1. Living fishes  The living fishes (not a monophyletic group) include: the jawless fishes (e.g. lampeys), the jawless fishes (e.g. lampeys), cartilaginous fishes (e.g. sharks and rays), cartilaginous fishes (e.g. sharks and rays), bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and the bony, lobe-finned fishes (e.g. lungfishes, coelacanth). the bony, lobe-finned fishes (e.g. lungfishes, coelacanth).

2. 16.1

3. 16.2

4. Living jawless fishes  The living jawless fish once were included in the “Agnatha” along with ostracoderms because they lack the gnathostome characters of jaws and two sets of paired fins.  Today it is apparanet that the extinct ostracoderms are more closely related to the gnathostomes than are the living agnathans.

5. Living agnathans  There are slightly more than 100 species of living jawless fishes or Agnathans (the term agnathan does not represent a monophyletic group).  These belong to two classes the Myxini (hagfishes) and the Cephalaspidomorphi (lampreys).

6. Characteristics of living agnathans  Lack jaws (duh!)  Keratinized plates and teeth used for rasping  Vertebrae absent or reduced  Notochord present  Dorsal nerve cord and brain  Sense organs include taste, smell, hearing, vision.

7. Hagfishes: class Myxini  Hagfishes are a marine group of deep-sea, cold- water scavengers.  They use their keen sense of smell to find dead or dying fish and invertebrates and rasp off flesh using their toothed tongue.  As they lack jaws, they gain leverage by knotting themselves and bracing themselves against whatever they’re pulling.

8. Hagfishes  Hagfishes feed using two horny plates located either side of their tongue that are covered in sharp tooth-like structures.  When the tongue is everted the plates are spread apart and when the tongue is retracted the plates come together and mesh.

9. 16.3

10. Hagfishes  Hagfishes are considered the sister group of all vertebrates because they lack any trace of vertebrae.  They also have many other primitive characteristics including simple kidneys and only one semicircular canal on each side of the head.

11. Hagfishes  Hagfishes are unusual in that they have body fluids, which are in osmotic equilibrium with the surrounding sea. This is unknown in other vertebrates, but common in invertebrates.  They are also unusual in having a low pressure circulatory system that has three accessory hearts in addition to a main heart.

12. Hagfishes  Hagfishes have a remarkable (and revolting) ability to generate enormous quantities of slime, which they do to defend themselves from predators.  A single individual can fill a bucket with slime.

13. Lampreys: Class Cephalaspidomorphi  Lampreys are similar in general size and shape to hagfishes, but are more closely related to gnathostomes than are hagfishes.  Lampreys possess vertebral structures called arcualia, tiny cartilaginous skeletal elements that are homologous with the neural arches of vertebrates.

14. Lampreys  Unlike hagfishes, lampreys possess large well developed eyes and have two semicircular canals.  They also are not isosmotic. Instead well- developed kidneys and chloride cells in the gills regulate the concentration of body fluids and allow lampreys to live in a wide range of salinities.

15. Lampreys  The lamprey’s mouth is located at the base of the oral hood (a fleshy suction cup lined with teeth).  The oral hood allows the lamprey to latch on tight to its prey and once attached the lamprey is very hard to dislodge.

16. Lampreys  Lampreys occur in both marine and fresh waters and about half of all species are ectoparasites of fish (the others are non-feeding as adults and live only a few months).  Lampreys spawn in streams and the larvae (ammocoetes) live and grow as filter feeders in the stream for 3-7 years before maturing into an adult. Feeding adults live a year or so before spawning and dying.

17. 16.5

18. Lampreys  Parasitic lampreys have a sucker-like mouth with which they attach to fish and rasp away at them with their keratinized teeth.  The lamprey produces an anticoagulant as it feeds to maintain blood flow. When it is full the lamprey detaches, but the open wound on the fish may kill it. At best the wound is unsightly and largely destroys the fish’s commercial value.

