CHAPTER. Chordates 24-1

Transcription

1 CHAPTER Chordates 24-1

2 24-2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

3 Overview Diversity Fish has many usages extending beyond what are actually considered fishes today (e.g., starfish, etc.) A modern fish Aquatic vertebrate with gills, limbs (if present) in the form of fins, and usually with a skin covered in scales of dermal origin Fishes Do not form a monophyletic group In an evolutionary sense, can be defined as all vertebrates that are not tetrapods 24-3

4 Diversity Common ancestor of fishes is also an ancestor of land vertebrates Therefore in pure cladistics, would make land vertebrates fish - a nontraditional and awkward usage Approximately 24,600 living species Include more species than all other vertebrates combined Adapted to live in medium 800 times denser than air Can adjust to the salt and water balance of their environment 24-4

5 Diversity Gills are efficient at extracting oxygen from water that has 1/20 the oxygen of air Lateral line system detects water currents and vibrations, a sense of distant touch Evolution in an aquatic environment both shaped and constrained its evolution Fish refers to one or more individuals of one species Fishes refers to more than one species 24-5

6 24-6 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7 Ancestry and Relationships of Major Groups of Fishes History Descended from an unknown free-swimming protochordate ancestor about 550 million years ago Earliest fish-like vertebrates: Group of agnathan fishes Agnathans Include extinct ostracoderms and living hagfishes and lampreys Hagfishes lack vertebrae Lampreys have rudimentary vertebrae 24-7

8 Ancestry and Relationships of Major Groups of Fishes Included in subphylum Vertebrata because they have a cranium and other vertebrate homologies Unique enough to be assigned to separate classes Remaining fish have paired appendages and join tetrapods as a monophyletic lineage of gnathostomes Gnathostomes Appear in the Silurian fossil record with fully formed jaws No intermediate forms between agnathans are known The Devonian is called the Age of Fishes 24-8

9 Ancestry and Relationships of Major Groups of Fishes One group, the placoderms, became extinct in the Carboniferous and left no direct descendants Cartilaginous Fishes Lost heavy dermal armor and adopted skeleton of cartilage Flourished during the Devonian and Carboniferous Almost became extinct at end of the Paleozoic Increased in numbers in the early Mesozoic, diversifying to form a modern shark assemblage 24-9

10 Ancestry and Relationships of Major Groups of Fishes Acanthodians Well represented in the Devonian Became extinct by the lower Permian Resemble bony fish but have heavy spines on all fins except the caudal fin Probably the sister group of bony fishes 24-10

11 24-11 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

12 Ancestry and Relationships of Major Groups of Fishes Bony Fishes Dominant fishes today Two distinct lineages Ray-finned fishes Lobe-finned fishes Ray-finned fishes radiated to form modern bony fishes Lobe-finned fishes are a relict group with few species today Include the sister group of the tetrapods 24-12

13 Overview Living Jawless Fishes Living jawless fishes include hagfishes and lampreys About 43 species of hagfishes are known and about 41 species of lamprey are described Members of both groups lack jaws, internal ossification, scales, or paired fins Both groups share porelike gill openings and an eel-like body Recent molecular analysis shows lampreys and hagfishes forming a monophyletic unit Many zoologists do not support this grouping thus we take the view that hagfishes form the sister group of a clade that includes lampreys and gnathostomes

14 24-14 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15 24-15 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

16 Class Myxini: Hagfishes Entirely marine Scavengers and predators of annelids, molluscs, dead or dying fishes, etc. Enters dead or dying animal through orifice or by digging inside using keratinized plates on tongue Nearly blind Locate food by an acute sense of smell and touch To provide leverage Ties knot in tail and passes it forward to press against prey Special glands along body secrete fluid that Living Jawless Fishes becomes slimy in contact with seawater

17 Living Jawless Fishes Body fluids are in osmotic equilibrium with seawater Circulatory system includes three accessory hearts in addition to a heart behind gills Reproduction of Hagfishes Copenhagen Academy of Science offered a prize over a century ago for information on hagfish breeding habits it remains unclaimed In some species, females outnumber males by 100 to 1 Females produce small numbers of large, yolky eggs 2-7 centimeters in diameter 24-17

