Download 2- MARINE ORGANISMS

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2- MARINE ORGANISMS
The oceans are representing the largest living space on the planet today and
throughout Earth's history. The creatures that inhabit the wide sea are diverse in both
shape and form. The oceans and the coastal seas provide a complex variety of
environments for living organisms. At the surface, conditions range from polar to
tropical, over depth from light to total and constant darkness and the sea floor may be
rock, sand, or rood. Organisms of the marine environment have much in common
with the organisms of the land, but they also have very different survival problems
and have developed unique solutions to cope with them. Life in the water ranges from
microscopic, single celled organisms to the largest fish and the greatest whale.
CLASSIFICATIN OF ORGANISMS
A wide variety of the organisms inhabit the environment of the ocean. All organisms
are classified and placed in groups to promote identification and to increase
understanding of the relationships that exists among them.
KINGDOMS OF LIFE IN THE SEA
The Five-Kingdom System
Since the time of Carolus Linnaeus (1707–1778), many new organisms have been
discovered, and many classification systems have been proposed. Some of the
systems have merit; all try to make sense of the great diversity of life on our planet.
However, most scientists now use the classification system that is composed of five
kingdoms. (See the Table)
TABLE THE FIVE-KINGDOM CLASSIFICATION SYSTEM
Monera: This group, commonly called the monerans, includes the bacteria and
blue-green bacteria (formerly called blue-green algae). Bacteria are structurally
simple microorganisms that are especially significant as decomposers, breaking down
organic compounds into nutrients that can be used by other organisms. They are also
an important food source and help degrade pollutants. Some species are autotrophic,
and account for much of the oceanic primary production. All bacteria are singlecelled; all lack a nuclear membrane and thus their nuclear material is dispersed
throughout the cell. Scientists call organisms that lack a nuclear membrane
prokaryotes.
Cyanobacteria are photosynthetic bacteria widely distributed in the marine
environment.
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Protista: The kingdom of protists includes mostly single-celled, or unicellular,
organisms, although some are composed of many cells or live together in small
colonies. All protists have their nuclear material enclosed within a membrane; that is,
they have a nucleus. Scientists call organisms with this feature eukaryotes. (All
organisms on Earth other than those in the kingdom Monera are eukaryotes.) Tiny
protozoa (animal-like organisms) and many kinds of algae (plantlike organisms)
make up this group.
Diatoms
Diatoms (phylum Bacillariophyta) are unicellular, though many species aggregate
into chains or star-like groups. These protists usually float and drift near the ocean
surface.
Diatoms are part of the ocean’s community of plankton (meaning “wanderers” that
drift, rather than swim) and are more accurately classified as phytoplankton (“plant
wanderers”). Some diatoms (called encrusting diatoms) live attached to solid
substrates, while others alternate between attached and free-floating forms. The cell
walls of diatoms are made of silica. After their death, they sink to greater depths
building up siliceous sediments called the siliceous ooze.
Dinoflagellates (dino for “whirling” and flagellum for “whip”)
Members of another protist group often found near the ocean’s surface are the
dinoflagellates. Three types of dinoflagellates are shown in the next Figure. They are
classified in phylum Dinoflagellata or Pyrrophyta, which means “red (or fire)
algae.” How do dinoflagellates compare with diatoms? Dinoflagellates have two
flagella. Dinoflagellates also possess chloroplasts and, like diatoms, are able to make
and store their own food. Many dinoflagellate species have an eyespot that is sensitive
to light. They use the eyespot to move toward the light, thus increasing their ability to
make food. Unlike diatoms, dinoflagellates are also able to take in food. The cell
walls of dinoflagellates and diatoms differ in structure and composition.
Dinoflagellate cell walls are made of cellulose like those of plants, not of silica like
those of diatoms. A cellulose cell wall is not as transparent as a diatom’s glassy cell
wall.
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Seaweeds
What do ice cream and toothpaste have in common? It may surprise you to learn that
both products contain substances that come originally from seaweed. Seaweeds are
multicellular algae that live in the sunlit waters of the ocean. Organisms that are
multicellular are made up of more than one cell. More than 500 species of
macroscopic algae live in the ocean.