19. Sea lamprey close up of sucker and teeth

20. 16.4

22. Lampreys  Because attached lampreys cannot have a through-flow of water they have to ventilate their gills in a tidal fashion.  Water is drawn in and pumped out of the gill slits, which is not very efficient, but is a necessary compromise.

23. Introduced sea lampreys  Landlocked sea lampreys made their way into the Great Lakes around 1918 and caused the complete collapse of the lake trout fishery by the 1950’s.  Lamprey numbers fell as their prey base collapsed and control efforts were introduced. Trout numbers have since recovered somewhat, but wounding rates are still high.

24. Sea lampreys in Lake Champlain  Lake Champlain also has large populations of sea lampreys which spawn in the creeks that empty into the lake.  Until recently, lampreys were believed to have been introduced into Lake Champlain, but genetic analyses indicate the population was established perhaps as much as 11,500 years ago by lampreys that migrated up the St. Lawrence.

25. Sea lampreys in Lake Champlain  As is the case elsewhere there has been a campaign to control lamprey numbers primarily by using lampricides in steams.  Controls do reduce lamprey wounding rates and after control rates have fallen from 60-70 wounds per 100 fish examined to as low as 30 wounds/fish.

26. Early jawed vertebrates  The origin of jaws was a hugely significant event in the evolution of the vertebrates and the success of the Gnathostomes [the jawed vertebrates, “jaw mouth”] is obvious.  The first jawed vertebrates were the placoderms heavily armored fish which arose in the early Devonian (about 400mya).  They also possessed paired pelvic and pectoral fins that gave them much better control while swimming.

27. 15.13 Early jawed fishes of the Devonian (400 mya).

28. Evolution of Jaws  Vertebrate jaws are made of cartilage derived from the neural crest, the same material as the gill arches (which support the gills).  Jaws appear to have arisen by modification of the first cartilaginous gill arches, which aid in gill support and ventilation.

29. Evolution of Jaws  The advantages of possessing jaws are obvious.  However, structures must benefit the organism at all times or they will not be selected for.  What use would a proto-jaw have been before being fully transformed?

30. Evolution of Jaws  Mallatt (1996,1998) has suggested that jaws were originally important for gill ventilation, not grasping prey.  Gnathostomes have much higher energy demands than agnathans. They also possess a series of powerful muscles in the pharynx. These muscles allow them to both pump water across the gills and suck water into the pharynx.

31. Evolution of Jaws  It is likely that selection initially favored enlargement of the gill arches and the development of new muscles that enabled them to be moved and so pump water more efficiently.  Once enlarged and equipped with muscles it would have been relatively easy for the arches to have been modified into jaws.

32. Evolution of Jaws  Being able to close the mouth would have enabled the muscles of the pharynx to squeeze water forcefully across the gills.  Selection would have favored any change in gill arches and musculature that enhanced water movement over the gills.  Thus, Mallatt suggested that the mandibular branchial arch enlarged into protojaws because it allowed the entrance to the pharynx to be rapidly opened and closed.

33. Evolution of Jaws  Selection would have favored enlargement and strengthening of the mandibular arch to tolerate the forces exerted on it by the strong pharyngeal muscles.  Once the proto-jaws can be rapidly closed they can also take on a grasping function and new selective forces would quickly have driven jaw elaboration.

34. 15.12 Note resemblance between upper jaw (palatoquadrate cartilage) and lower jaw (Meckel’s cartilage) and gill supports immediately behind in this Carboniferous shark

35. Evolution of Jaws  Equipped with jaws for grabbing and holding prey and powerful pharyngeal muscles that could suck in prey gnathostomes could attack moving prey.  An enormous diversification of gnathostomes followed.

36.  Four major groups of fish are present in the Devonian, two now extinct groups (the placoderms and acanthodians) and two living (the Chondrichthyians, [sharks and relatives] and the Osteichthyians, [bony fishes]).

