Agnathan and Gnathostome fishes

Outline

• 1. Review
• Deuterostome, Chordate relationships
• Chordate characteristics
• Early chordate evolution
• 2. Origin of the Craniates
• 2 key features - neural crest cells, placodes
• Other changes
• 3. Relationships of the "fishes"
• 4. General aspects of life in water
• Respiration
• Locomotion

Outline

• 5. Basal relationships of the Craniates
• Class Myxini - hagfishes (agnathan/jawless)
• Vertebrata - synapomorphies
  • 2 types of skeleton
• "Ostracoderms" - extinct agnathan/jawless
• Class Cephalospidamorpha - lampreys (agnathan/jawless)
• Superclass Gnathostomata - jawed fishes
  • Origin and evolution of jaws
• Class Placodermi (extinct jawed fishes)
• Class Chondrichthyes (sharks, rays, dogfishes, ratfishes)
  • Fins
  • High speed locomotion
• Class Osteichthyes (bony fishes)
  • Subclass Actinopterygii (ray finned fishes)
  • Locomotion
  • Feeding
  • Subclass "Sarcopterygii"
• Review of innovations, locomotory and feeding specializations

Brief Review: Deuterostome Relationships

Phylum Chordata

• Three major groups
• Clearly monophyletic
• Primitive similarities
• Gill slits
• Dorsal, hollow nerve cord
• Synapomorphies
• Notochord
• Post anal tail
• Endostyle

Early Chordate Evolution

• Phase 1: Origin of ...
• Gill slits, dorsal nerve cord
• Base of hemichordates/chordates
• Phase 2: Origin of …
• Notochord
• Endostyle
• Post-anal tail, swimming phase
• Base of chordates
• Phase 3: Origin of …
• Myomeres/segmentation
• Distinctly "craniate-looking" circulation
• Base of cephalochordates and craniates

Early Chordate Evolution

The Origin of Craniates

• Transition from "Protochordates" to craniates marked by a dramatic change in …
• Activity level
• Sensory systems
• Locomotor efficiency
• Feeding strategies
• Size

The Origin of Craniates:
Two key features

• 1) Neural crest cells
• Gill arch supports
  • More efficient respiration
  • Eventually jaws
• Some bone, eventually
  • More effective protection
  • More effective locomotion
  • Parts of teeth
• Myelinating cells, many neurons
  • More effective movement
  • More effective digestion

The Origin of Craniates:
Two key features

• 1) Neural crest cells
• 2) Placodes
• Contribute to much of the sensory system, including:
• Ear, lateral line
  • Balance, hearing, pressure
• Olfactory
  • Sense of smell
• Optic
  • Lens of eye

The Origin of Craniates:
Other changes

• Respiration/circulation
• Respiratory structures
• Respiratory pigments
• Heart
• Muscular gut
• Cartilage and eventually bone for protection and support
• (Recall: Cephalochordates
  Respiration/Circulation
• Closed circulation
• No heart, just contractile vessels
• Pattern similar to "fishes"
  • Ventral aorta -> Gill bars ->
    Dorsal aortae -> Body -> …
• No blood pigments)

Relationships of "Fishes"

• "Fishes" Paraphyletic
• "Agnatha"
  • Myxini - Hagfishes
  • Cephalaspidomorphi
• Chondrichthyes
  • Sharks, rays, etc.
  • "Cartilaginous" fishes
• Actinopterygii
  • Ray-finned fishes
• "Sarcopterygii"
  • Coelacanths
  • Lungfishes

General aspects of life in Water:
Respiration

• Water as a respiratory medium, relative to air
• Low in O₂ (typically about 1/30th)
• Viscous (50x), therefore difficult to move
• Dense (800x), therefore difficult to move
• Diffusion is very slow, so distances must be short
• Must move 23,000 times as much of the mass of water as air to get an equivalent amount of O2 !

General aspects of life in Water:
Respiration

• Water flow is unidirectional over the gills
• Saves the energy of reversing water flow
• Water flow is nearly continuous
• Less acceleration in water
• Gill surface area is large
• Filaments, lamellae
• Diffusion distance is small
• Blood and water flow in countercurrent fashion!

General aspects of life in Water:
Locomotion

• Staying off the bottom
• Generating lift using fins
• Being neutrally buoyant
  • Buoyant tissues - oils, fats
  • Lung or Swim bladder
  • Swimbladders are under pressure
    • Gas gland
    • Rete mirable - counter current

General aspects of life in Water:
Locomotion

• Staying off the bottom
• Generating thrust
• Aquatic locomotion very complicated
• Drag related to
  • Profile
  • Amount of undulation
  • Shape of propulsive fins

General aspects of life in Water:
Locomotion

• Staying off bottom
• Generating thrust
• Tradeoffs in locomotion
• Acceleration
  • Small profile, large propulsive surface, flexible body
• Maneuverability
  • Deep profile, broad surface, short body
• Cruising
  • Small profile, stiff body, high/narrow tail (high aspect ratio)

Basal Relationships Of Craniates

Class Myxini

• Very odd animals!
• 43 species; scavengers
• Development poorly known
• Well developed notochord
• Lack scales, paired fins
• Lack vertebrae!
• Primitive absence of bone!
• Produce copious mucus

Class Myxini - Mucus

• Protection from predators, competitors
• Knotting behavior

Vertebrata - Synapomorphies

• Vertebrae (at least rudimentary)
• Capacity to form bone
• Bone secondarily lost in lampreys
• Primitive jawless fishes had great amounts of bone forming an exoskeleton

Two types of skeleton

• Dermal skeleton
• Not preformed in cartilage
• Formed external armor in early fishes
• Retained in modern vertebrates
  • Facial, pectoral bones
  • Teeth!
• Endoskeleton
• Preformed in cartilage

Why develop bone?