These algae are classified according to the color of the pigments in their cells. The
three main groups classified in this way are the green algae, brown algae, and red
algae.
People harvest seaweeds as a useful source of food, food additives, and agricultural
fertilizer. The annual harvest of North American kelp weighs some 20,000 tons.
Three types of green algae.
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Three types of brown algae.
Three types of red algae.
PROTOZOANS
Thousands of species of protozoans are found living on the surface of marine
substrates and in the bottom sediments. These one-celled organisms are classified
within the protist kingdom, along with the algae. The protozoa are subdivided into
three major groups: the Ciliophora, Zoomastigina, and Sarcodina.
The Ciliophora are the largest group of protozoa, composed of thousands of
freshwater and marine species, all having cilia.
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The Zoomastigina consist of a group of animal-like protists that move through the
water by means of whiplike flagella. Included in this group are the euglena and the
dinoflagellates, which take in (as well as make their own) food.
The Sarcodina are the protozoan group that includes the forams and radiolarians, as
well as the amoebas. They live on the surface of substrates and move by means of
cytoplasmic extensions called pseudopods (meaning “false feet”). This kind of
movement, typical of the amoeba, is called ameboid movement.
Foraminiferan: The foraminiferan (meaning “hole-bearing”), or foram for short,
is a unicellular protist. Forams are encased in a shell, or test, made up of calcium
carbonate (CaCO3). Parts of a foram’s cytoplasm flow out through holes, or pores,
in the shell and form a sticky surface for catching food.
Radiolarian. A unicellular zooplankton. It is transparent because its cell wall
(like that of a diatom) is composed of silica. Long spines branch out from a
radiolarian’s body, like the spokes of a wheel, for added buoyancy and protection.
Radiolarian
Foraminiferan
Fungi: The fungi include both unicellular and multicellular eukaryotic organisms
that are not able to make their own food. Fungi absorb their nutrients from dead
organic material and live tissues. Like plant cells, fungal cells are enclosed by a rigid
cell wall. Unlike plants, fungi do not contain the green pigment chlorophyll. Fungi
play an important role in breaking down dead organisms and recycling organic
material. Hundreds of species of fungi exist, both on land and in the ocean.
Plantae: Plants are multicellular, eukaryotic organisms that are able to make their
own food out of simple chemical substances. Sea grasses are the only land plants that
have fully adapted to life under the sea. They grow submerged, and their flowers
produce pollen grains that drift through the water to reach and pollinate other flowers.
They release their seeds underwater, too.
Animalia: Animals are multicellular, eukaryotic organisms. Unlike plants, animals
are not able to make their own food, but must instead eat plants or other animals to
obtain their nutrients. Most animals are capable of movement. We categorize animals
in two main groups: those that lack a backbone and skull (the invertebrates) and
those that have a backbone and skull (the vertebrates). Both groups are widely
represented by animals that live in marine environments.
MARINE INVERTEBRATES
More than 95 percent of marine animal species do not have backbones; they are
invertebrates. They range in size from the miniature fauna living in the seabed
sediment and tiny zooplankton drifting in surface waters, to giant squid that reach 18
m long. Marine invertebrates belong to about 30 different groups called phyla
(singular phylum), with each phylum-containing species sharing many features in
common.
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SPONGES, ROTIFERS, AND BRYOZOANS
Each belong to distinct phylum of marine invertebrates. Although each group of
species is unique enough to be placed in its own phylum, they do share some
characteristics. Rotifers and bryozoans are both microscopic, but sponges can grow
quite large. Sponges and bryozoans are both sedentary, but rotifers are capable of
movement. However, the animals found in all three groups are multicellular, bottomdwelling invertebrates.
Sponges
Sponges (phylum Porifera meaning “pore bearing”) are the simplest multicellular
invertebrates. In structure a sponge is little more than a sac with pores. Inside, lining a
central cavity, are cells with beating hairlike structures called cilia that create
miniature water currents. These draw food particles into the sac through the pores.
