Page 258 - (DK) Ocean - The Definitive Visual Guide

256 ANIMAL LIFE

Animal Life

ANIMAL LIFE FIRST APPEARED IN THE OCEAN over
DOMAIN Eucarya
one billion years ago. It has since diversified into a vast
KINGDOM Animalia
array of different organisms. The range of scale among
PHYLA About 30
marine animals is immense: the smallest invertebrates
SPECIES Over 1.5 million
are over half a million times smaller than the largest
whales. Despite this huge disparity, animals all share two key features. First,
they are heterotrophs, meaning they obtain energy from food. Second, they INVERTEBRATE
This yellow tube
are multicellular, which distinguishes them from single-celled life forms. sponge, from the sea
off Belize, is a typical
sessile invertebrate.
Instead of moving to
Marine Animal Diversity find food, it filters out
particles of food by
Animals are classified into 30 or more major groups (phyla), all of which include at least pumping water
some marine animals. Twenty-nine of these phyla are composed of animals without through its pores.
backbones (invertebrates), each phylum representing a completely different body plan.
Only one phylum, the chordates, contains animals with backbones (vertebrates). In salt
water, vertebrates include fish, reptiles, birds, and mammals—
animals that are often described as the dominant forms of
ocean life. However, in terms of abundance and diversity,
invertebrates have a stronger claim to this title.
Invertebrates exist in all ocean
VERTEBRATE
habitats and outnumber marine Active predators, such
vertebrates by a million to one. as this barracuda, need
They include an array of fixed sharp senses and rapid
CHANGING SHAPE (sessile) animals, such as corals and reactions to catch prey.
Most invertebrates change shape as they Unlike invertebrates, they
develop. Feather stars start as drifting larvae, sponges. They also form most of the have fast-acting nerves
which eventually attach themselves to corals zooplankton, a drifting community and well-developed
or rock before changing into swimming adults. of tiny animals and animal-larvae. brains.
Support and Buoyancy

On land, most animals have hard skeletons to counteract gravity’s pull.
Life is different in the sea, because water is denser than air. It buoys up
soft-bodied animals, such as jellyfish, enabling them to grow large. They
use internal pressure to keep their shape, the same principle that works
in balloons. Animals with hard body parts, such as fish and mollusks, are
often denser than water, and would naturally sink.
BUBBLE RAFT To combat this, many have a buoyancy device. Bony
The violet sea snail fishes have an adjustable gas-filled swim bladder,
stays afloat by producing while squid have an internal float made of chalky
bubbles of mucus.
The mucus slowly material, containing many gas-filled spaces. Some
hardens, forming a surface dwellers, such as the violet sea snail, have
permanent raft. gas-filled floats that prevent them from sinking.

Groups and Individuals
Among marine animals, there is a social spectrum COLONY ON THE MOVE LONE GIANT
from species that live on their own to those that form A diver films a pyrosome colony The whale shark (below)
permanent groups. The whale shark is a typical solitary in the sea off Florida. It consists is a solitary species with
of thousands of tiny soft-bodied
a pantropical range.
species, spending its entire life on its own apart from animals called tunicates, joined It only congregates in
when it breeds. It can do this because its huge size together to form a tube. particular regions during
means it has few natural predators. Smaller fish often the breeding season.
form shoals, which reduce each fish’s chances of being
singled out for attack. Many invertebrates, from corals
to tunicates, live in permanent groups, known as
colonies. In most coral colonies, the individual
animals, or polyps, are anatomically
OCEAN LIFE units, even though they are joined. NUMBERS
identical and function as independent
SAFETY IN
Crowded together
Other animal colonies, such as the
in a ball, gregarious
Portuguese man-of-war, are made
striped catfish
of individuals with distinct forms.
(right) make a
Each form carries out a different
confusing target
task, like parts of a single animal.
for predators.