Primitive Fishes

Primitive Fishes Origin of fishes A. From What? B. When? C. How? D. Where? Fish evolved from primitive Chordates Phylum Chordata characteristics: 1. Dorsal, hollow nerve chord 2. Notochord - flexible dorsal rod for support - present at some stage in all chordates (usually in embryonic development) - remnant present in adults of many fishes: sharks, rays, sturgeons 3. Pharyngeal gill slits Chordate subphyla: OLD classification • Urochordata: tunicates • Cephalochordata: lancelets • Vertebrata: vertebrates NEW classification • Urochordata: tunicates • Cephalochordata: lancelets • Craniata: craniates - present in embryos of all vertebrates How did fish evolve? Neoteny = retention of larval features into the adult stage Garstang Hypothesis: Tunicates gave rise to Cephalochordates (or directly to primitive fishes via neotony (paedogenesis) Stages: 1) ancestors (tunicates?) had: a) sessile adult stage b) free swimming larval stage for dispersal 2) larval stage became more active; more vertebrate-like 3) eventually, larvae capable of reproduction evolved (= neoteny) 1 Ascidian “tadpole” larva Lancelet (a cephalochordate) Pharyngeal gill slits Lancelet (adult Cephalochordate) Dorsal hollow nerve cord Notochord Ammocoete of lamprey (a fish) How first fishes may have evolved from primitive chordate ancestors first fish lancelet proto-vertebrate, development of a cranium adult with tunicate larval features adult tunicate When did fish first evolve? Earliest accepted chordate • Chordates date from early to mid Cambrian (544 mya) • first good craniate fossils - late Cambrian/Ordovician (500 mya) • BUT these first fossil bones were of external armor, characteristic of early jawless fishes (no internal bones) • so, vertebrates probably originated during the early Cambrian explosion (600 mya) Pikaia gracilens - Middle Cambrian Burgess shale 2 Where did fish evolve? Characteristics of fishes (& other craniates) Subphylum Craniata Generally believed to be in ocean because: Characteristics: 1. distinct cranium: skull with brain 2. notochord does not extend forward of brain 3. cartilage or bone present 4. brain well developed 5. chambered heart • is the habitat of other chordates • marine deposits contain most early vertebrate fossils Two superclasses within Craniata: • Agnatha – jawless (=“no jaw”) • Gnathostomata – jawed fishes (=“jaw mouth”) only two living groups of Agnathans: • Myxini (hagfishes) • Petromyzontida (lampreys) all other Craniates are Gnathostomes jaws no jaws Agnatha Jawless Fishes Subphylum Craniata Superclass Agnatha Class Myxini Order Myxiniformes - hagfish Class Petromyzontida Order Petromyzontiformes - lamprey “Group” Ostracoderms - Extinct 1) first jawless fishes were Ostracoderms (“shell skin”) • • artificial designation - not a monophyletic group now extinct 2) two main classes of Ostracoderms: • Class Cephalaspidomorphi = ancestor to lamprey • Class Pteraspidomorphi = ancestor to hagfish? 3 Ostracoderms (“shell skin”) Drepanaspis, a heterostracan ostracoderm (about 400 mya) 1) Characteristics a) first fossils have well developed external bone, no internal bone: Cambrian (500 mya) b) dominant for 100 my (gone by 380 mya) c) first fossils: marine; later marine and freshwater 2) Features a) no jaw, muscular feeding pump (filter feeders) b) body armor - true bone c) some had paired fin-like appendages, but not true fins with bony support d) heterocercal tail Hemiclaspis, a cephalaspid Phebolepis, a thelodont ostracoderm (about 410 mya) - small, had scales instead of bony plates Ostracoderms LIVING AGNATHANS: I. Agnathans (Superclass Agnatha) (jawless fishes) Hagfishes Class Myxini (refers to copious amounts of “slime"); Order Myxiniformes Ostracaderms jaws Lampreys Class Petromyzontida Order Petromyzontiformes - "stone mouth" no jaws Agnatha 4 Living Jawless Fishes (Agnathans) Hagfish - Myxiniformes First fossils of lampreys and hagfish appeared after most modern fishes and even tetrapods Myxiniformes Hagfish Lamprey - Petromyzontiformes Petromyzontiformes Lamprey Characteristics of both hagfish and lampreys a) jawless b) single gonad c) skeleton cartilaginous or fibrous (no vertebrae) d) no paired fins e) no body armor f) single median nostril g) series of round gill openings, no true gill arches Hagfishes Myxinidae; ~ 70 spp. Phylum: Chordata “Superclass Agnatha” Class Myxini Order Myxiniformes Comparison of Agnathan characteristics Character mouth vertebrae armor paired fins Ostracoderms hagfishes sucking, no teeth lampreys teeth on tongue teeth on oral disk & tongue yes no no yes (true bone) no scales no scales not true fins no no yes rudimentary yes mucous ? copious no reproduction ? direct larvae eyes Hagfishes – interesting features • SLIME! -- a 50 cm hagfish can fill 8 liter bucket in minutes • scavengers • ties in knots to feed and rid self of mucus • degenerate eyes • teeth only on tongue • barbels • cutaneous & gill respiration 5 Hagfish produce large eggs and are direct developers (no larval phase) Hagfish