Sponges show a variety of interesting shapes, sizes, and colors. The bath sponge
(Euspongia), which lives in warm tropical waters, may be most familiar to you. The
traditional bath sponge which is the skeleton of the sponge is now threatened because
of overharvesting. The beautiful Venus’s flower basket (Euplectella), which lives at
great depths, is a tubular sponge composed of a delicate network of glassy spicules.
The vase sponge (Ircinia), found on sandy bottoms near coral reefs, grows vertically.
Some basket sponges, found in tropical waters, grow so large that a person could sit
inside of them.
Rotifers
Meaning “wheel bearer”, dozens of rotifer species live in the moist sands along the
shore and in the gravel of aquarium tanks.
(Rotifers are also common in freshwater.)
Rotifers, which are in their own unique
phylum Rotifera, are able to change the shape
of their body. For example, when they swim
by means of their cilia, rotifers can telescope
their bodies to facilitate movement. Some
rotifers are predatory, while others scavenge
on debris. When attached to a substrate, the
rotifer’s crown of beating cilia in its head
region gives the animal the appearance of a
miniature spinning wheel. The moving cilia
create a water current that pulls floating food
toward the rotifer’s mouth.
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Bryozoans
Another benthic organism, which is sometimes mistaken for a sponge, is the
bryozoan (meaning “moss animal”). The bryozoan, classified in its own phylum
Bryozoa, is a microscopic multicellular animal that lives within a box- or vase-shaped
compartment made of calcium carbonate or chitin. Branching colonies composed of
hundreds of individual bryozoans cover the surfaces of rocks, seaweeds, and shells.
Colonies of different types of bryozoans have different shapes.
Cnidarians
JELLYFISH / SEA ANEMONES / CORALS / HYDROIDS
JELLYFISH
Jellyfish are typical members of the phylum Cnidaria. Members of this phylum, called
cnidarians. The jellyfish is not, in fact, a fish. Jellyfish differ from true fish in many
ways; the most fundamental difference is that the jellyfish, as an invertebrate.
Jellyfish are members of the class Scyphozoa. All members of this class have an
umbrella-shaped structure called the medusa, with tentacles hanging down from it.
SEA ANEMONES
Sea anemones, which look like colorful underwater flowers, are members of the class
Anthozoa (meaning “flower animal”). Unlike jellyfish, the adult sea anemone lives as
a polyp, attached to a substrate by means of a muscular foot.
The sea anemone is a sessile cnidarian
Structure ofthe jellyfish, a typical cnidarian.
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CORALS
One of the most spectacular structures in nature is a coral reef. Some are huge, like
the Great Barrier Reef off the coast of Australia, which is about 2000 km long and 80
km wide. However, the organism responsible for the formation of reefs is so small. In
fact, the coral animal is so tiny that some can be seen only through a microscope. Yet,
different species of coral animals are capable of building a variety of structures that
range in size from small to the massive reefs that can pose a danger to ships. Corals
come in a variety of shapes and sizes, often resembling familiar objects for which
they are named.
Coral Types and Structure
There are two types of corals: stony (hard) corals and soft corals. The stony corals are
made up of limestone (calcium carbonate), and they can form massive stony
structures. The soft corals are composed of a fibrous protein, which gives them
flexibility. Underwater, the soft corals look more like plants than animals, as they
sway back and forth with the waves and currents. The basic structure of the coral
animal is the coral polyp,
shown in the Figure. As
you can see, the coral polyp
resembles a very small sea
anemone. Due to their
similarities in structure and
function, corals are placed
in the class Anthozoa along
with the sea anemones.
However,
unlike
sea
anemones, which live alone
or in small groups, coral
polyps live in large groups
as
colonial
animals,
attached to one another by a
thin
membrane.
The
membrane connects the
polyps’ digestive systems,
so there is nutritional sharing among them.
HYDROIDS
Another type of cnidarian that may resemble a plant is the hydroid. Hydroids, which
are members of the class Hydrozoa are actually colonial animals, made up of many
individual polyps that function together as a single organism. There are some
exceptions to the rule; for example, the hydroid Tubularia is not colonial.
