17'2 Fishes

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17.2 Fishes
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Objectives

• Describe the main characteristics and adaptations
  of fishes.
• Distinguish between the three classes of fishes.
• Compare and contrast the feeding methods of
  jawless fishes and cartilaginous fishes.

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Characteristics of Fishes

• Like all vertebrates, fishes have a backbone that
  replaces the notochord during the embryo stage.
• Fishes also have gills.
• Gills are organs that take up dissolved oxygen
  from water.
• The gills are located on either side of a fish's
  head.

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• As a fish swims, water passes over its gills.
• Oxygen in the water diffuses into cells at the
  surface of the gills.
• At the same time, carbon dioxide diffuses out of
  blood vessels and into the passing water.
• Recall that all chordates have gill slits at some
  stage of their development.
• Unlike most other vertebrates, in fishes gill
  slits become permanent gills.

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• The 31,000 different species of fishes are
  grouped into three classes.
• Fishes in all three classes share the following
  characteristics
• Most fishes are ectotherms.
• Most fishes have streamlined bodies and use fins
  for locomotion.
• Most fishes have a very good sense of smell.
• All fishes have highly developed nervous systems.

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Jawless Fishes

• The first fishes to appear on the earth where
  covered by thick, bony plates.
• Like the fishes of today, they had backbones and
  were considered vertebrates.
• However, these early fishes lacked jaws.
• Scientists infer that like their invertebrate
  ancestors, they probably were filter feeders.

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• A few species of fishes without jaws still exist
  today.
• They are the lampreys and the hagfish.
• These jawless fishes do not have the bony plates
  of the first fishes, but they are placed in the
  same class.
• Hagfish seem to have changed little since they
• first evolved.

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• Unlike their more ancient relatives, lampreys and
  hagfish are not filter feeders.
• They are parasites and scavengers respectively.
• Lampreys use suckers to attach to their hosts.
• Once attached, they rip into their host's flesh
  using rasplike tongues.
• They then feed upon the blood and body fluids of
• the host.

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Cartilaginous Fishes

• Gently move the outer part of your ear back and
  forth with your fingers.
• Your ear holds its shape but is flexible because
• of a type of tissue called cartilage (KART uhl
  ihj).
• Cartilage is a firm but flexible connective
  tissue that occurs at most joints of your
  skeleton.

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• Members of the second class of fishes have
  skeletons made up entirely of cartilage.
• They are called cartilaginous (KART uhl AJ uh
  nuhs) fishes.
• This group of fishes includes sharks, rays, and
  skates.

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• Unlike the jawless fishes, cartilaginous fishes
  have jaws.
• The jaws evolved from the first pair of skeletal
  rods that supported the gills of jawless fishes.

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• Most cartilaginous sharks are carnivores.
• Their sharp senses and sharp teeth make them well
  adapted for life as predators.
• Some of the largest sharks, however, are filter
  feeders.
• The whale shark strains water to remove plankton.

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• Cartilaginous fishes are more dense than ocean
  water, so they must keep swimming to stay above
  the sea floor.
• In some, swimming is also what makes a current of
  water pass over the gills, supplying the fish
  with oxygen.

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Bony Fishes

• The greatest number of fish species are bony
  fishes.
• The bony fishes first appeared around the same
  time as the cartilaginous fishes, more than 350
  million years ago.

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• As their name implies, bony fishes have skeletons
  made of bone.
• Other adaptations also help them live in many
  kinds of water habitats.

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• Buoyancy Control - Bony fishes have an organ
  called a swim bladder.
• This organ helps them conserve energy as they
  swim through the water.
• The swim bladder is similar to a balloon.

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• Fishes use their swim bladders to adjust their
  depth in the water.
• Inflating the swim bladder with just the right
  amount of air keeps the fish from sinking to the
  bottom of the ocean or floating to the surface.
• The swim bladder, therefore, allows the fish to
  maintain a buoyancy that is just right in water.

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• Buoyancy is the force of fluid pushing an object
  up.
• Buoyancy acts against the weight of an object.
• It makes an object seem like it weighs less in a
  fluid.
• If the weight of an object under water is greater
  than the
• bouyant force, the object will sink.

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• If the weight is less than the buoyant force, the
  object will rise to the surface and float.
• If you have been swimming, you may know what it
  is like to float.
• If the weight is equal to the buoyant force, the
  underwater object will stay at any level.
• This is what happens to a fish.

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• Scales and Fins - The outside of a bony fish is
  covered
• with a layer of smooth scales.
• The scales overlap each other.
• The scales also are covered with a thin layer of
  mucus.

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• The mucus and overlapping of the scales are
  adaptations that help the fish move through the
  water with little resistance.
• Also notice the different kinds of fins on the
  bony fish.
• The fins are adapted for swimming and guiding the
  fish.

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• Gills and Lungs Unlike the cartilaginous
  fishes, boney fishes do not have to swim to move
  water over their gills.
• They can pump water through the mouth and into
  the gill chamber.
• The water is pumped by movements of a special
  flap of tissue in the gill chamber

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• Most bony fishes depend only upon gills for their
  gas
• exchange.
• However, some bony fishes have both gills and
  lungs.
• One of these fishes, called the African
  lungfish, uses its lungs to help meet its oxygen
  needs.
• When it needs to take in more oxygen, the
  lungfish swims to the surface of the water and
  "gulps" fresh air.

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End