Phylum Chordata

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Phylum Chordata

• The chordates are a group of particular interest
  to us as we belong to it, being members of the
  subphylum Vertebrata.
• The chordates include all of the vertebrates
  (fish, amphibians, reptiles, mammals and birds),
  but also two non-vertebrate subphyla the
  Urochordata and the Cephalohordata.

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Phylum Chordata

• The chordates were in the 19th century considered
  to have been derived from protostome ancestors
  (the annelid, mollusc, arthropod group).
• However, a better understanding of embryology
  shows that chordates are deuterostomes and the
  invertebrates they are most closely related to
  are Echinodermata and the Hemichordata.

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Characteristics of the Chordata

• Chordates are
• bilaterally symmetrical
• triploblastic
• have a well developed coelom
• have a complete digestive system

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Five distinctive characteristics of the chordates

• Five distinctive characteristics separate the
  chordates from all other phyla
• Notochord
• Single, dorsal, tubular nerve cord
• Pharyngeal pouches or slits
• Endostyle
• Postanal tail
• Not all of these characteristics are apparent in
  adult organisms and may appear only in the
  embryonic or larval stages.

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Notochord

• Notochord the notochord is a flexible, rodlike
  structure. It extends the length of the body and
  is an anchor point for muscles.
• The notochord bends without shortening so it
  permits the animal to undulate.

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Notochord

• In nonvertetbrates and the jawless vertebrates
  the notochord is present throughout life.
• However, in the jawed vertebrates it is replaced
  by the vertebral column the remnants of the
  notochord being found in the intervertebral
  disks.

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Single, dorsal, tubular nerve cord

• In most invertebrates the nerve cord, if present,
  is ventral to the gut.
• In chordates, in contrast, the nerve cord is
  dorsal to the gut and notochord. The nerve cord
  passes through the neural arches of the
  vertebrae, which protect it.
• The nerve cord is enlarged in vertebrates into a
  brain, which is surrounded by a bony or
  cartilaginous cranium.

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Pharyngeal pouches and slits

• Pharyngeal slits occur in aquatic chordates and
  lead from the pharyngeal cavity to the outside.
• The pharyngeal slits are used as a filter feeding
  device in protochordates (i.e., Urochordata
  (Tunicates)) and Cephalochordata (lancelets e.g.
  Amphioxus).
• Water containing food is drawn in through the
  mouth by cilia and exits via the pharyngeal slits
  where the particles are trapped in mucus.

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Amphioxus
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Pharyngeal pouches and slits

• In vertebrates the pharyngeal arches have been
  modified into gills by the addition of a rich
  blood supply and thin gas permeable walls.
• The contraction of muscles in the pharynx drive
  water through the gills.

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Pharyngeal pouches and slits

• In amniotes an opening may not form and rather
  than slits only grooves called pharyngeal pouches
  develop.
• In tetrapods these pouches give rise during
  development to a variety of structures including
  the middle ear cavity, eustachian tube, and
  tonsils.

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Endostyle or thyroid gland

• The endostyle is found in protochordates and in
  lamprey larvae. It is located on the floor of
  the pharynx and secretes mucus, which is used to
  trap particles.
• The endostyle works with the pharyngeal slits in
  filter feeding.

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Endostyle or thyroid gland

• Some cells in the endostyle secrete iodinated
  proteins and are homologous with
  iodinated-hormone secreting thyroid gland, which
  is found in adult lampreys and vertebrates.

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Postanal tail

• The postanal tail, some musculataure and the
  notochord enable larval tunicates and amphioxus
  to swim.
• The postanal tail evolved to allow organisms to
  swim and its efficiency has been enhanced by the
  addition of fins. The postanal tail is present
  only in vestigial form in humans (the coccyx)
  although tails as a whole are widespread amoing
  vertebrates.

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Amphioxus
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Classification of the Chordata

• There are three subphyla in the Chordata
• Subphylum Urochordata tunicates
• Subphylum Cephalochordata lancelets
• Subphylum Vertebrata fish, amphibians, reptiles,
  birds, mammals, etc.

