BIOL 2003 Lecture Notes - Lecture 15: Agnatha, Branchial Arch, Ostracoderm
Ostracoderms
October 30, 2015
Agnatha = hagfish, lamprey, and ostracoderms
Ostracoderms – devionian armoured fish
• Heterostracans
o Bone – exoskeleton
o No jaws – agnathans
o Filter feeders, detritus
• Osteostracans
• Anaspida *very small light scales; once thought to be ancestor of jawless fish
Hagfish(Myxiniformes) – earliest branch of vertebrates
• Fossil: 300 million years ago
• Branched off before ostracoderms
• Very soft body so fossils are difficult to find
• Defense mechanism – secrete mucous
• Around 70 species
• Simple body form – little variation
• No scales, no bone
• No vertebrae
• No paired fins
• No jaw, but include effective mouth parts
o Protrusible tongue
o Keratinous teeth
• Use muscles at bottom on throat to push tongue out which then spreads apart so teeth can latch onto
prey (like polychaetes)
• Knots for feeding = flexible notochord
o Longitudinal support
o Very flexible
o Can tie itself into a knot and then move that up its body to pull things off or in
o Hagfish at home: deep sea scavengers (ex. Whale carcasses)
• In need of conservation:
o Do not replace themselves quickly
o Easily exploited: leather and food
Lamprey Life Cycle:
• Adult form has special mouth parts for parasitizing fish = ectoparasites
• 29 species
• Migrate towards lakes
• Attach to fish
• Reproduction occur in streams
• Lampreys and hagfish feed in similar ways
• Larvae are filter-feeders and burrow into the sediment (parallel to cephalochordates but do not have
close phylogeny)
• Half parasitic; half filter feeders *all filter feeders
• Mouth part forms sucker dish with keratinous teeth (generates suction)
• Creates hole inside fish to sucks out body fluids like blood and lymph
• On Lake Trout:
o More eradication efforts than conservation
o Problems for endemic non-parasitic lamprey (lampricides)
o Food of English royalty
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