sparkle
in the short summer darkness. Very striking as an instance of the
insurgence of life are the sea-skimmers (Halobatidæ), wingless insects
related to the water-measurers in the ditch. They are found hundreds of
miles from land, skimming on the surface of the open sea, and diving in
stormy weather. They feed on floating dead animals.
The drifters or easygoing swimmers--for there is no hard and fast
line--are represented, for instance, by the flinty-shelled Radiolarians
and certain of the chalk-forming animals (Globigerinid Foraminifera); by
jellyfishes, swimming-bells, and Portuguese men-of-war; by the
comb-bearers or Ctenophores; by legions of minute Crustaceans; by
strange animals called Salps, related to the sedentary sea-squirts; and
by some sluggish fishes like globe-fishes, which often float idly on the
surface.
Open-sea animals tend to be delicately built, with a specific gravity
near that of the sea-water, with adaptations, such as projecting
filaments, which help flotation, and with capacities of rising and
sinking according to the surrounding conditions. Many of them are
luminescent, and many of them are very inconspicuous in the water owing
to their transparency or their bluish colour. In both cases the
significance is obscure.