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Subphylum Urochordata

• The Urochordata (tunicates named for the tough
  tunic that surrounds the adult) look like most
  unpromising candidates to be chordates and
  relatives of the vertebrates.
• The largest group, the ascidians or sea squirts
  (Class Ascidiacea) as adults are marine, sessile,
  filter feeding organisms that live either
  solitarily or in colonies.

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Ciona intestinalis (a solitary sea squirt)
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Synoicum pulmonaria  a colonial sea squirt
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Ascidians

• Adult ascidians lack a notochord and there is
  only a single ganglion in place of the dorsal
  nerve cord.
• Of the five characteristics of chordates adults
  possess only two pharyngeal gill slits and an
  endostyle, both of which they use in filter
  feeding.

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Ascidians

• The adult sea squirt draws water in through an
  incurrent siphon and pushes it back out an
  excurrent one.
• Food particles are filtered out in the pharyngeal
  slits with mucus from the endostyle used to trap
  particles.

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Larval Ascidian

• Even though the adult ascidian hardly resembles a
  chordate its larva does.
• Larval ascidians are very small and tadpole-like
  and possess all five chordate characteristics
  previously outlined.

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Young larval ascidian
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Larval Ascidian

• The larval ascidians role is to disperse and to
  achieve this it is free swimming. However, it
  has only a short larval life (minutes to a couple
  of days) and does not feed during this time.
• Instead it searches for a place to settle and
  then attaches and metamorphoses into an adult.

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Ascidian metamorphosis

• During metamorphosis the notochord disappears,
  the nerve cord is reduced to a single nerve
  ganglion and a couple of nerves.

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Other Urochordate classes

• Besides the ascidians there are two other classes
  of the Urochordata the Larvacea and Thaliacea.
• Both are small, transparent planktonic forms.
  Thaliaceans are cylindrical or spindle shaped
  whereas larvaceans are tadpolelike and resemble
  an ascidian larva.

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Garstangs hypothesis of chordate larval evolution

• In the 1920s it was proposed that the
  vertebrates were derived from an ancestral
  ascidian that retained its characteristics into
  adulthood (the process by which juvenile
  characteristics are retained into adulthood is
  referred to as paedomorphosis).

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Garstangs hypothesis of chordate larval evolution

• Garstangs hypothesis is supported by
  embryological evidence, but more recently
  molecular analyses have suggested that sessile
  ascidians are a derived form and that the free
  living larvaceans are more likely to be the
  closest relatives.

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Subphylum Cephalochordata

• The cephalochordates are the lancelets, which are
  small (3-7 cm long) laterally compressed fishlike
  animals that inhabit sandy sediments of coastal
  waters. They lack a distinct head and have no
  cranium.
• They are commonly referred to as Amphioxus as
  this was the original genus name. There are 29
  species, five of which occur in North American
  coastal waters.

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Amphioxus

• Amphioxus is a filter feeder.
• Water enters the mouth and then passes through
  the pharyngeal slits, where food is trapped in
  mucus. Cilia then move the food to the gut.

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Amphioxus
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Amphioxus

• Amphioxus is interesting because it displays the
  basic chordate characteristics in a simple and
  obvious form because of its transparency.
• Amphioxus is considered to be the closest living
  relative of the vertebrates because it shares
  several characteristics with vertebrates that
  Urochordates do not possess.

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Amphioxus characteristics shared with vertebrates

• Characteristics amphioxus shares with vertebrates
  include
• Segmented myomeres (muscle blocks)
• Dorsal and ventral aortas
• Branchial (gill) arches (blood vessels running
  over the gills).

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Amphioxus characteristics not shared with
vertebrates

• Amphioxus however lacks several characteristics
  that biologists think the ancestor of vertebrates
  possessed. These include
• Tripartite brain (with forebrain, midbrain and
  hindbrain) protected by a cranium
• Chambered heart
• Muscular gut and pharynx
• List continues on next slide

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Amphioxus characteristics not shared with
vertebrates

• Various special sensory organs (eyes, chemical
  and pressure receptors)
• Neural crest (ectodermal cells that are found on
  the embryonic neural tube and are engaged in the
  formation of the cranium, tooth dentine, some
  endocrine glands and Schwann cells, provide
  myelin insulation to nerve cells).