18 Living Jawless Fishes Class Petromyzontida: Lampreys Diversity All lampreys in Northern Hemisphere belong to the family Petromyzontidae Marine lamprey Petromyzon marinus Occurs on both Atlantic coastlines Grows to a length of one meter 20 species of lampreys in North America Half belong to nonparasitic brook-dwelling species 24-18

19 24-19 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

20 Living Jawless Fishes Reproduction and Development Ascend freshwater streams to breed Marine forms are anadromous Leave the sea as adults to spawn upstream In North America All spawn in winter or spring Males build nest by lifting stones with oral discs and using body vibrations Female anchors to a rock and male attaches to her head As eggs are shed into nest, the male fertilizes them Adults die soon thereafter 24-20

21 Living Jawless Fishes Eggs hatch in two weeks into ammoceotes larvae Lives first on yolk supply and drifts downstream to burrow into sandy areas Suspension-feeder until it metamorphoses into an adult Change to an adult involves eruption of eyes, keratinized teeth replacing the hood, enlargement of fins, maturation of gonads and modification of gill openings 24-21

22 Living Jawless Fishes Parasitic Lampreys Marine, parasitic lampreys migrate to the sea Other species remain in freshwater Attach to a fish by a sucker-like mouth Sharp teeth rasp through flesh as they suck fluids Inject anticoagulant into a wound When engorged, lamprey drops off but wound may be fatal to fish Parasitic freshwater adults live 1 2 years before spawning and dying Anadromous forms live 2 3 years Nonparasitic lampreys do not feed Digestive tract degenerates as an adult They spawn and die 24-22

23 24-23 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24 Living Jawless Fishes Sea Lamprey Invasion of the Great Lakes Region No lampreys were in the U. S. Great Lakes west of Niagara Falls until the Welland Ship Canal was deepened between 1913 and 1918 Sea lampreys moved first through Lake Erie to Lakes Huron, Michigan, and Superior Lampreys preferred lake trout Destroyed this commercial species along with overfishing 24-24

25 Living Jawless Fishes Then rainbow trout, whitefish, lake herring, chubs, and other species were destroyed Lamprey populations declined both from depletion of food and control measures Chemical larvicides in spawning streams Release of sterile males 24-25

26 24-26 Class Chondrichthyes: Cartilaginous Fishes Overview About 970 living species Smaller and more ancient group Well-developed sense organs, powerful jaws, and predaceous habits helped them survive True bone is completely absent throughout the class Phosphatized mineral tissues retained in teeth, scales, and spines Nearly all are marine Only 28 species live primarily in freshwater After whales, sharks are the largest living vertebrates, reaching 12 meters in length

27 Class Chondrichthyes: Cartilaginous Fishes Subclass Elasmobranchii: Sharks, Skates and Rays 13 living orders of elasmobranchs with about 937 total species described Order Carcharhiniformes Contains the coastal tiger and bull sharks and the hammerhead Order Lamniformes Contains large, pelagic sharks such as the white and mako shark 24-27

28 24-28 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

29 Class Chondrichthyes: Cartilaginous Fishes Order Squaliformes Contains dogfish sharks Order Rajiformes Contain skates Order Myliobatiformes Contains several groups of rays (stingrays, manta rays, etc.) 24-29

30 Class Chondrichthyes: Cartilaginous Fishes Form and Function Sharks are among the most gracefully streamlined of fishes Body is fusiform Thrust and lift provided by an asymmetrical heterocercal tail Vertebral column turns upward and extends into dorsal lobe of caudal fin Fins include Paired pectoral and pelvic fins One or two median dorsal fins One median caudal fin Sometimes a median anal fin 24-30

31 24-31 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

32 Class Chondrichthyes: Cartilaginous Fishes In males, the medial part of the pelvic fin is modified to form a clasper used in copulation Paired nostrils are anterior to mouth Lateral eyes are lidless Behind each eye is a spiracle Remnant of the first gill slit Tough, leathery skin with placoid scales Reduce water turbulence Detect prey at a distance by large olfactory organs sensitive to one part per 10 billion 24-32

33 24-33 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

34 Class Chondrichthyes: Cartilaginous Fishes Nostrils to each side of the hammerhead shark may improve stereo-olfaction Prey may also be located from long distances sensing low frequency vibrations in the lateral line Consists of neuromasts in interconnected tubes and pores on side of body At close range, switch to vision Most have excellent vision even in dimly lit water Up close, sharks are guided by bioelectric fields that surround all animals 24-34

