Fish

Biological diversity - fish

• phylum Chordata - Chordates 1st appear - fossil record, cambrian explosion (500 mya)
• Pikaia - similar appearance - modern lancelets
• 4 defining anatomical features - chordates (synapomorphies)

• notochord - stiffened, flexible rod cartilage. internal support+movement
• vertebrates - modified - development intravertebral discs

• dorsal hollow nerve cord - nerve cord expands - brain, anterior - cephalisation. begins - infolding - dorsally located ectoderm (hollow)

• Pharyngeal slits - perforate pharynx. feeding+respiration. aquatic chordates - gills. terrestrial chordates - jaw, neck, head

• postanal tail - muscular tail extends beyond anus. may be lost - development

• phylum Chordata - 3 subphyla. 2 invertebrate subphyla - cephalochordata (lancelets)+ Tunicata (seasquirts+salps). 1 vertebrate subphyla - vertebrata

• subphylum cephalo chordata - Amphioxus/"lancelets"
• resemble fish - no developed brain+few sensory organs
• live buried - sand
• filter feed: "cirri" - current through pharyngeal slits
• closed circulatory system, simple nervous system, segmented trunk musculature+respond - light

• subphylum Tunicata - 4 classes, 3000 species = Ascidiacae (sea squirts), Sorberacea (sea tulips), Thaliacea (salps)+Appendicularia (larvaceans)

• subphylum Tunicata - sister group - vertebrates
• Chordate features - larvae = muscular postanal tail, notochord, dorsal nerve chord, pharyngeal "basket" slits
• larvae = motile - settle - substrates+undergo tissue reorganisation - adult form 

• subphylum Tunicata - share feature - vertebrates = neutral crest cells
• cells near dorsal margins - neutral tube - folding 
• migrate, give rise - pigment cells - epidermis (UV protection)
• craniates - variety - jaw structures

• subphylum Vertebrata: class Myxini
• Hagfish - 30 species 
• rudimentary cartilaginous skull, no vertebrae, no jaws
• raspinf keratinous structures - use feed - carrion+injured fish
• produce slime = antipredator strategy. cloggs gills - predators 
• all species = marine 

• subphylum Vertebrata: class Petromyzontida
• Lampreys - 40 species
• more developed skull, cartilaginous arches extend around notochord, no true vertebrae+no jaws
• rasping keratinous teeth feed - live fish
• migrate between freshwater+marine (anadromous)

• subphylum vertebrata: Agnathans
• 2 interpretations - taxonomic relationships - agnathans (jawless) - gnathostomes (jaws)
• A = vertebrae hypothesis - lampreys = more morphological features - common - gnathostomes vertebrates 
• B = cyclostome hypothesis - feeding apparatus suggets lampreys+hagfish more related. B supported - molecular analysis

• subphylum Vertebrate: the Gnathostomes
• earliest fossil vertebrates = all jawless
• Ostracoderms - defensive bony plates - skin+cartilaginous vertebrae
• late Devonian (450 mya) = first jawed vertebrates (Placodermi+Acanthodii)
• jaws evolved - cartilaginous gill arches

• subphylum Vertebrata: class Placodermi
• 1st jawed fish - giant, armoured predators
• Dunkleosteus…