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Subphylum Vertebrata

• The vertebrates are a large and diverse group
  including the fishes and tetrapods.
• Vertebrates possess the basic chordate
  characteristics, but also a number of novel
  homologous structures.
• An alternative name for the group Craniata is
  actually a better descriptor for the entire group
  because all members possess a cranium, but some
  jawless fishes lack vertebrae.

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Important developments of the Vertebrates

• Musculoskeletal system. Vertebrates possess an
  endoskeleton, which is much more economical in
  materials than the exoskeleton of invertebrates.
• It forms a jointed scaffolding for the attachment
  of muscles. Initially the endoskeleton probably
  was cartilaginous (it still is in jawless fishes
  and sharks) and later became bony in many groups.

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Important developments of the Vertebrates

• Bone is stronger than cartilage, which makes it a
  better material to use for muscle attachment in
  places where mechanical stress may be high.
• Bone may have evolved initially as a means of
  storing minerals and was later adapted for use in
  the skeleton.

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Important developments of the Vertebrates

• Various aspects of vertebrate physiology have
  been upgraded also to meet increased metabolic
  demands.
• For example the pharynx, which was used for
  filter feeding in primitive chordates has had
  muscles added that make it a powerful water
  pumping organ.
• With the conversion of the pharyngeal slits to
  highly vascularized gills the pharynx has become
  specialized for gas exchange.

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Important developments of the Vertebrates

• The ancestors of vertebrates switched from filter
  feeding to more active feeding, which required
  movement and the ability to sense the environment
  in detail.
• With these changes came the need for a control
  center to process information. The anterior end
  of the nerve cord consequently became enlarged
  into a brain.

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Important developments of the Vertebrates

• The vertebrate brain in fact developed into a
  tripartite brain (with a forebrain, midbrain, and
  hindbrain) that was enclosed within a protective
  cranium of bone or cartilage.

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Important developments of the Vertebrates

• Sense organs have also become highly developed
  among the vertebrates.
• These include complex eyes, pressure receptors,
  taste and smell receptors, lateral line receptors
  for detecting water vibrations, and
  electroreceptors that detect electrical currents.

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Important developments of the Vertebrates

• The development of the head in vertebrates with
  its array of sense organs appears to have been
  driven by the evolution of new embryonic tissues
  that give rise to cells that play an important
  role in the formation of sensory structures.

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Important developments of the Vertebrates

• A factor that may have played a major role in the
  evolution of the vertebrates is the duplication
  of Hox genes.
• Hox genes play a major role in embryonic
  development and vertebrates have four copies,
  whereas invertebrates and amphioxus have only
  one.

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Important developments of the Vertebrates

• The duplication of the Hox genes appears to have
  occurred around the time vertebrates originated
  and it may be that this gene duplication freed up
  copies of these genes, which control development,
  to generate more complex animals.

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Early vertebrate ancestors

• Fossils of early chordates are scarce, but a few
  are known including Pikaia from the Burgess Shale
  (approx 580 mya) that appears to be an early
  cephalochordate and has a notochord and segmented
  muscles.

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15.8
Pikaia
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Early vertebrate ancestors

• Another fossil from China Haikouella lanceolata
  about 525mya.
• This fossil has a notochord, pharynx, and a
  dorsal nerve cord which are chordate characters,
  but also pharyngeal muscles, eyes, a head, gills
  and a brain which are vertebrate traits.

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Haikouella lanceolata
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Haikouella lanceolata
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Jawless early vertebrates

• A wide variety of armored jawless fishes called
  ostracoderms are known from the Ordovician
  (approximately 490-440 mya) up to near the end of
  the Devonian period (about 360 mya).
• These fish in many cases lack paired fins and so
  probably were not precision swimmers.

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Ostracoderms
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Jawless early vertebrates

• The ostracoderms were heavily armored and jawless
  with narrow, fixed mouths. They appear to have
  been mainly filter feeders that used their
  pharyngeal muscles to pump water.
• Ultimately, the ostracoderms were outcompeted by
  fish that possessed the next big evolutionary
  development jaws.