Marine Worms
FLATWORMS AND RIBBON WORMS / ROUNDWORMS AND SEGMENTED
WORMS / GIANT TUBE WORMS AND ARROW WORMS
Tens of thousands of marine invertebrate species are worms. Most live in or on the
seafloor, but a few, such as the arrow worms (phylum Chaetognatha), float in the
plankton.
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Flatworms
The flat-body form is the distinguishing trait used to classify these worms in the
phylum Platyhelminthes (meaning “flatworms”). Although some flatworms are
microscopic, many others can be seen without a microscope. In fact, some species can
reach nearly 20 meters in length! There are more than 10,000 species of flatworms.
Include Planarian, Tapeworms and Trematodes.
Ribbon Worms
The largest free-living worm in the sea is the ribbon worm (Cerebratulus lacteus),
shown in the next Figure. The nearly 1000 species of ribbon worms are classified in
the phylum Nemertea. Ribbon worms live in the intertidal zone. Most are free-living,
although some live inside the shells of clams and oysters. Ribbon worms burrow in
the sand and also swim with a gently undulating motion among animals that live
encrusted on rocks.
Roundworms
There are more than 10,000 species of roundworms. Very tiny roundworms live in the
sand and mud at the bottom of the water column. Roundworms are classified in the
phylum Nematoda; they are often referred to as nematodes.
A marine roundworm.
The ribbon worm.
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Segmented Worms
The worm’s body is divided into compartments, or segments. Referred to as the
segmented worms, or annelids, earthworms and all other annelids are classified in
the phylum Annelida (meaning “little rings”). There are three classes of annelids,
comprising more than 10,000 species. Earthworms, and their aquatic relatives that
feed on organic matter in sediments, belong to the class Oligochaeta (meaning “few
bristles”).
The Sandworm
A common marine annelid is the clamworm, or sandworm (Nereis). (See Figure)
Sandworms live in muddy sands. Sandworms belong to the class Polychaeta (meaning
“many bristles”).
The Bloodworm and Other Polychaetes
Another segmented worm found in marine sediments is the bloodworm (Glycera).
(See the Figure) Like the sandworm, the bloodworm is a polychaete. You can see a
bloodworm’s blood through its skin, hence its common name.
Polychaete worms also include a variety of other
worms. Some polychaete worms have developed a
different strategy to avoid being eaten—they live
inside a tube, which they make themselves. Most of
these tube dwelling worms are small and threadlike in
appearance. The fan worm (Sabella) constructs its
hard tube by mixing sand grains with mucus, which it
secretes from special sacs (See the right Figure). The
parchment worm (Chaetopterus variopedatus)
secretes a tube formed from a tough fibrous material,
in which it lies buried in sediments. Both of these
worms thrust their feathery gills out of the tube into
the water to obtain oxygen, give off carbon dioxide,
and capture plankton.
Another polychaete tube worm, called the Atlantic
tube worm (Hydroides), secretes a hard tube
composed of calcium carbonate, which is cemented to
the surfaces of mollusks, rocks, and corals.
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The Leech
An example of a segmented worm without bristles is the leech,
which is placed in class Hirudinea (See Figure.) Some leeches are
free-living, while others are parasites that can be found attached
to the gills and mouths of fish. The leech attaches to its host by
means of two suckers, located at the anterior (front) and posterior
(back) ends of its body. Sharp teeth in the sucker at the anterior
end pierce the host’s skin. The leech then draws blood from its
host. At the same time, the leech secretes a chemical
anticoagulant, called hirudin, into the wound. This prevents the
host’s blood from clotting, thus enabling the blood to flow freely
into the leech.
GIANT TUBE WORMS AND ARROW WORMS
Giant Tube Worms
These are worms that live in protective tubes, and some
of these grow quite large. An interesting group of deepsea gutless worms belongs to the phylum Pogonophora.
In 1977, a new species of worm belonging to this phylum
was discovered living near hot-water vents on the deep
seafloor, thousands of meters below the ocean’s surface.