37. Placoderms  The Placoderms are armored fishes that appear to be basal to other gnathostomes.  The oldest known are from the early Silurian. Large, heavy plates of dermal bone covered the front half of the body and small bony scales covered the rest.

38. http://universe-review.ca/I10-29-placoderm.jpg http://tea.armadaproject.org/Images/deaton/deaton_5placoderm.JPG.jpg

39. Placoderms  Most placoderms did not possess true teeth (although late forms do, evolved independently of the other gnathostomes).  Instead they had toothlike structures called tooth plates that were extensions of the dermal jawbones.

40. Arthrodires  More than half of all known placoderms are arthrodires (“jointed necks”).  Arthrodires had modified joints between the head shield and trunk shield, which gave them an enormous gape and made them ferocious predators

41. http://www.palaeos.com/Vertebrates/Units/050Thelodonti/Images/Gnathostomata1.jpg Dunkleosteus upper Devonian. 10 meters long.

43. Placoderms  Placoderms like ostracoderms declined rapidly in the mass extinctions of the late Devonian.  A few forms survived for about 5 million years beyond the last ostracoderms, but the group was extinct by the end of the Devonian.

44. Acanthodians  The other extinct group of fishes is the acanthodians, which appear closely related to the bony fishes.  Acanthodians (from the Greek acantha meaning a spine) are named for the spines they had in front of their numerous fins (as many as six pairs in addition to the pelvic and pectoral pairs).

45. http://higheredbcs.wiley.com/legacy/college/levin/0471697435/ chap_tut/images/nw0273-nn.jpg http://people.eku.edu/ritchisong/RITCHISO//Acanthodian.gifo Acanthodians

46. Acanthodians  Acanthodians had fusiform bodies and heterocercal tails and so were likely midwater fishes.  Acanthodians became extinct in the early Permian.

47. Chapter 4. The challenges of living in water  All vertebrates inhabit one or other of two fluid media: air and water.  These differ greatly in their physical characteristics.

48. Air vs. water  Density: water is 800 times denser than air.  Because water is dense, aquatic animals don’t need strong weight bearing skeletons. Gravity has little impact on their body structure.  In contrast, gravity is a constant challenge for terrestrial animals.

49. Air vs. water  Viscosity: water is 18 times more viscous than air. Viscosity measures how easily a fluid moves across a surface.  Because of this difference aquatic animals have to be much more streamlined than those that live in the air.  Because air flows easily, tidal ventilation is possible in lungs. In water, it is difficult and very rare.

50. Air vs. water  Oxygen content: Oxygen makes up about 20.9% of the volume of air (209ml of O 2 in a liter of air). Water is never more than 50ml per liter and is often 10ml or less.  Low O 2 content is another reason fish don’t use tidal ventilation. Because of the low O 2 content of water, fish gills have evolved to be very efficient at extracting O 2.

51. Air vs. water  Heat Capacity: The specific heat of water (amount of heat needed to change the temperature of one gram of water by one degree) is 3400 times greater than that of air.  Thus, water resists temperature change. It heats and cools slowly. Hence an aquatic animal has a more stable thermal environment than an air-living one.

52. Air vs. water  Heat Conductivity: Water conducts heat almost 24 times as quickly as air.  Because water is such a good conductor there is little variation in temperature within a body of water. If water gets too hot, a fish must go to deeper water and that may not always be possible.

53. Air vs. water  Electrical conductivity: water is an electrical conductor, but air is not (except at high voltages).  Electricity therefore can be (and is) used by aquatic animals to detect other animals and also as a weapon.

54. Pressure effects  Water is much denser than air and pressure changes with increasing depth are very important to fishes.  Every 10m increase in depth in water increases the pressure experienced by 1 atmosphere. Thus, a fish at 100m experiences 10 atmospheres of pressure.