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Early jawed vertebrates

• The origin of jaws was a hugely significant event
  in the evolution of the vertebrates and the
  success of the Gnathostomes the jawed
  vertebrates, jaw mouth is obvious.
• The first jawed vertebrates were the placoderms
  haevily armored fish which arose in the early
  Devonian (about 400mya) and possessed not only
  jaws, but paired pelvic and pectoral fins that
  gave them much better control while swimming.

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15.13
Early jawed fishes of the Devonian (400 mya).
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Jaws

• Jaws arose by modification of the first
  cartilaginous gill arches, which aid in gill
  support and ventilation.
• It is believed that selection favored enlargement
  of these gill arches and the development of new
  muscles that enabled them to be moved and so pump
  water more efficiently.
• Once enlarged and equipped with muscles it would
  have been quite easy for the arches to have been
  modified into jaws.

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15.12
Note resemeblance between upper jaw
(palatoquadrate cartilage) and lower
jaw (Meckels cartilage) and gill supports
immediately behind in this Carboniferous shark
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Living fishes

• The living fishes (not a monophyletic group)
  include
• the jawless fishes (e.g. lampeys),
• cartilaginous fishes (e.g. sharks and rays),
• bony, ray-finned fishes (most of the bony fishes
  such as trout, perch, pike, carp, etc) and
• the bony, lobe-finned fishes (e.g. lungfishes,
  coelacanth).

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Living jawless fishes

• There are a little more than 100 species of
  living jawless fishes or Agnathans (the term
  agnathan does not represent a monophyletic
  group).
• These belong to two classes the Myxini
  (hagfishes) and the Cephalaspidomorphi (lampreys).

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Characteristics of agnathans

• Lack jaws (duh!)
• Keratinized plates and teeth used for rasping
• Vertebrae absent or reduced
• Notochord present
• Dorsal nerve cord and brain
• Sense organs include taste, smell, hearing,
  vision.

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Hagfishes class Myxini

• Hagfishes are a marine group of primarily
  scavengers.
• They use their keen sense of smell to find dead
  or dying fish and invertebrates and rasp off
  flesh using their toothed tongue.
• As they lack jaws, they gain leverage by knotting
  themselves and bracing themselves against
  whatever theyre pulling.

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Hagfishes

• Hagfishes are unusual in that they have body
  fluids, which are in osmotic equilibrium with the
  surrounding sea. This is unknown in other
  vertebrates, but common in invertebrates.
• They are also unusual in having a low pressure
  circulatory system that has three accessory
  hearts in addition to a main heart.

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Hagfishes

• Hagfishes have a remarkable (and revolting)
  ability to generate enormous quantities of slime,
  which they do to defend themselves from
  predators.
• A single individual can fill a bucket with slime.

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Lampreys Class Cephalaspidomorphi

• Lampreys occur in both marine and fresh waters
  and about half of all species are ectoparasites
  of fish (the others are non-feeding as adults and
  live only a few months).
• Lampreys spawn in streams and the larvae
  (ammocoetes) live and grow as filter feeders in
  the stream for 3-7 years before maturing into an
  adult. Feeding adults live a year or so before
  spawning and dying.

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Lampreys

• Parasitic lampreys have a sucker-like mouth with
  which they attach to fish and rasp away at them
  with their keratinized teeth.
• The lamprey produces an anticoagulant as it feeds
  to maintain blood flow. When it is full the
  lamprey detaches, but the open wound on the fish
  may kill it. At best the wound is unsightly and
  largely destroys the fishs commercial value.

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Sea lamprey close up of sucker and teeth
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16.4
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Introduced sea lampreys

• Landlocked sea lampreys made their way into the
  Great Lakes around 1918 and caused the complete
  collapse of the lake trout fishery by the 1950s.
• Lamprey numbers fell as their prey base collapsed
  and control efforts were introduced. Trout
  numbers have since recovered somewhat, but
  wounding rates are still high.