These giant tube worms (Riftia pachyptila) measure up
to 1 meter long and live clustered in water that is rich in
hydrogen sulfide. The tubes in which they live (which
are made of proteins and minerals) may measure up to 2
meters in length. (See Figure)
Marine biologists have discovered unique bacteria that
live inside the giant tube worms. These bacteria are able
to use the hydrogen from the hydrogen sulfide in the
water and combine it with carbon dioxide from seawater
to produce sugars. So the worms live by utilizing
carbohydrates made by the bacteria, which get their energy from the oxidation of
hydrogen sulfide. The process by which the bacteria produce energy-rich compounds
from inorganic chemicals is called chemosynthesis.
Arrow Worms
Near the surface of the ocean, lives the tiny, transparent arrow worm (Sagitta),
shown in the next Figure. The arrow worm is classified in the phylum Chaetognatha
(meaning “bristlejaw”). Arrow worms are just a few centimeters long. They have tiny
fins that enable swimming, but they mostly drift as part of the plankton community.
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Mollusks
BIVALVES / GASTROPODS / CEPHALOPODS / OTHER MOLLUSKS
BIVALVES
All mollusks have soft, bilaterally symmetrical bodies composed of a head, foot, and
coiled visceral mass (internal organs), and most have either an external or internal
shell. The clam (Mercenaria) is a common bivalve. There are more than 15,000
species of clams. Most clams live buried in sand in the intertidal and subtidal zones.
One of the oldest living clam specimens known is a giant clam (Tridacna gigas) from
the tropical waters of the South Pacific; it is over 60 years old and weighs more than
half a ton.
Bivalves have a variety of adaptations that, by securing them to a substrate, enable
them to filter feed more effectively. The mussel (Mytilus), shown below, lives in the
turbulent intertidal zone, where there is constant wave action. Tough byssal threads,
which are made of a fibrous protein, attach the mussel firmly to rocks and other hard
substrates. A gland inside the mussel’s foot secretes the byssal threads, which extend
out from the mussel. Sticky pads at the end of the threads enable the mussels to cling
to surfaces.
The oyster (Crassostrea), shown in the next Figure, also lives attached to a substrate.
The shells of the oyster are rough and uneven. The flat upper shell fits like a lid on top
of the more curved lower shell. The lower shell secretes a cement that adheres to
rocks and other hard substrates. Dental scientists are interested in the chemical
properties of oyster cement, because it might be useful in developing a new type of
filling for teeth. Besides secreting their shells and cement, under the proper conditions
oysters and other bivalves can produce natural pearls.
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GASTROPODS (meaning “stomach-foot”)
The gastropods are a diverse group, comprising about two thirds of all mollusk
species. These mollusks are also referred to as univalves, meaning “one shell,” which
is one of their distinguishing characteristics. All gastropods are placed in the class
Gastropoda (meaning “stomach-foot”). The snails shown below are representative of
a typical gastropod.
GASTROPOD DIVERSITY
Flat-shelled gastropods; the shells in the previous figures are spiral in shape,
whereas those in the next Figure are more flat. The biggest of the flat-shelled
gastropods is the abalone (Haliotis), which inhabits the rocky Pacific Coast, where it
grazes on algae that grow on rocks. Just as the oyster can produce pearls, the
abalone’s secretes this pearly lustrous material, which is called mother-of-pearl. (This
material is often used to make jewelry.) A common gastropod of the intertidal zone is
the slipper shell (Crepidula). The underside of the slipper shell resembles a slipper or
shoe, hence its name.
Another flat-shelled gastropod that inhabits the rocky coast in the intertidal zone is the
limpet. The limpet shell is shaped like a slightly flattened cone. Some species, such as
the keyhole limpet (Diodora), have a hole at the top of the shell, which allows wastes
to exit.
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An unusual type of gastropod is shown in the next Figure, the sea slug (Dendronotus),
also called a nudibranch. The sea slug and the sea hare (Aplysia) are gastropods that
either lack shells or have reduced shells.
Like the cephalopods (see below), some sea hares can release dark ink to confuse, and
escape from, predators.
CEPHALOPODS (meaning “head-foot”)
Unlike the gastropods and bivalves, these mollusks are excellent swimmers. The
swimming mollusks belong to the class Cephalopoda (meaning “head-foot”), with
their prominent features being the head and the tentacles (the “foot”). Referred to as
the cephalopods, they swim by a kind of jet propulsion.