55. Pressure effects  Because of pressure effects on the use of gas-filled structures as buoyancy aids fish have had to evolve a variety of adaptations to remain at or near neutral buoyancy.  In contrast, in air extreme changes in altitude are required before significant effects of reduced air pressure are felt.

56. Obtaining oxygen in water: Gills  Fish exchange oxygen and carbon dioxide through the use of gills.  The gills of teleost fish (the largest group of bony ray-finned fish) are enclosed in pockets of the pharynx behind the mouth (the opercular cavities).  A flap of tissue (the operculum) protects the gills and also maintains the streamlining of the body.

57. Gills  Within the opercular cavity are a series of gill arches and from each gill arch project two sets of gill filaments.  On each gill filament are numerous, small and thin-walled projections called secondary lamellae. Gas exchange takes place at the secondary lamellae.

58. Countercurrent exchange Figure 42.21 Gill arch Water flow Operculum Gill arch Blood vessel Gill filaments Oxygen-poor blood Oxygen-rich blood Water flow over lamellae showing % O 2 Blood flow through capillaries in lamellae showing % O 2 Lamella 100% 40% 70% 15% 90% 60% 30% 5% O2O2

59. Gills  Water flow is one way through the gills.  Flaps just within the mouth and at the margins of the operculae prevent backflow.  Many fish (especially less active ones or resting ones) depend on the pumping action of the mouth and opercular cavities (called buccal pumping) to maintain a steady flow of water across the gills

60. Ram Ventilation  For fast swimming predatory fish buccal pumping would be inadequate to supply their gas exchange needs.  In fishes such as tuna, mackeral and swordfishes the ability to pump water has been reduced or lost.  Instead these fish depend on ram ventilation. They swim with their mouths open which creates a steady flow of water across the gills.

61. Northern Bluefin Tuna www.nytimes.com http://www.glaucus.org.uk/mackerel.jpg

62. Counter current exchange  The one-way flow of water across the gills is exploited by the fish to maximize oxygen extraction.  The lamellae of the gills are richly supplied with blood, which flows in a countercurrent direction to the flow of water maximizing the amount of oxygen extracted.

63. 16.25

64. Counter current exchange  Because the direction of blood flow is opposite the direction of the flow of water there is always an oxygen gradient between the water and the blood. Hence, oxygen always flows from the water into the blood.  Gills are very efficient and can extract up to 85% of the dissolved oxygen in the water.

65. Counter current exchange  All counter current exchangers work on the principle of maintaining a concentration gradient along the length of the structure.  In the gill, blood entering the lamellae is deoxygenated and it encounters water that has had much of its oxygen removed. However, the concentration gradient ensures the water gives up oxygen to the blood.

66. Counter current exchange  As the blood flows through the lamellae its oxygen concentration increases, but because of the countercurrent arrangement, it is always encountering water with a higher oxygen content than is in the blood so the blood continues to gain oxygen until it is saturated.

67. Counter current exchange  Countercurrent exchangers are widespread among vertebrates.  For example, they are found in the flippers of whales (to reduce heat loss from the body), in the lungs of birds (to maximize oxygen extraction), in the salt glands of seabirds (to concentrate salt) and as we will see shortly in the swim bladder of fishes (to maintain high gas pressure in the swim bladder).

68. How fish obtain oxygen from the air  Some fish that live in water with low oxygen content cannot obtain enough oxygen to survive using their gills alone.  These fish supplement their oxygen intake by using lungs or other accessory respiratory structures.

69.  Tropical Asian anabantid fish (which include the common pet fish tetras and gouramis) have vascularized chambers in the rear of the head called labyrinths.  The fish gulp air at the surface and it is transferred to the labyrinth where gas exchange takes place.

70. Tetra. http://animal-world.com/encyclo/fresh/characins/ images/SerpaeTetraWFCh_C2418.jpg Pearl Gourami http://www.thekrib.com/Fish/gourami.jpg

71. Lungs  Lungs obviously are most associated with tetrapods, but they evolved in fish millions of years before the first tetrapods evolved.  In fact lungs have evolved independently multiple times in different lineages of fish.

