Fishes
    A. Originated in the sea
    B. 69% of earth covered with water creates many habitats
    C. 25,000 species of living fishes
    D. Evolution of Fishes
        1. Eary Crainiates
            a. Late Cambrian to middle Ordovician
            b. Aquatic filter feeders
            c. advantages over proto chordates
            d. muscular suction for filter feeding rather than cilia
            e. more food - grow larger
        2. Ostracoderms - no jaws
        3. Agnathans (early Carboniferous)
        4. Gnathostomes (mid Silurian)
         a. Chondrichthyes (Sharks, Skates, & Rays) > early Devonian
               b. Osteichthes (Bony fishes)
 

Agnathans (Jawless Fishes)

I. Appear in early carboniferous

II. Characterized by
    a.  no jaws
    b.  no paired appendages
    c.  circular mouths (cyclostomata)

III. Two distinct Groups  - Hagfishes and Lampreys

    A.  Hagfish (Myxinoidea)
        i.  no bone (no vertebral column)
        ii.  entirely marine species
        iii. 40 species in 6 genera
        iv. world wide distribution
        v.  live in mud burrow colonies in deep regions of the continental shelf
        vi. eat polycheate worms and shrimp (probably mole like)
        vii. also attracted to bait and other foods (super scavangers) (knot feeding)
        viii. accessory hearts, not coordinated
        ix.  slime defenses
        x.  recently decimated by eel-skin industry

    B.  Lampreys (Petromyzontidae)
        i.  40 species
        ii.  possess vertebra
        iii.  most are anadromous (up stream to breed)
        iv.  adults do not feed or migrate
        v.  worldwide distribution  except tropics and polar regions
        vi.  feed on vertebrate fluids
        vii. Reproduction -
                    a. spawn after temperature triggered migration attach to rock and thrash about to make nest (oxygenates) male
                        and female wrap around - fertilize eggs  die
                    b. larvae hatch - drift downstream - burrow into mud  and filter feed for 3 to seven years
                    c.  metamorphosis in parasitic juvenile
        viii. expansion into the great lakes devastated comercial fishing stocks
        ix.  now controlled with traps and pesticides

Gnathostomes (jawed fishes)

A.  Jaws (major inovation!)
    1. allows for expansion of feeding modes and substrate manipulation
    2.  arose from modification of a gill arch
    3. internally supported paired appendages
    4.  a variety of other neural, muscular and endocrine differences

B. Origin of Fins
    1.  three dimensions to control  - extremely important in adaptating to conditions
        a.  yaw (swinging right to left
        b.  pitch (up and down)
        c.  roll (rotation around the body axis)
    2.  fins projecting horizontally from body control roll
    3.  move fins forward  - controls pitch
    4. vertical fin controls yaw

Chondrichthyes (cartilaginous fishes)

I. Appear in early devonian
II.  Two groups: Holocephali (ratfish and chimaeras) and Elasmobranchii (sharks, skates and rays)

    A. Holocephali (whole head) (30 species)
        1. little agreement @ phylogony
        2. single gill opening
        3. ratfish or chimaera (mythological beast - fantasy critter)
        4. ratfish from long tail
        5. found deeper than 80 m
        6. pavement like teeth for crushing mollusks
        7. defensive dorsal spine with poison gland
        8. lay leathery eggs

    B.  Elasmobranchii (plate gilled)
        1. five to seven gill openings
        2. Squalomorph sharks (80 sp)
            i. smaller brain size
            ii. more ancestral
            iii. dog fish, cookie-cutter , basking, megamouth
            iv. deep / cold water  (except dogfish)
            v. some bioluminescence
        3. Galeomorph sharks (280 sp)
            i. modern
            ii. nurse, mackerel (great white)(tiger sharks)(hammerheads)
            iii. dominant carnivores in warm shallow seas
        4. Eight key adaptations for evolutionary success
            i. Buoyancy
                a. reduce weight (cartilaginous)
                b. no gas filled bladder
                    - sharks without liver has density of 1.062 - 1.089 gms/ ml
                    - seawater is 1.030 mg/ml
                    - shark liver is 0.95 mg/ml  (may be as much as 25% of body mass)
                    - adjust oil content adjusts buoyancy  (shark liver oil)
            ii.  Respiration
                a. spiracles anterior to gill openings (draw in water)
            iii.  External Covering
                a.  placoid scales - give lightweight protective coat
                b.  increase hydrodynamic efficiency (reduce turbulence)
                c.  grow larger as individual grows
(tenoid scales of bony fish serve the same function but didn’t evolve until several hundred million years later)
            iv. Feeding
                a.  Jaw attachment
                    -  only loosely attached
                    -  both upper and lower can move up or down relative to the brain case
                    -  hyostylic
                        >  allows for multiple jaw positions (cranial kinesis)
                        >  can drop away from brain case and open wide to bite organisms larger than itself
                b. Teeth
                    -  triangular and serrated (modified placoid scales which are continually shed)
                    -  shark bites - 2800kg (cm2)
                    -  protrudes jaw to set teeth further in - thrashes to create lateral motion
                    -  saws off a chunk of flesh.  Can eat both large and small prey
                    -  often kill by exsanguination (hold prey as it bleeds to death)
                    -  largest sharks eat small things
                    -  basking sharks  - teeth are whip like - form strainers like baleen
                    -  whale sharks use gill rakers to collect food
                    -  megamouth - mouth lined with reflective crystals - bioluminescent?
            v.  Movement
                a.  heterocercal tail increases propulsion
            vi.  Sensory systems
                a.  lateralalis system  (vibration)
                    -  detect struggling fish
                    -  helicopter rotor vibrations
                b.  ampullae of Lorenzini - electro reception
                    -  sensitive to 0.01 microvolts
                    -  detect electric fields
                    -  muscle contraction generate changes in electrical potential
                    -  use for prey detection and possibly navigation
                    -  chemo reception well refined (1 part in 10 billion)
                c. sharks probably find prey by mechano and chemo reception but rely on electroreception for final attack
                d. vision - teatum lucidum (layer of guanine crystals0
                    -   reflects light (eyeshine)
                    -  controlled on bright light by expansion of cells with melanin pigment
            vii.  Osmoregulation
                a. become isotonic with sea water by retaining nitrogenous wastes
                b. rectal gland excretes sodium and chloride
            viii.  Reproduction
                a. internal fertilization
                b. males wrap around females and insert claspers
                c. sometimes bite to hold on
                d. relatively small number of offspring with greater parental investment
        5. Skates and Rays Hypotremata  (456 sp)
            i.  benthic
            ii. durophagous habits
            iii. swim with large pectoral fins
            iv. reduced placoid scales improve flexibility
            v. remaining senticles are enlarged along dorsal surface (some venomous)
            vi. some have tissues in tail which emit electric discharge
            vii. id conspecifics
                a. torpedo rays discharge up to 200 V to stun prey