Cephalopods include; octopus, squid, Nautilus, and cuttlefish. The octopus (meaning
“eight feet”) has eight tentacles, whereas the squid (Loligo) has ten tentacles (two of
which are longer than the others).
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The chambered nautilus, which inhabits the deep waters of the South Pacific Ocean,
has a spiral-shaped shell that is divided into compartments.
The cuttlefish has an internal shell composed of calcium carbonate, known as the
cuttlebone (which is either coiled or flat, depending on the species), adds support to
the cuttlefish’s soft, streamlined body.
OTHER MOLLUSKS
Chitons
Unlike many mollusks, chitons have no eyes or tentacles on their heads. Also, notice
the overlapping shells, a characteristic not found in any of the other mollusks. The
chiton is assigned to its own class, Polyplacophora (meaning “many plates”). They
vary in size from 1 to 40 cm in length.
Scaphopods
Another small class of mollusks, called Scaphopoda, consists of the tusk shells,
named for their tapering shell shape.
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Crustaceans
THE LOBSTER / THE CRAB / THE SHRIMP AND OTHERS CRUSTACEANS/
DIVERSITY AMONG THE ARTHROPODS
Members of phylum Arthropoda are commonly called the arthropods (meaning
“jointed feet”). The characteristic movable limbs, which give the phylum its name, are
referred to as jointed appendages. The tough body
covering, or outer skeleton, is the animal’s exoskeleton.
It is made of chitin, a type of carbohydrate. Chitin
varies from flexible to hard in different arthropod
species. The exoskeleton functions not only as a
protective cover for arthropods but also as a place of
attachment for their muscles. The arthropods are so
diverse in appearance that scientists differ on whether to
divide the group into several classes, subphyla, or
completely separate phyla. For our purposes, we refer to
the major subdivisions as classes, the most important in
marine habitats being the class Crustacea (meaning they
have a “crust” or “shell”).
Crustaceans such as lobsters, crabs, and shrimps have
five pairs of legs located under their carapace.
Therefore, they are referred to as the decapods
(meaning “ten legs”).
The claws, which are used in food-getting, are the first
pair of legs; the four other pairs are the walking legs.
The Lobster
Two common lobster species are the northern lobster
(Homarus americanus) and the spiny lobster (Panulirus
argus). The northern lobster, has two large claws, which
are absent in the spiny lobster.
The Crab
There are many species of crabs alive in the world today. Look at the three crabs
shown in the next Figure. They look very different from one another. What do you
think is the reason for this? Crabs live everywhere—on land and in the sea, and from
surface waters down to the great ocean depths. The great diversity that exists among
them is due to the fact that crabs have successfully adapted to these many different
habitats.
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THE SHRIMP AND OTHERS CRUSTACEANS
Crustaceans range in size from nearly microscopic to absolutely huge. You are more
aware of the larger ones, such as the lobster and crab, because they are popular
seafood items.
The Shrimp
The shrimp looks somewhat like a small version of the lobster. The pink Gulf shrimp
(Penaeus duorarum), which grows up to 17 cm long, is caught for the seafood
industry by fishing trawlers.
Copepods and Krill
The most abundant crustacean in the ocean is actually the tiny copepod (Calanus),
there are more than 1000 species of copepods in the sea. The copepod, which is less
than half a centimeter long, eats mainly diatoms.
A cold-water relative of the copepod is a shrimplike animal called krill (Euphausia).
(Although it resembles a shrimp, the krill has more than 10 legs, so it is not classified
with the decapods.) Krill grow to about 5 cm in length. Like the copepod, the krill is a
planktonic animal that eats diatoms and floats in large masses near the ocean’s
surface. Krill are the principal food source for the filter-feeding (baleen) whales and,
along with the copepod, provide food for countless fish, birds, and seals.
Amphipods and Isopods
Whereas copepods and krill float in the open ocean, Amphipods and Isopods are other
shrimplike crustaceans are found near the edge of the sea, living in or on a substrate.
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The Barnacle
An unusual crustacean that is often mistaken for a mollusk is the barnacle. The acorn,
or rock, barnacle (Balanus) lives attached to rocks and other hard surfaces. Its
overlapping, sharp calcium carbonate plates, which resemble a mollusk’s shell,
protect the animal inside.