72. Lungs  Embryonically, lungs develop as out- pocketings of the pharyngeal region of the gut.  In lungfishes and tetrapods lungs develop from the ventral surface of the gut.  However, in gars (a primitive bony fish) lungs develop on the dorsal surface as is also true in teleosts.

73. South American Lungfish http://www.ucmp.berkeley.edu/vertebrates/sarco/lungfish1.jpg

74. Australian Lungfish

75. Longnose gar http://www.biokids.umich.edu/files/12296/gar_large.jpg http://animals.nationalgeographic.com/staticfiles/NGS/ Shared/StaticFiles/animals/images/primary/gar.jpg

76. . Lungs  Increased surface area increases the efficiency of lungs.  Ridges and pockets in the wall of the lung increase surface area and these alveolar lungs are found in lungfishes and tetrapods (both groups also have paired lungs).  Gars in contrast have a single alveolar lung whereas bichirs (a group of African air-breathing fish) have paired non-alveolar lungs, but one lobe is smaller than the other.

77. Armored Bichir http://www.aquarticles.com/images/Gallo/Armoured%20bichir%202.gif http://www.fbas.co.uk/Bichir.jpg

78. Swim bladder  Teleosts have evolved extremely fine control over their buoyancy and can remain neutrally buoyant, which provides large energy savings.  Most pelagic teleosts have a swim bladder, which evolved from paired lungs of Devonian fishes.  Swim bladders are found mainly in fish that occur in the upper 200m of the water column.  The swim bladder’s wall is impermeable to gases, but can expand a lot.

79. Swim bladder  A gas-filled bladder is affected by depth changes so the fish must be able to add and remove gas to remain neutrally buoyant.  Gas can be secreted into or removed from the swim bladder so that the fish remains at neutral buoyancy.

80. Swim bladder  Some fishes (e.g. trout, goldfish) gulp or release air by opening a pneumatic duct that connects to the esophagus. These fishes are referred to as physostomous (Greek phys = bladder, stom = mouth)  More derived teleosts (physoclistic; Greek clist = closed) have discarded the pneumatic duct and instead secrete gas into the swim bladder using a gas gland.

81. Gas gland  When arterial blood arrives at the gas gland it enters a layer of tissue called the secretory epithelium and here lactic acid is released.  This decreases pH which causes oxygen to be released by the hemoglobin because the hemoglobin’s oxygen affinity (Bohr effect) and oxygen capacity (Root effect) are reduced.

82. Gas gland  The release of oxygen raises the partial pressure of oxygen in the blood above that in the swim bladder and so the oxygen flows into the swim bladder.

83. Rete mirabile  In deep sea fish a very high gas pressure must be maintained to resist the pressure of the water.  For example, at 2000 meters gas at a pressure of 200 atmospheres (more than the oxygen pressure in fully charged steel cylinder) must be maintained in the swim bladder even though the oxygen pressure in the fish’s blood is only 0.2 atmospheres (oxygen pressure at sea level).

84. Rete mirabile  Why doesn’t the oxygen in the swim bladder flow out into the blood?  Because of a structure called a rete mirabile (miraculous net), which stops this loss.

85. Rete mirabile  The swim bladder is supplied with blood via an artery. Before the artery reaches the swim bladder it divides into an enormous number of thin, parallel capillaries that run parallel to but in the opposite direction to a similar array of venous capillaries.

86. Rete mirabile (below)

87. Rete mirabile  Let us assume the swim bladder contains gas at 100 atmospheres. Venous blood leaving the swim bladder thus contains oxygen at that pressure.  As the venous capillary leaves the swim bladder it runs parallel to incoming arterial blood which contains blood with a slightly lower partial pressure of oxygen.