35 24-35 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

36 Class Chondrichthyes: Cartilaginous Fishes Electroreceptors, the ampullae of Lorenzini, are located on the shark s head Upper and lower jaws equipped with sharp, triangular teeth that are constantly replaced Mouth opens into large pharynx, containing openings to gill slits and spiracles Short esophagus runs to stomach Liver and pancreas open into short, straight intestine Spiral valve in intestine slows passage of food and increases absorptive area 24-36

37 24-37 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

38 Class Chondrichthyes: Cartilaginous Fishes Rectal gland secretes NaCl and assists the opisthonephric kidney Heart chambers provide standard circulatory flow through gills and body Elasmobranchs retain nitrogenous compounds in the blood Raises blood solute concentrations and eliminates the osmotic inequality between blood and seawater 24-38

39 Class Chondrichthyes: Cartilaginous Fishes Reproduction and Development Fertilization is internal Maternal support of embryo is variable Includes oviparous, ovoviviparous, and viviparous species Mermaid s purse Horny capsule encasing eggs laid by some oviparous species 24-39

40 Class Chondrichthyes: Cartilaginous Fishes Form and Function of Rays More than half of elasmobranchs are rays Most specialized for benthic life Dorsoventrally flattened body and enlarged pectoral fins used to propel themselves Respiratory water enters through large spiracles on top of the head Teeth adapted for crushing prey Molluscs, crustaceans, and sometimes small fish Stingrays Have whiplike tail with spines and venom glands Electric rays Have large electric organs on each side of head 24-40

41 24-41 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

42 24-42 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

43 Subclass Holocephali: Chimeras Small subclass Remnants of a line that diverged from the earliest shark lineage 33 extant species Fossil chimaeras First appeared in the Carboniferous, reached a zenith in the Cretaceous and early Tertiary, and then declined Mouth lacks teeth Equipped with large flat plates for crushing food Upper jaw fused to the cranium Feed on seaweed, molluscs, echinoderms, Class Chondrichthyes: Cartilaginous Fishes crustaceans, and fish

44 24-44 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

45 Osteichthyes: Bony Fishes Origin, Evolution and Diversity In early to middle Silurian, a lineage of fishes with bony endoskeletons gave rise to a clade that contains 96% of living fishes and all living tetrapods 3 features unite bony fishes and tetrapod descendants Endochondral bone replaces cartilage during development Lung or swim bladder is present Evolved as an extension of gut Have several cranial and dental characters unique to clade 24-45

46 24-46 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

47 Osteichthyes: Bony Fishes Do not define a natural group and is a term of convenience rather than a valid taxon Bony operculum and branchiostegal rays associate them with acanthodians By the middle Devonian, bony fishes developed into 2 major lineages Ray-finned fishes, class Actinopterygii, radiated to form modern bony fishes Lobe-finned fishes, class Sarcopterygii, include lungfishes and the coelacanth Operculum increased respiratory efficiency Helped draw water across gills 24-47

48 Osteichthyes: Bony Fishes Gas-filled structure off the esophagus Helped in buoyancy and also in hypoxic waters In fishes that use these pouches for respiration Pouches are called lungs In fishes that use these pouches for buoyancy Pouches are called swim bladders Specialization of jaw musculature improved feeding 24-48

49 Class Actinopterygii: Ray-finned Fishes Diversity Over 27,000 species of ray-finned fishes Palaeoniscids Earliest forms Small, had large eyes, a heterocercal caudal tail, and interlocking scales with an outer layer of ganoin Single dorsal fin and numerous bony rays derived from scales stacked end to end Fossils have been found as early as the late Silurian Flourished throughout the late Paleozoic Distinct from lobe-finned fishes and saw the ostracoderms, etc. decline 24-49

50 24-50 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

51 24-51 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

52 24-52 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

53 Class Actinopterygii: Ray-finned Fishes Bichirs In the clade Cladista Have lungs, heavy ganoid scales, and other characteristics similar to the palaeoniscids 16 species of which all live in the freshwaters of Africa Chondrosteons 27 species of freshwater and anadromous sturgeons and paddlefishes Dam construction, overfishing, and pollution have led to their decline 24-53