Barnacles attach to almost any substrate, from a ship’s hull to a whale’s skin. Like the
yellow boring sponge, the barnacle is a type of encrusting organism. (See the next
Figure.) Ships must be dry-docked periodically to be scraped clean of barnacles,
because they add weight, increase friction, and thus hinder a ship’s ability to move
smoothly and quickly. In effect, a barnacle encrusted ship uses more fuel to move
through the water.
Another barnacles is the gooseneck, or goose, barnacles (Pollicipes) for the long
stalks by which they are attached to the substrate, they are able to bend with the
currents to capture large plankton that drift by.
Horseshoe crab
Two unusual groups of arthropods found in or near the sea are the horseshoe crab and
the marine insects. The horseshoe crab is not a true crab; it lacks antennae and
mouthparts, and it has six pairs of legs, or appendages. (A true crab has five pairs of
legs.) The horseshoe crab is actually more closely related to spiders and scorpions
than to crustaceans. As such, it is placed in it own class, Merostomata.
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Arthropods also include the marine insects. Unlike the crustaceans, insects have only
three pairs of legs, and a body made up of three segments (head, thorax, and
abdomen). Insects also differ from some other arthropods in that they have just one
set of antennae and one pair of eyes. They are placed in their own class, Insecta,
which comprises nearly a million known species. The most familiar of marine insects
is the marsh mosquito.
Echinoderms
STARS IN THE SEA / ADAPTATIONS IN THE SEA STAR / SEA URCHINS
AND SAND DOLLARS / ECCENTRIC ECHINODERMS
STARS IN THE SEA
Sea stars or starfish are bottom-dwelling invertebrates. They are not fish at all; they
have neither scales nor a backbone. In fact, starfish, or sea stars, are types of
echinoderms—spiny-skinned animals that lack body segmentation but have radial
symmetry (usually five-part) and an internal skeleton. In radial symmetry, all similar
body parts are regularly arranged around the central point of an animal’s body.
There are more than 5000 species of echinoderms, which are placed in five main
classes: sea stars; sea urchins and sand dollars; brittle stars; sea lilies and feather stars;
and sea cucumbers.
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MARINE VERTEBRATES
PROTOCHORDATES AND JAWLESS FISHES / CARTILAGINOUS FISHES /
BONY FISHES
Protochordates (meaning “first chordates”)
During some stage of their development they possess a hollow dorsal nerve cord, a
notochord, and pharyngeal gill slits. (The notochord is a flexible, rodlike structure that
supports the spinal cord.) The presence of these structures in these animals means that
they are chordates, related to such animals as fish, and classified within the phylum
Chordata. However, unlike fish, they lack certain advanced structures and so are
referred to as primitive (invertebrate) chordates, or protochordates (meaning “first
chordates”), a subgroup of the chordate phylum.
The largest group of protochordates is the tunicates (subphylum Urochordata). All
3,000 species are marine. Sea squirts or ascidian (class Ascidiacea) are referred to as
tunicates because they are covered by a clear, tough membrane that resembles a
tunic. They are the only sessile, or attached, chordates. Another primitive chordate is
the acorn worm (Saccoglossus). Superficially, this animal looks like any other
invertebrate, since it resembles a worm.
acorn worm
tunicate
A third primitive chordate is the lancelet (Amphioxus).
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The higher chordate subgroup includes all the vertebrates: fish, amphibians, reptiles,
birds, and mammals. In vertebrate embryos, the notochord develops into a backbone.
JAWLESS FISHES
Most fish possess movable mouthparts
called jaws, but a few species of fish do
not. There are only two types of jawless
fishes alive today—the sea lamprey and
the hagfish. These fishes, which are placed
in the class Agnatha (meaning “without
jaws”), live as parasites. Some jawless
fishes can grow up to one meter in length.
The sea lamprey (Petromyzon marinus),
uses the sucking disk on its mouth to
attach to living host fish.