88. Rete mirabile  Oxygen thus flows from the venous capillary to the arterial capillary.  Along its entire length from the swim bladder the gas pressure in the venous capillary is falling as it gets further from the swim bladder, but the pressure is always higher than that in the parallel arterial capillary so gas always flows from the venous capillary to the arterial capillary.  Thus the rete acts as a trap that keeps gas in the swimbladder.

89. Ovale  To release gas from the swimbladder, fish use a structure called the ovale.  The ovale is a muscular valve that connects the swim bladder to a capillary bed. When the ovale is opened the high pressure of oxygen in the swimbladder causes it to diffuse into the capillary bed and enter the blood stream.

90. Deep Sea fishes  Many deep sea bony fishes deposit oils and lipids in the gas bladder. Others have lost the gas bladder entirely.  Fish that migrate over a large vertical distance tend to depend more on oils for buoyancy than gas because oils are incompressible and thus unaffected by pressure changes.

91. Buoyancy in sharks  Cartilaginous fish do not possess a swim bladder. To compensate they store large amounts of low density oils in their enlarged livers (which may represent 25% of their body mass).  Sharks also have high concentrations of urea in their blood, which also reduces their buoyancy.

92. Buoyancy in sharks  Pelagic sharks have the largest livers and contain the most oil. Liver tissue has an average density of 0.95 g/ml  With the liver removed the tissue density of a shark is about 1.06-1.09 g/ml (water is 1g/ml), but with the liver included average density falls to approximately 1.007 g/ml.  A 460 kg tiger shark thus has an effective weight of only about 3.5 kg.

93. Large liver of a great white shark

94. Buoyancy in sharks  Bottom-dwelling sharks such as nurse sharks can afford to be more negatively buoyant.  They have smaller livers and there is less oil deposited in the liver.

95. Tiger Shark http://www.kidzone.ws/sharks/photos/ Nurse sharks

96. Diving mammals  For air-breathing vertebrates lungs and pressure pose a different series of challenges.  Air contained within the lungs becomes pressurized with increasing water depth.  Under high pressure nitrogen in the lungs is forced into the blood stream.

97. Diving mammals  As an animal ascends the pressure falls and the nitrogen gas comes out of solution.  If the animal comes up too quickly, bubbles of nitrogen may form in the tissues causing decompression sickness (“the bends”).

98. Diving mammals  To avoid this problem, diving mammals breathe out before they dive and their thoracic cavity actually collapses at about 150m depth which forces air out of the lungs.  To avoid an accumulation of nitrogen over time whales and seals do not perform multiple long dives, but alternate deep dives with surface time, which allows nitrogen to leave the blood stream.

99. Vision in water  Air and water have different refractive indices so light bends as it passes through an air water boundary.  Thus, to an observer on land an object in water appears closer than it really is.

100. Vision in water  The corneas of terrestrial and aquatic organisms both have a refractive index of about 1.37.  For terrestrial vertebrates light is bent when it passes through the air-cornea interface. Thus, the cornea can play a major role in focusing.

101. Vision in water  For aquatic vertebrates there is too little difference in the refractive indices of water and the cornea for the cornea to assist in focusing.  In fish thus the lens is largely responsible for focusing.  As a result, fish have spherical lenses with a high refractive index and the whole lens is moved in and out to focus.

102. Vision in water  Light is absorbed by water and disappears with depth. It also can be scattered by suspended particles. Thus, vision is often of limited use.  Hence, fish must often depend on other senses.

103. Other sensory systems in water  For fish a distinction between taste and smell is pointless. Chemoreception is a better term.  In fish chemoreceptors often occur over the entire body and very low concentrations can be detected.  Both sharks and salmon can detect odors at concentrations of less than 1 part in a billion and home in on the source of a smell.

104. Touch  In terrestrial vertebrates in the inner ear, hair cells play a major role in hearing.  Similar clusters of these cells in fish form neuromast organs that are distributed over the head and body.  In jawed fishes (and amphibian larvae) the neuromast organs are often arranged in one or more canals (the lateral line system) that runs along the side of the body.

