Page 216 - (DK) Ocean - The Definitive Visual Guide

214 INTRODUCTION TO OCEAN LIFE

Swimming and Drifting

MOST OF THE OCEAN’S LIVING SPACE IS NOT ON THE SEABED but in the water
column and out in the open ocean—areas known as the pelagic zone. Salt water
provides support, as well as the nutrients that allow many plants and animals to live
in the water column without ever going near the seabed. Some animals live at the
interface between ocean and air, or alternate between both environments, because
it is more energy-efficient. The water surface, water column, and seabed are all
interconnected, and many animals move between these habitats.

Plankton
The sunlit, surface layers of the ocean are home to many tiny plants and
animals (plankton) that drift with the water currents. Phytoplankton
consist of bacteria or plantlike chromists (see p.234) that can
photosynthesize and make their own food. Along with fixed seaweeds
and seagrasses, phytoplankton form the basis of
ocean food webs. Zooplankton consists of animals,
most of which are very small and feed on the
phytoplankton. However, jellyfish can grow to a huge
size. Many deep-sea forms have strange shapes and
soft bodies that are very delicate. Some zooplankton,
such as arrow worms, comb jellies, and copepods,
live permanently in the plankton, hunting and
grazing (holoplankton), while others are simply the
larval and dispersal stages of animals, including crabs,
worms, and cnidarians (meroplankton) that will
TEMPORARY PLANKTON
spend part or all of their adult lives on the seabed. Most temporary zooplankton
Many planktonic organisms have elegant spines, are the larvae of animals that,
long legs, or feathery appendages that help them as adults, live on the seabed.
PLANKTONIC LARVA float. Tropical zooplankton generally have more The common jellyfish, however,
The eggs of the has a planktonic adult stage
common shore crab of these than their temperate or polar equivalents (shown above), and a fixed,
hatch into floating, because warm water tends to be less dense and asexual, juvenile stage (right).
spiny zoea larva. viscous, and so provides less support.

Nekton
Fish and most other free-living marine animals can all swim, even if
only for short distances, over the seabed. However, some animals spend
their whole lives swimming in the open ocean and are collectively called
nekton. This group includes many fish and all whales, dolphins, and other
marine mammals, turtles, sea snakes, and cephalopods. There are also some
representatives from other groups such as swimming crabs and shrimp.
Most nektonic animals are streamlined,
TYPICAL NEKTON FEATURE
Dusky dolphins are typical of nektonic and there is a remarkable similarity in
animals, most of which are vertebrates shape between some dolphins and open-
(animals with backbones). ocean nektonic fish such as tuna.

The Ocean–air Interface

Some animals live at the interface between air and water, either floating at the
surface or alternating between the two environments. Oceanic birds such as
albatrosses, petrels, gannets, and tropic birds spend their whole lives out at sea.
They eat, sleep, preen, and even mate on the ocean surface. Large rafts of such
seabirds are particularly vulnerable to oil spillages. Other diving seabirds, such
as terns, alternate between hunting at sea and resting on land. Just as these birds
plunge down into the water to catch fish, so some sharks lunge out of the water
to catch birds and turtles. Flying fish launch into the air to escape their predators.
OCEAN LIFE body projecting into the air. The by-the-wind sailor is a small, colonial cnidarian FLYING AND DIVING
Some planktonic animals live permanently at the water surface with part of their
that is supported by a sail-like float and transported by wind blowing against its
The brown pelican is one of
vertical sail. Drifting with it on a raft of mucous
several species that dive or dip
DRIFTING AT THE INTERFACE
bubbles is the violet sea snail, which also feeds
down from the air into the
The large gas-filled float of the
on it. There are even surface-living insects, of
Portuguese man-of-war supports the
water to catch fish. It uses its
the genus Halobates, that drift the oceans.
capacious beak as a scoop.
whole colony at the water surface.