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Sea lampreys in Lake Champlain

• Lake Champlain also has large populations of sea
  lampreys which spawn in the creeks that empty
  into the lake.
• Until recently, lampreys were believed to have
  been introduced into Lake Champlain, but genetic
  analyses indicate the population was established
  perhaps as much as 11,500 years ago by lampreys
  that migrated up the St. Lawrence.

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Sea lampreys in Lake Champlain

• As is the case elsewhere there has been a
  campaign to control lamprey numbers primarily by
  using lampricides in steams.
• Controls do reduce lamprey wounding rates and
  after control rates have fallen from 60-70 wounds
  per 100 fish examined to as low as 30
  wounds/fish.

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Class Chondrichthyes cartilaginous fishes

• The class Chondrichthyes has two subclasses
• Elasmobranchii, which includes the sharks and
  rays.
• Holocephali the chimaeras ratfish and
  ghostfish.

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Two species of ray
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Hammerhead Shark
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Class Chondrichthyes

• The Chondrichthyes are an ancient group that
  although not as diverse as the bony fishes have
  persisted largely unchanged for hundreds of
  millions of years.
• There are about 850 living species, all of which
  have cartilaginous skeletons, even though they
  are descended from ancestors that had bone.

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Class Chondrichthyes

• The Chondrichthyes well-developed jaws, highly
  developed sense organs, powerful swimming ability
  and streamlined shape have enabled them to thrive
  as marine predators for more than 350 million
  years, as other groups have come and gone.

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Great White Shark
Hammerhead sharks
Whale shark
Two skates
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Diversity of sharks
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Sharks

• Sharks represent a little less than half of the
  elasmobranchs and most are specialized predators.
• The largest species is the whale shark, which is
  a plankton feeder, but most of the others are
  predators of fish, marine mammals, crustaceans
  and whatever else they can catch.

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Sharks

• Sharks are very well streamlined, but are heavier
  than water (because they lack a swim bladder) and
  sink if not swimming forward.
• Sharks increase their buoyancy by having a large
  oil-filled liver that reduces their density, but
  not enough to prevent them from sinking.

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Large liver of a great white shark
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Sharks

• Sharks have an asymmetrical heterocercal tail and
  the vertebral column extends into the dorsal
  lobe.
• The tail provides both lift and thrust, while the
  large flat pectoral fins also provide lift to
  keep the head up.

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Sharks

• Sharks have skin covered in dermal placoid
  scales, which are small tooth-like structures
  (with enamel, dentine and pulp just like real
  teeth).
• These scales give sharkskin a tough, leathery and
  abrasive feel.
• The scales are modified in the mouth to produce
  the rows of replaceable teeth characteristic of
  sharks.

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Sand tiger shark (note multiple rows of teeth)
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Sharks

• Sharks use a variety of senses to track detect
  prey. They have highly developed olfactory
  senses and can detect minute quantities of blood
  in the water.
• They are also able to detect vibrations in the
  water using a lateral line system.

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Lateral line system

• The lateral line system consists of a series of
  fluid-filled canals that open to the outside.
• Inside in the canals are sensory cells called
  neuromasts that are very sensitive to vibrations
  in the water

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Organs of Lorenzini

• Sharks are also able to detect the faint
  bioelectric fields that surround all animals.
  This allows them to locate prey buried in sand or
  sense prey at night.
• The bioelectric detectors are called ampullary
  organs of Lorenzini and are found in the sharks
  head.

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Reproduction

• Reproduction in all Chondrichthyes is internal
  and the male uses modified pelvic fins called
  claspers to insert sperm.
• The presence or absence of claspers makes it easy
  to distinguish male from females.

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Great white shark claspers
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Reproduction

• All skates and some sharks are oviparous and lay
  eggs soon after fertilization.
• Other sharks are ovoviviparous and the eggs
  develop within the mothers body and hatch either
  in her or just after being released from her.

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Egg case of cat shark
Embryo of deep sea cat shark. There is a very
large yolk sac to support the embryos growth.
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Reproduction

• The remaining sharks are viviparous and the
  offspring are nourished by a placenta,
  unfertilized eggs or smaller siblings.