105. Lateral line system  The fluid-filled canals of the lateral line system are open to the outside.  The neuromasts are located inside in the canals and are very sensitive to vibrations in the water.  The hair cells in the neuromasts have cilia embedded in a gelatinous structure (the cupula). When the cupula is displaced the cilia bend and a nerve impulse is triggered.

107. Lateral line system  The neuromast cells can detect water currents of as little as 0.025mm/sec.  Because neuromast cells are distributed across the body, differences in arrival time of pressure waves can be used to locate the source of a disturbance (e.g. an insect on the surface of the water).

108. Electroreception  Many fishes, especially sharks can detect electric fields.  By detecting the faint bioelectric fields that surround all animals sharks can locate prey buried in sand or sense prey at night.

109. Organs of Lorenzini  The bioelectric detectors are called ampullary organs of Lorenzini and are found in the shark’s head.  In rays they are also on the pectoral fins.  The receptor is connected to a surface pore by a canal that is filled with an electrically conductive gel.

110. Organs of Lorenzini  Because the canal runs quite deep under the epidermis the sensory cell can detect when there is a difference between the electrical potential in the surrounding tissue and in the distant pore opening.  The electroreceptors are very sensitive and can detect minor changes in the electrical field around the shark.

112. Organs of Lorenzini  The threshold for detection is less than 0.01 microvolts per cm, which is comparable to the best commercially available voltmeters.  The electrical activity sharks detect is due to muscle contraction, the firing of motor nerves and also potential differences due to chemical differences between organisms and their surroundings.

113. Regulation of the internal environment  Organisms are not impermeable and aquatic vertebrates face considerable challenges in regulating their internal environments.  Fish in freshwater environments face the problem of being flooded with water, whereas those is seawater can be drained of water.

114. Kidneys  Kidneys play a central role in regulating the internal environment.  The functional unit of the kidney is the nephron (of which there are usually thousands to millions) each of which produces urine.

115. Kidneys  The blood is first filtered through a cluster of capillaries called the glomerulus to produce a non-selective filtrate.  The filtrate is then processed so that essential metabolites (amino acids, glucose, etc.) and water are retrieved.  The final fluid which may differ greatly in its composition depending on circumstances is urine, which is voided to the outside.

116. Marine fishes  Marine bony fishes are hypoosmotic to sea water a nd lose water by osmosis and gain salt by both diffusion and from food they eat.  These fishes balance water loss b y drinking seawater and actively excrete salt through their gills. They produce little urine.

117. Gain of water and salt ions from food and by drinking seawater Osmotic water loss through gills and other parts of body surface Excretion of salt ions from gills Excretion of salt ions and small amounts of water in scanty urine from kidneys (a) Osmoregulation in a saltwater fish

118. Freshwater fishes  Freshwater animals constantly take in water from their hypoosmotic environment  They lose salts by diffusion.  Freshwater animals maintain water balance b y excreting large amounts of dilute urine  Salts lost by diffusion a re replaced by foods and uptake across the gills

119. Uptake of water and some ions in food Osmotic water gain through gills and other parts of body surface Uptake of salt ions by gills Excretion of large amounts of water in dilute urine from kidneys (b) Osmoregulation in a freshwater fish

120. Nitrogen excretion  Proteins and nucleic acids both contain nitrogen.  When these substances are metabolized they are broken down to ammonia. Ammonia is very soluble in water, but also toxic and must be excreted quickly.  Because ammonia can be lost through the gills easily most fish excrete ammonia.

121. Nitrogen excretion  In vertebrates nitrogen is also excreted as urea and uric acid.  Both are less toxic than ammonia.

122. Nitrogen excretion  Urea is produced from ammonia and has two advantages. It increases the osmotic concentration of the blood so marine waters dehydration is reduced.  Urea is less toxic than ammonia so it can be so it can be stored when there is too little water for it the urea to be excreted.