mucous is composed of proteins and carbohydrates that bind to water Line of mucous glands along ventral surface What is the slime for? • suffocate prey fishes? • protection from digestive enzymes? • discourage other scavengers? • secure burrow walls? • defense from predators? hagfish feeding technique & slime removal representative species with a great name: Myxine glutinosa (Atlantic hagfish) • Myxini = slime • glutinosa = glutinous or gluey 6 What can you do with a hagfish? Lampreys ~ 38 spp. hagfish are important scavengers hagfish are food Hagfishes vs. Lampreys Lamprey – interesting features Similarities: Character Phylum: Chordata “Superclass” Agnatha Class Petromyzontida Order: Petromyzontiformes a) oral disk in adults, with horny teeth on disk and tongue Hagfishes Lampreys Differences: notochord YES YES lingual teeth YES YES Character functional eyes NO YES single nostril YES YES dorsal fins NO YES 1 2 jaws NO NO # semicircular canals vertebrae NO NO parasitic NO YES paired fins NO NO eel-like body YES YES Hagfishes Lampreys b) two types: freshwater or anadromous -- anadromous (migratory) and parasitic - larvae stay in streams for 4+ yrs - adults migrate to ocean or lake, mature and feed (as parasites) for 2 years -- freshwater (non-migratory) and non-parasitic - adults also stay in streams - larval stage 6 yrs - non-feeding adult for 6 months 7 lamprey oral disk lamprey life cycle Lampetra tridentata (Pacific lamprey) convergence with salmon migration from freshwater to ocean Larval Lampreys (Ammocoetes) migration into freshwater • no teeth, mouth surrounded by an oral hood • blind • filter-feeders that capture particles with mucus emergence from streambed death of spawned adults metamorphosis • extended larval period • like lancelets (Amphioxiforms) Note: not recognized as lampreys for a long time “ammocoete” Lampetra tridentata (Pacific lamprey) Lampetra tridentata (Pacific lamprey) spawning act pit nest male squeezes eggs out of female • isolating mechanism between species pairs? (size differences) • group spawning in nonparasitic species 8 Ecology: invasion of Great Lakes by a fish parasite: Sea Lamprey Control Methods Petromyzon marinus (sea lamprey) • invaded Great Lakes • decimated fish populations - up to 56% mortality of lake trout TFM (Larval Lampricide) Migration Barriers & Traps - up to 75% mortality of whitefish Sister species in lampreys: Sterile Males (bisazir) Paired (sister) or Satellite Species • parasitic form (large adult) • free-living form (small adult) parasitic non-parasitic Ø evolved independently many times in different groups of lampreys (parallel evolution) Ichthyomyzon gagei (southern brook lamprey) Lampetra aepyptera (least brook lamprey) Ichthyomyzon castaneus (chestnut lamprey) Lampetra aepyptera (least brook lamprey) Next big advancement....JAWS Gnathostomata: jawed fishes “Perhaps the greatest of all advances in vertebrate history was the development of jaws and the consequent revolution in the mode of life of early fishes” - Romer 1962 9 Jaws evolved from modified gill arches Gill slits Gnathostomes diverged from Agnathans soon after fishes first evolved skull Evolutionary advantages of jaws: this is still a hypothesis: no intermediate fossils have been found • manipulate food (e.g., cut, grind, and crush; allows addition of new items to diet) • use for defense • manipulate non-food items (aid in nest building) red: mandibular arch blue: hyoid arch 5 classes of Gnathostomes: Phylum Chordata Subphylum Craniata Subphylum Craniata Superclass Gnathostomata extinct Class Placodermi Class Chondrichthyes extinct Class Acanthodii Class Sarcopterygii Class Actinopterygii Class Acanthodii (“stout spined”) spiny sharks – first jawed fishes in fossil record – from 450 mya until 280 mya “plate skin” “cartilage fish” “stout spine” “flesh fin” “ray fin” – first marine, then invaded freshwater • all Gnathostomes are a monophyletic group • all share: - jaws - two sets of paired fins - three semi-circular canals in their inner ears Acanthodi features • stout median and paired spines (rows down ventral surface) • cartilaginous skeleton • large head and large eyes • small and minnow-like • ganoid scales • true teeth on jaws • strong swimmers: up in water column (pelagic); not benthic like ostracoderms and most placoderms • unclear if more closely related to modern bony fishes or sharks (but closer to them than placoderms are) 10 Placoderm features Placodermi (“plate skinned”) • first fossils from late Silurian (420 mya), dominated in late Devonian (400mya) • jaws but no ability to extrude them • no teeth replacement, and teeth structure different from derived fishes (just bony plates) • bony armor (plate-like) • first in marine, then freshwater • true paired fins, but no anal fin • died out 280 mya • some very large (10 m) arthrodire placoderm 375 mya Bothriolepis, an antiarch placoderm (375 mya) Dunkleosteus, a giant arthrodire placoderm 375 mya, up to 6 m in length Ostracaderms jaws no jaws Agnatha Craniata 11