54 24-54 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55 Class Actinopterygii: Ray-finned Fishes Neopterygians Appeared in late Permian and radiated extensively during the Mesozoic During the Mesozoic, one lineage gave rise to the modern bony fishes: Teleost 2 surviving early neopterygians are the bowfin and gars Gars and bowfin gulp air and use the vascularized swim bladder to supplement gills 24-55

56 24-56 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

57 Class Actinopterygii: Ray-finned Fishes Teleosts Constitute 96% of all living fishes and half of all vertebrates Range from 10 millimeters to 17 meters long, and up to 900 kilograms in weight Survive from 5,200 meters altitude in Tibet to 8,000 meters below the ocean surface Some can live in hot springs at 44 o C while others survive under Antarctic ice at - 2 o C Some live in salt concentrations three times seawater Others found swamps devoid of oxygen 24-57

58 24-58 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

59 Class Actinopterygii: Ray-finned Fishes Morphological Trends Heavy dermal armor replaced by light, thin, flexible cycloid and ctenoid scales Some eels, catfishes, and others lost scales Increased mobility from shedding armor helps fish avoid predators and improved feeding efficiency Fins changed to provide greater mobility and serve a variety of functions Braking, streamlining, and social communication Homocercal tail allowed greater speed and buoyancy Jaw changed to increase suctioning and protrusion to secure food 24-59

60 Class Sarcopterygii: Lobe-finned Fishes Diversity Ancestor of tetrapods Group of extinct sarcopterygian fishes called rhipdistians Early sarcopterygians Had lungs, gills, and a heterocercal-type tail During the Paleozoic, tail became a symmetrical diphycercal tail Had powerful jaws, heavy, enameled scales with a dentine-like material called cosmine, and strong, fleshy, paired lobed fins Today, clade is represented by 8 fish species 6 species of lungfishes and 2 species of coelacanths 24-60

61 24-61 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

62 24-62 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

63 Class Sarcopterygii: Lobe-finned Fishes Lungfish Australia lungfishes, unlike close relatives, rely on gill respiration and cannot survive long out of water South American and African lungfish can live out of water for long periods of time Rhipidistians and the coelacanth were termed crossopterygian: polyphyletic group no longer used Rhipidistians flourished in late Paleozoic and then became extinct Include ancestors of tetrapods 24-63

64 Class Sarcopterygii: Lobe-finned Fishes The Coelacanth Arose in the Devonian, radiated, reached a peak in the Mesozoic, and dramatically declined Thought to be extinct 70 million years, a specimen was dredged up in 1938 More were caught off coast of the Comoro Islands and in Indonesia Living coelacanth is a descendant of Devonian freshwater stock Tail is diphycercal with a small lobe between the upper and lower caudal lobes Young coelacanths born fully formed after hatching from eggs up to 9 cm in diameter 24-64

65 Structural and Functional Adaptations of Fishes Locomotion in Water Speed Most fishes swim maximally at ten body lengths per second Larger fish therefore swims faster Short bursts of speed are possible for a few seconds Mechanism Trunk and tail musculature propels a fish Muscles are arranged in zigzag bands called myomeres Have the shape of a W on the side of fish Internally the bands are folded and nested Each myomere pulls on several vertebrae 24-65

66 24-66 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

67 24-67 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

68 Structural and Functional Adaptations of Fishes Fish undulations move backward against the water Produces a reactive force with two parts Thrust pushes fish forward and overcomes drag Lateral force makes the fish s head yaw Large and rigid head minimizes yaw Swaying body generates too much drag for fast speed Fast fish are less flexible and generate all thrust with caudal fins Fast oceanic fish have swept-back, sickle-like tail fins, similar to high-aspect ratio wings of birds 24-68

69 24-69 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

70 Structural and Functional Adaptations of Fishes Economy Swimming is the most economical form of motion because water buoys the animal Energy cost per kilogram of body weight for traveling one kilometer is 0.39 Kcal for swimming, 1.45 Kcal for flying, and 5.43 for walking Yet to be determined how aquatic animals can move through water with little turbulence 24-70

71 Structural and Functional Adaptations of Fishes Neutral Buoyancy and the Swim Bladder Fish are slightly heavier than water To keep from sinking, a shark must continually move forward Fins keep it angled up Shark liver has a special fatty hydrocarbon, or squaline, that acts to keep the shark a little buoyant Swim bladder, as a gas-filled space, is the most efficient flotation device 24-71

