Page 220 - (DK) Ocean - The Definitive Visual Guide

218 INTRODUCTION TO OCEAN LIFE

Zones of Ocean Life HUMAN IMPACT
SHIFTING ZONES

NO PART OF THE OCEAN IS DEVOID of organisms, from polar The northern and southern
geographical limits of many
seas to the tropics and from coasts and the seashore to the deepest shallow-water marine species are
depths. The seabed and the water column above it both support dictated by water temperature.
Most species breed and disperse
a huge variety of life. However, marine organisms are distributed only within certain temperature
unevenly both horizontally and vertically. As on land, climate (mainly limits. Climate change is slowly
raising water temperatures and in
temperature) and food play a large part in determining distributions the Northern Hemisphere, records
and biodiversity. In the harsh environment at the poles, there is less have shown that some warm-
water species are extending their
coastal life than in the warm tropics, but beneath the surface, Antarctic ranges farther north. Similarly,
seas support rich marine communities. Although there is life at every some cold-water species may be
expected to retreat farther north.
depth, most creatures can only survive within particular depth zones
at particular pressures, temperatures, and light regimens. TROPICAL INVADER
Warm-water triggerfish stray as far north as
southern Britain and have now begun to breed
Geographical Zones there. With continued ocean warming, they
may become a native species.
Seawater temperatures are much more stable than those on land because water loses
and gains heat more slowly than does air. However, the distribution of marine coastal
and continental shelf communities still follows a global pattern, with distinct polar,
temperate, and tropical ecosystems. Coastal salt marsh in temperate parts is replaced in
the tropics by mangroves. Kelp forests only grow in cool waters but extend into the
tropics in places where cold water upwells from the deep, such as off the coast of
Oman on the Arabian peninsula. Planktonic species and bottom-living species with
planktonic larvae might be expected to occur anywhere that ocean currents take them.
However, a boundary between water
masses with different physical
characteristics may present as
effective a barrier in an ocean
as mountains do on land.
Below a certain depth, there
are fewer such barriers, and
conditions are stable and
similar worldwide, so
deep-sea animals often have
very wide distributions.
KEY
CLIMATIC ZONES equatorial temperate
The shape and tilt of our planet results in differences
in the amount of solar radiation reaching land and tropical subpolar
ocean at different latitudes. This produces
large-scale climatic zones that ring Earth. subtropical polar

Endemic Species

Some marine organisms, especially pelagic species, have a wide MALDIVES ANEMONEFISH
global distribution, since there are few barriers to their dispersal. This endemic fish is not a
Others live in restricted geographical ranges and are said to be strong swimmer. It does not
endemic to a particular sea, island, or country. The most remote have planktonic larvae and
lives only in the Maldives
patches of habitat, such as small oceanic islands, tend to have and Sri Lanka in the Indian
the most endemic species. This is Ocean. Its host anemone
because animals in their dispersive has a wider distribution,
stages, such as eggs and larvae, may because its larvae disperse
on ocean currents.
survive only for short periods and so never
reach distant shores. The Red Sea holds many
endemic fish species. It is connected to the Indian Ocean only by a
narrow channel and so is effectively isolated. Endemic fish are often
those that cannot or do not swim
OCEAN LIFE beyond their home islands. their eggs on rocks under their
GALAPAGOS PENGUIN
far. Anemonefish, for example, lay
This penguin species lives only
around the Galápagos islands.
anemones and the young search for
The cold, upwelling Cromwell
Current keeps them cool in spite
new anemones on the same reef.
of the tropical climate. They
Flightless marine birds such as
are isolated by the surrounding
penguins are likewise restricted in
warm waters, so cannot disperse
their ability to colonize new areas.