123. Nitrogen excretion  The production of urea was an important trait that facilitated the invasion of the land.  Lobe-finned fishes however probably evolved urea production because it reduced osmotic dehydration.  Uric acid requires very little water for excretion and is the main form of nitrogen waste in dry environments.

124. Temperature regulation  Because of the high heat capacity and heat conductivity of water it is difficult for organisms to maintain a difference between their body temperature and the surrounding water temperature.  In air, in contrast, it is comparatively easy to do so.  For fish, however, there is much less variation in the temperature of water over time and many fish live in water that hardly changes temperature over a year.

125. Temperature regulation  Historically the terms poikilotherm (variable heat) and homeotherm (same) were used to categorize organisms into those whose body temperatures varied over time or stayed constant.  However, often organisms don’t fit neatly into these categories (e.g. hibernating mammals let their temperatures fall)  Ectotherm and Endotherm however are better terms as they describe the sources of heat and most organisms use a combination.

126. Temperature regulation of aquatic vertebrates: fish  Many fishes display regional heterothermy in which they keep the core of the body much warmer (up to 15ºC) than the surrounding water.  In some sharks such as the mako and great white countercurrent heat exchangers keep the core 5-10ºC warmer than the water

127. Temperature regulation of aquatic vertebrates: fish  Tuna have myoglobin rich swimming muscles which produce a lot of heat and are kept at about 30ºC, again by a rete system. Tuna and sharks also use rete in the brain and eyes to retain heat in those organs.  In billfishes (swordfish, marlin, sailfish) the superior rectus muscle of the eye has evolved into an exclusively heat generating structure that keeps the brain and eye warm.

128. Swordfish, marlin, sailfish http://www.fishingmaui.com/gamefish/blue_marlin_hawaii.jpg Blue Marlin

129. Temperature regulation of aquatic vertebrates: fish  Being able to keep portions of the body warm is a big advantage to these fish.  Because of their high core temperature tuna muscles can work more efficiently and the fish can swim much faster.  It also allows the fish to enter cold water that would otherwise affect their body functions. Swordfish, which diver deeper and spend more time in cold water, have better heater organs than do marlin and sailfish which spend less time in cold water.

130. Temperature regulation of aquatic vertebrates: mammals  Because aquatic mammals breathe using lungs they don’t risk losing heat through blood flow to gills and can keep the whole body at a high temperature.  Fully aquatic mammals such as cetaceans (whales and dolphins) and seals use a thick layer of blubber as insulation and countercurrent heat exchangers limit heat loss from the flippers.

131. Temperature regulation of aquatic vertebrates: mammals  Semi-aquatic mammals (e.g. beavers and otters) use thick, water-repellant fur coats to trap air, which is a good insulator.  Similarly, diving birds trap air in their feathers for the same purpose.

132. Importance of body size  Because surface area increases as a square function of a linear dimension, but volume increases as a cube function, larger animals have proportionally less surface area than smaller animals of the same shape.  It is not surprising therefore that selection has favored large body size in marine mammals and birds.

133. Importance of body size  Large body size also plays a big role in temperature regulation of leatherback turtles (the largest of all marine turtles at up to 850 kg).  Leatherbacks are the most specialized turtles and have replaced their shell with a a leathery body covering.

134. http://mvyps.org/~John_Nelson/01033434-000F4C52.3/Turtle_leatherback-jso1.jpg Leatherback Turtle

135. Importance of body size  Leatherbacks are pelagic and range from Alaska and Norway south to the tips of South Africa and South America where water is often frigid.  The turtle’s large body size and heat exchangers in the flippers however, enable the animal to maintain a body temperature 18ºC higher than surrounding water.  Other species of turtles are no more than half the size of leatherbacks and are confined to much warmer waters because they cannot maintain a large temperature difference between themselves and the environment.