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Skates and rays

• More than half of all elasmobranchs are skates
  and rays.
• They have characteristically dorsoventrally
  flattened bodies and greatly enlarged pectoral
  fins, which they swim with using a wavelike
  motion.

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Blue spotted ray
Manta Ray
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Skates and rays

• The spiracles are much larger in rays than in
  sharks because water for the gills enters
  exclusively through them as the mouth is usually
  buried in the sand.

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Skates and rays

• Skates and rays are usually well camouflaged and
  sit on the bottom. A few species are dangerous
  because of their sharp and barbed tail
  (stingrays) or because they can generate severe
  electric shocks (electric rays).
• Their teeth are for crushing prey and they mainly
  feed on molluscs and crustaceans.

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Subclass Holocephali Chimaeras

• Chimaeras are a small group (about 35 species) of
  deep sea cartilaginous fishes known commonly as
  ratfish or ghostfish.
• They have a large head, plate-like grinding
  teeth, a cover over the gills and lack both a
  spiracle and stomach.
• The tail is thin and not much use in swimming.
  Instead chimaeras depend on flapping their
  pectoral fins for much of their movement.

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Male spotted ratfish
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Bony fishes Osteichthyes

• The term osteichthyes does not describe a
  monophyletic group, but is a term of convenience
  to describe the fishes whose skeletons are made
  of bone that replaces cartilage during embryonic
  development.
• There are two classes the Actinopterygii (the
  ray-finned fishes) and the Sarcopterygii (the
  lobe-finned fishes)

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General characteristics of bony fish

• Skeleton made of bone of endochondral origin
  (derived from cartilage).
• Paired and median fins supported by dermal rays.
• Respiration mainly by gills. Gills covered with
  operculum.
• Swim bladder often present.
• Complex nervous, circulatory and excretory
  systems present

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Class Actinopterygii (ray-finned fishes)

• This is by far the larger of the two living
  classes of fishes with more than 27,000 species.
• Ancestral ray finned fishes in the Devonian were
  small and heavily armored with ganoid scales
  (thick, bony non-overlapping, relatively
  inflexible scales) and heterocercal tails (shaped
  like that of modern sharks).

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Chondrosteans

• A few relic species (the chondrosteans) still
  possess such characteristics.
• These include sturgeon, paddlefish and the
  African bichir.

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Teleosts

• The vast majority of modern fishes are teleosts.
• They have replaced the heavy armored scales of
  their ancestors with much lighter more flexible
  scales that overlap each other and also have
  evolved homocercal symmetrical tails.

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Swim bladder

• Teleosts also have evolved extremely fine control
  over their buoyancy and can remain neutrally
  buoyant, which provides large energy savings.
• Most pelagic teleosts have a swim bladder, which
  evolved from paired lungs of Devonian fishes.
• Gas can be secreted into or removed from the swim
  bladder so that the fish remains at neutral
  buoyancy.

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Swim bladder

• Some fishes (e.g. trout) can gulp or release air
  by opening a pneumatic duct that connects to the
  esophagus.
• More advanced teleosts have discarded the
  pneumatic duct and instead secrete gas into the
  swim bladder using a gas gland or absorb it
  through a highly vascularized part of the swim
  bladder called the ovale.

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Gas gland

• When arterial blood arrives at the swim bladder
  lactic acid is released by the gas gland, which
  causes oxygen to be released by hemoglobin.
• This raises the partial pressure of oxygen in the
  blood above that in the swim bladder and so the
  oxygen flows into the swim bladder.

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Rete mirabile

• In deep sea fish a very high gas pressure must be
  maintained to resist the pressure of the water.
• For example, at 2000 meters gas at a pressure of
  200 atmospheres (more than the oxygen pressure in
  fully charged steel cylinder) must be maintained
  in the swim bladder even though the oxygen
  pressure in the fishs blood is only 0.2
  atmospheres (oxygen pressure at sea level).