• Protection
• Support
• Calcium Metabolism
• Sensory structures

"Ostracoderms" - Extinct jawless fishes

• Appear 500 mya
• Well developed dermal skeleton
• Hence the name
• From neural crest

Class Cephalospidamorphi

• Represented by lampreys
• Formerly more diverse
• Lack bone
• Secondary loss
• Lack paired fins
• Primitive absence
• Ventilation by muscular pump
• Larvae superficially similar to Amphioxus (Cephalochordata) in appearance, habits

Class Cephalospidamorphi

• Represented by lampreys
• Lack bone
• Lack paired fins
• Ventilation by muscular pump
• Larvae superficially similar to Amphioxus
  (Cephalochordata) in appearance, habits
• After metamorphosis …
• Non-feeding
• Parasitic

Superclass Gnathostomata

• Incredibly diverse
• Includes:
  • Placoderms (extinct)
  • Chondrichthyes
  • Actinopterygii
  • "Sarcopterygii"
  • Tetrapoda
• Clearly monophyletic

Superclass Gnathostomata

• Incredibly diverse
• Clearly monophyletic
• Synapomorphies include:
• Two pairs of paired appendages
• Jaws
• Teeth

Jaws - Origin and Evolution

• Evolved from gill arches
• Serial homology
• "First" arch
• Forms jaw
• "Second" arch
• Supports jaw joint
• Spiracle in between

Class Placodermi

• Early gnathostomes (jawed fishes), now extinct
• Large, active predators
• Retain bone
• Especially dermal head shield
• Endoskeleton weak, cartilaginous

Class Chondrichthyes

• Two groups
• Elasmobranchii
• Holocephali
• Bone secondarily lost
• Present in some fossils
• Includes the largest fishes
• Whale sharks > 15 m
• Basking sharks 10 m

Class Chondrichthyes

• Two groups
• Elasmobranchii
• Holocephali
• Bone secondarily lost
• Present in some fossils
• Includes the largest fishes
• Whale sharks > 15 m
• Basking sharks 10 m

Class Chondrichthyes - Fins

• Paired fins
• Supported by cartilage rays
• Usually low on body
• Sometimes highly modified
  • E.g., skates and rays
• Caudal fin
• Usually heterocercal
• Generates lift
• Balanced by lift from pectorals

Class Chondrichthyes:
High speed locomotion

• Tail becomes
• Lunate
• High aspect ratio
• Homocercal
• Partial endothermy
• Retention of heat in
  • Body
  • Brain
  • Eyes

Class Osteichthyes: The Bony fishes
Subclass Actinopterygii: The ray-finned fishes

• Very Diverse
• About 25,000 spp.
• Sister-group of coelacanths/lungfishes/ tetrapods
• Shared possesion of lungs
• Modified to swim bladder in most teleosts

Actinopterygian Trends:
Locomotion

• Reduction of scales
• Reduce buoyancy problems
• Pectoral fins no longer need to generate lift
• Strong internal skeleton

Actinopterygian Trends:
Locomotion

• Reduction of scales
• Strong internal skeleton
• Tail hetero - to homocercal
• More forward thrust

Actinopterygian Trends:
Locomotion

• Reduction of scales
• Strong internal skeleton
• Tail hetero - to homocercal
• Pectoral fins moved up, pelvic fins forward
• Much greater maneuverability

Actinopterygian Trends:
Locomotion

• In High speed fishes
• Lunate tail, high aspect ratio
• Narrow-necking
• Stiff body

Actinopterygian Trends
Locomotion

• In High speed fishes
• Lunate tail, high aspect ratio
• Narrow-necking
• Stiff body
• Partial endothermy
• Seen in fast sharks too!

Actinopterygian Trends:
Feeding

• Increased mobility of skull and jaws
• Upper jaw detached
• Jaw protrusion
• Circular gape
• Feeding very fast
• Ram to suction feeding

Some Important Innovations

• Origin of Craniates
• Neural crest, placodes and derivatives
• Muscular pharynx, heart, etc.
• Origin of Vertebrates
• Bone, vertebrae
• Origin of Gnathostomes
• Jaws, teeth, paired appendages
• Origin of Osteichthyes
• Lungs

Locomotory Specializations

• High speed cruising evolved within sharks and Actinopterygians
• High aspect ratio tail
• High metabolic rate
• Within Actinopterygians
• Hetercercal to homocercal tail
• Dramatic reduction in scales
• Increased stiffening of the internal skeleton
• Migration of fins, increasing maneuverability
• Lung -> closed swimbladder

Feeding Specializations

• Increase in mobility of skull
• Especially in Actinopterygians
• Circular gape, upper jaw protrusion