Unlike most other fish, the lamprey does not have any scales. The Pacific hagfish
(Eptatretus stouti) and the Atlantic hagfish (Myxine glutinosa) actually use the sharp
teeth in their round mouths to burrow into the bodies of dead or dying fish.
CARTILAGINOUS FISHES
Fish whose entire skeleton is composed of cartilage are called cartilaginous fishes.
They are placed in the class Chondrichthyes (meaning “cartilage fishes”). There are
about 800 species of cartilaginous fishes, as compared with more than 25,000 species
of bony fishes. They include sharks and dogfishes (about 330 species) and flattened
skates and rays (about 450 species).
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BONY FISHES
More than 95 percent of all fish on Earth belong to the class Osteichthyes (meaning
“bony fishes”). As the name implies, the bony fishes have a skeleton that is made up
of bone. They are called teleosts (from the Greek teleios for “perfect” and osteon for
“bone”). Bony fishes are found in every type of aquatic environment, from lakes and
rivers to tropical reefs and polar oceans.
Bony fish have a protective covering of scales, which are loosely attached to the skin.
Bony fish feel slimy to the touch because their skin, which is living, secretes a
protective mucus coating over their scales. The mucus serves two functions: it acts as
a barrier against infection and it reduces friction so the fish can move easily through
the water.
Strange Shapes of Fish
One of the most curious looking fish in the ocean is the sea horse (Hippocampus).
sea horse
The anglerfish, typical of various deep-sea fishes, has
special adaptations for feeding in the depths, such as a
large mouth, huge spiky teeth, and a light-producing
lure to attract prey.
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The huge mola, or ocean sunfish, the biggest bony fish
in the ocean, attaining a body length of 3 meters and a
weight of up to 2275 kg
The coelacanth (Latimeria chalumnae) grows to nearly
2 meters long
Major differences between cartilaginous and bony fishes
Marine Reptiles
Of about 8,000 living species of reptiles, only about 80 live in seawater or brackish
water (diluted seawater). Marine reptiles include sea snakes, sea turtles, two species
of crocodile, and a lizard. Marine reptiles are found mainly in tropical and subtropical
habitats.
snake
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MARINE BIRDS
Marine birds have a much wider range than reptiles; they are found from polar seas to
tropical shores. Birds that depend on the ocean for their survival are commonly
referred to as seabirds. There are nearly 9000 species of birds; all are in the class
Aves. Fewer than 350 (or 4 percent) are seabirds. Not all birds fly, but they all do
share several important characteristics.
Seabirds have adaptations for marine life, such as webbed feet for swimming or
paddling and salt glands that empty into the nostrils to expel excess salt. Seabirds
spend part of their time feeding in, on, or above the sea, but all species nest on land.
Three examples of common shorebirds
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pelican
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Marine Mammals
CETACEANS: WHALES AND DOLPHINS / SEALS AND OTHER MARINE
MAMMALS
CETACEANS: WHALES AND DOLPHINS
Whales and dolphins belong to the order of mammals called Cetacea. There are about
80 different species of cetaceans. The largest cetaceans are the whales and the
smallest cetaceans are the dolphins and porpoises, which are actually small whales
that range from 2 to 4 meters in length.
Baleen Whales
The whales are classified into two main groups—the baleen whales and the toothed
whales. The baleen whales, which belong to suborder Mysticeti, are filter feeders that
eat plankton and small fish. They include such species as the blue, finback,
humpback, right, and gray whales.
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Toothed Whales
The toothed whales, which belong to suborder Odontoceti, include all other whales
(such as the sperm, killer, pilot, and beluga), dolphins, and porpoises. These whales
have peg-like teeth on their jaws, with which they catch prey such as fish, seals,
penguins, and squid.
Seals and Sea Lions
To move efficiently through water, another group of marine mammals has paddlelike
appendages, or flippers, and a torpedo-shaped body. These fin-footed carnivorous
marine mammals are classified in the suborder Pinnipedia (meaning “wing-foot” or
“feather-foot”), which includes the seals, sea lions (and fur seals), and walruses.
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The Sea Otter
Sea otters are closely related to the land-dwelling weasels and minks. They are the
smallest of the marine mammals, growing to only about a meter in length.
Manatees and Dugongs
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