72 Structural and Functional Adaptations of Fishes Swim bladder arose from paired lungs of primitive Devonian bony fishes Absent in tunas, some abyssal fishes, and most bottom dwellers Fish controls depth by adjusting volume of gas in swim bladder Due to pressure, as a fish descends, the bladder is compressed making the total density of the fish greater As a fish ascends, the bladder expands making the fish lighter and it will rise ever faster 24-72

73 24-73 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

74 Structural and Functional Adaptations of Fishes Gas is removed in 2 ways Primitive physostomous fishes Pneumatic duct connecting swim bladder and esophagus Advanced teleosts are physoclistous Pneumatic duct is lost and gas must be secreted into the blood from a vascularized area Both types require gas to be secreted into the bladder from the blood via the gas gland A network of capillaries, rete mirabile, is a countercurrent exchange system used to trap gases Gas gland secretes lactic acid that forces hemoglobin to release its load of oxygen 24-74

75 Structural and Functional Adaptations of Fishes Rete capillaries are short in surface-dwelling fish and very long in deep-sea fishes A few shallow-water-inhabiting physostomes gulp air to fill the swim bladder Even at 2400 meters, the bladder is inflated, mostly with oxygen, but also with variable amounts of nitrogen, carbon dioxide, argon, and even some carbon monoxide, and the pressure within the air bladder must exceed 240 atmospheres 24-75

76 Structural and Functional Adaptations of Fishes Hearing and Weberian Ossicles Fish, like other vertebrates, detect sounds as vibrations in the inner ear. A teleost group, ostariophysans Possess Weberian ossicles Allow them to hear faint sounds over a much broader range than other teleosts 24-76

77 24-77 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

78 Structural and Functional Adaptations of Fishes Respiration Fish gills are filaments with thin epidermal membranes folded into plate-like lamellae Gills are inside the pharyngeal cavity and covered with a movable flap, the operculum Operculum protects delicate gill filaments and streamlines body Pumping action by operculum helps move water through gills Although it appears pulsatile, water flow over gills is continuous 24-78

79 24-79 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

80 Structural and Functional Adaptations of Fishes Water flow is opposite to the blood flow Countercurrent exchange maximizes exchange of gases Some bony fishes remove 85% of the oxygen from water passing over gills Some active fishes use ram ventilation Forward movement is sufficient to force water across gills Such fishes are asphyxiated in a restrictive aquarium even if the water is saturated with oxygen 24-80

81 Structural and Functional Adaptations of Fishes Fishes Out of Water Lungs of lungfishes allow them to respire in air Eels can wriggle over land during rainy weather Use skin as major respiratory surface A bowfin uses gills at cooler temperatures and lung-like swim bladder at higher temperatures Electric eel has degenerate gills and gulps air through vascular mouth cavity Indian climbing perch spends most of its time on land, breathing air in special chambers 24-81

82 Structural and Functional Adaptations of Fishes Osmotic Regulation Freshwater is hypotonic to fish blood Water enters body and salt is lost by diffusion Scaled and mucous-covered body is mostly impermeable, but gills allow water and salt fluxes Freshwater fishes are hyperosmotic regulators Opisthonephric kidney pumps excess water out Special salt-absorbing cells located in epithelium actively move salt ions from water into fishes blood Systems are efficient Freshwater fish devote little energy to maintaining osmotic balance 24-82

83 24-83 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

84 Structural and Functional Adaptations of Fishes About 90% of bony fishes are restricted to either freshwater or seawater habitats Euryhaline fishes live in estuaries where salinity fluctuates throughout day Marine bony fishes are hypoosmotic regulators Blood is hypotonic to seawater Tend to lose water and gain salt Risks drying out 24-84

85 Structural and Functional Adaptations of Fishes To compensate for water loss, a marine teleost drinks seawater Brings in more unneeded salt Carried by blood to gills and secreted by special salt-secretory cells Divalent ions of magnesium, sulfate and calcium are left in intestine and leave body with the feces Some divalent ions enter bloodstream and are excreted by kidney by tubular secretion Glomeruli are small or missing 24-85