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Rete mirabile

• Why doesnt the oxygen in the swim bladder flow
  out into the blood?
• Because of a structure called a rete mirabile
  (miraculous net), which stops this loss.
• The swim bladder is supplied with blood via an
  artery. Before the artery reaches the swim
  bladder it divides into an enormous number of
  thin, parallel capillaries that run parallel to
  but whose contents flow in the opposite direction
  to a similar array of venous capillaries.

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Rete mirabile (below)
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Rete mirabile

• Let us assume the swim bladder contains gas at
  100 atmospheres. Venous blood leaving the swim
  bladder thus contains oxygen at that pressure.
• As the venous capillary leaves the swim bladder
  it runs parallel to incoming arterial blood which
  contains blood with a slightly lower partial
  pressure of oxygen.

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Rete mirabile

• Oxygen thus flows from the venous capillary to
  the arterial capillary.
• Along its entire length from the swim bladder the
  gas pressure in the venous capillary is falling
  as it gets further from the swim bladder, but the
  pressure is always higher than that in the
  parallel arterial capillary so gas always flows
  from the venous capillary to the arterial
  capillary.
• Thus the rete acts as a trap that keeps gas in
  the swimbladder.

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Respiration

• Fish obtain oxygen using gills, which consist of
  filaments covered with a thin epidermal membrane
  that is repeatedly pleated into thin, flat sheets
  of tissue called lamellae.
• The gills are found within the pharyngeal cavity,
  which is covered with a flap called the operculum.

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Respiration

• The operculum protects the gills and also
  maintains the streamlining of the body.
• Water enters the mouth and is pumped across the
  gills by movements of the pharynx and exits under
  the operculum.

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Respiration

• The lamellae of the gills are richly supplied
  with blood, which flows in a countercurrent
  direction to the flow of water maximizing the
  amount of oxygen extracted.
• The gills are very efficient and can extract up
  to 85 of the dissolved oxygen in the water.

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Respiration

• Certain highly active fish such as mackerel with
  high metabolic rates cannot obtain enough oxygen
  by pumping water through their gills.
• Instead they must swim forward constantly in
  order to drive water through their mouth and over
  the gills, a process called ram ventilation

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Lobe-finned fishes Class Sarcoptrygii

• Today the sarcopterygians are a very small group
  that includes only six species of lungfishes and
  two species of coelacanths.
• However, all of the tetrapods (four-legged
  vertebrates) are descended from a group of
  sarcopterygian fishes known as the rhipidistians.

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Lungfishes

• There are six species of lungfishes one South
  American, one Australian and four African
  species.
• As their name suggests, these fish, as all
  sarcopterygians do, possess lungs and can breathe
  air.

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Lungfishes

• The Australian lungfish can gulp air and survive
  being in oxygen poor water, but cannot live out
  of water.
• In contrast, the South American and African
  species can survive out of water for long periods
  of time.
• The African species live in seasonal steams and
  ponds that dry out, but the lungfish survives by
  burrowing into the mud and forming a cocoon in
  which it survives until the water returns.

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The discovery of living coelacanths

• Coleacanths were believed to have been extinct
  for perhaps 50 million years when one was caught
  by a South African fishing boat in 1938.
• The curator of a small museum, M.
  Courtney-Latimer, recognized the fish was unusual
  and she brought it to the attention of the
  icthyologist J.L.B. Smith who after some delay in
  arriving identified the fish.

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The discovery of living coelacanths

• Unfortunately, the delay in arriving meant the
  fish had badly decomposed and many important
  structures had been lost.
• Smith named the fish (Latimeria) in honor of
  Courtney-Latimer and then embarked on a 14-year
  quest to find another coelacanth.
• But it wasnt until 1952 that a second was caught
  off the Comoro Islands, north of Madagascar,
  which is where the fish occur naturally (the 1938
  fish apparently had drifted far from its normal
  range).

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Images from the rediscovery of the Coelacanth
off the Comoros 1952.
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The discovery of living coelacanths

• In 1998 another population of Latimeria but a
  different species was discovered off Indonesia
  (10,000km east of the Comoros.
• Coelacanths are large fish about 5 feet long and
  when they swim they move their pelvic and
  pectoral fins in the same pattern that tetrapods
  walk.

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