86 Structural and Functional Adaptations of Fishes Feeding Behavior Fish devote most of their time searching for food to eat and eating With the evolution of jaws Fish left a passive filter-feeding life and entered a predator-prey battle Most fish are carnivores Feed on zooplankton, insect larvae, and other aquatic animals Most fish do not chew food Would block water flow across the gills 24-86

87 24-87 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

88 24-88 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

89 Structural and Functional Adaptations of Fishes A few can briefly crack prey items with teeth, or have molar-like teeth in throat Most swallow food whole Easy with water pressure that sweeps food in when the mouth opens Some are herbivores Eat plants and algae Crucial intermediates in food chain Suspension feeders Crop abundant microorganisms of the sea 24-89

90 Structural and Functional Adaptations of Fishes Many plankton feeders swim in large schools and using gill rakers to strain food Omnivores feed on both plants and animals Scavengers feed on organic debris Detritovores consume fine particulate organic matter Parasitic fishes suck body fluids of other fishes Digestion follows the vertebrate plan A few lack stomachs 24-90

91 Structural and Functional Adaptations of Fishes Intestine tends to be shorter in carnivores and long and coiled in herbivores Stomach primarily stores food Intestine digests and absorbs nutrients Teleost fishes have pyloric ceca Apparently for fat absorption 24-91

92 Structural and Functional Adaptations of Fishes Migration Eels Have presented a life history puzzle for centuries Catadromous, developing to maturity in freshwater but migrating to sea to spawn In fall, adults swim downriver to the sea to spawn, but none return In spring, many young eels or elvers appear in coastal waters and swim upstream 24-92

93 24-93 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

94 Structural and Functional Adaptations of Fishes Grassi and Calandruccio (1896) reported that elvers were advanced juveniles True larval eels were tiny leaf-shaped, and transparent Johann Schmidt traced eel migrations by examining plankton nets from commercial fishermen Adult eels were tracked to the Sargasso Sea southeast of Bermuda At depths of 300 meters or more, eels spawn and die 24-94

95 Structural and Functional Adaptations of Fishes Minute larvae journey back to the streams of Europe and North America American eel larvae make trip back in 8 months since the Sargasso Sea is much closer to American coastline, whereas the European eel larvae take three years 24-95

96 Structural and Functional Adaptations of Fishes Homing Salmon Salmon are anadromous Grow up in sea but return to freshwater to spawn 6 species of Pacific salmon and 1 Atlantic salmon migrates Atlantic salmon makes repeated spawning runs Pacific species spawn once and die 24-96

97 24-97 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

98 Structural and Functional Adaptations of Fishes Pacific species of sockeye salmon Migrates downstream Roams the Pacific for 4 years Returns to spawn in headwaters of parents stream Young fish are imprinted on odor of their stream May navigate to stream mouth by sensing magnetic field of the earth or angle of the sun, and then smelling their way home Salmon are endangered by stream degradation from logging, pollution and hydroelectric dams 24-98

99 Structural and Functional Adaptations of Fishes Reproduction and Growth Most fishes are dioecious with external fertilization and external development Guppies and mollies represent ovoviviparous fish that develop in ovarian cavity Some sharks are viviparous with some kind of placental attachment to nourish young Most oviparous pelagic fish lay huge numbers of eggs Female cod may release 4 6 million eggs 24-99

100 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

101 Structural and Functional Adaptations of Fishes Near-shore and bottom-dwelling species lay larger, typically yolky, nonbuoyant and adhesive eggs Some bury eggs Many attach eggs to vegetation Some incubate eggs in their mouth

102 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

103 Structural and Functional Adaptations of Fishes Many benthic spawners guard eggs Male is usually the guard Freshwater fishes produce nonbuoyant eggs The more care provided, the fewer the eggs produced Freshwater fishes may have elaborate mating dances before spawning Egg soon takes up water, the outer layer hardens and cleavage occurs

104 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

105 Structural and Functional Adaptations of Fishes Blastoderm develops and yolk is consumed Fish hatches carrying a semitransparent yolk sac to supply food until it can forage Change from larva to adult may be dramatic in body shape, fins, color patterns, etc. Growth is temperature dependent Warmer fish grow more rapidly Annual rings on scales, otoliths, etc. reflect seasonal growth cycles Most fish continue to grow throughout life

106 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.