Vertebrate Life 9th Edition PDF - [88-93]

Table 4–1 The physical properties of freshwater and air at 20°C. Most of these properties change with temperature and atmospheric pressure, and some are aﬀected by the presence of solutes as well. Property Freshwater Air Comparison Density 1 kg. liter-1 0.0012 kg. liter-1 Water is about 800 times as dense as air. Dynamic viscosity 1 mPa. s 0.018 mPa. s Water is about 55 times as viscous as air. Oxygen content 6.8 ml. liter 209 ml. liter The oxygen content of freshwater decreases from about 15 ml. liter-1 at 0°C to about 5 ml. liter-1 at 40°C. Seawater contains less oxygen than freshwater—5.2 ml. liter-1 at 20°C. Heat capacity 4.18 kJ. kg-1. °K-1 0.0012 kJ. kg-1. °K-1 The heat capacity of water is about 3500 times that of air. Heat conductivity 0.58 W. m-1. °K-1 0.024 W. m-1. °K-1 Water conducts heat about 24 times as fast as air. -1 -1 Obtaining Oxygen in Water—Gills Most aquatic vertebrates have gills, which are specialized structures where oxygen and carbon dioxide are exchanged. Teleosts are derived ray-ﬁnned ﬁshes, and this group includes the majority of species of extant freshwater and marine ﬁshes. The gills of teleosts are enclosed in pharyngeal pockets called the opercular cavities (Figure 4–1). The ﬂow of water is usually unidirectional—in through the mouth and out through the gills. Flaps just inside the mouth and ﬂaps at the margins of the gill covers (opercula, singular operculum) of bony ﬁshes act as valves to prevent backﬂow. The respiratory surfaces of the gills are delicate projections from the lateral side of each gill arch. Two columns of gill ﬁlaments extend from each gill arch. The tips of the ﬁlaments from adjacent arches meet when the ﬁlaments are extended. As water leaves the buccal cavity, it passes over the ﬁlaments. Gas exchange takes place at the numerous microscopic projections from the ﬁlaments called secondary lamellae. The pumping action of the mouth and opercular cavities (buccal pumping) creates a positive pressure across the gills so that the respiratory current is only slightly interrupted during each pumping cycle. Some ﬁlter-feeding ﬁshes and many pelagic ﬁshes—such as mackerel, certain sharks, tunas, and swordﬁshes— have reduced or even lost the ability to pump water across the gills. These ﬁshes create a respiratory current by swimming with their mouths open, a method known as ram ventilation, and they must swim continuously. Many other ﬁshes rely on buccal pumping when they are at rest and switch to ram ventilation when they are swimming. The arrangement of blood vessels in the gills maximizes oxygen exchange. Each gill ﬁlament has two arteries, an aﬀerent vessel running from the gill arch to the ﬁlament tip and an eﬀerent vessel returning blood to the arch. Each secondary lamella is a blood space connecting the afferent and efferent vessels (Figure 4–2 on page 75). The direction of blood flow through the lamellae is opposite to the direction of water ﬂow across the gill. This arrangement, known as countercurrent exchange, assures that as much oxygen as possible diﬀuses into the blood. Pelagic ﬁshes such as tunas, which sustain high levels of activity for long periods, have skeletal tissue reinforcing the gill ﬁlaments, large gill exchange areas, and a high oxygen-carrying capacity per milliliter of blood compared with sluggish bottom-dwelling fishes, such as toadfishes and flat fishes (Table 4–2 on page 76). Obtaining Oxygen from Air—Lungs and Other Respiratory Structures Although the vast majority of ﬁshes depend on gills to extract dissolved oxygen from water, ﬁshes that live in water with low oxygen levels cannot obtain enough oxygen via gills alone. These ﬁshes supplement the oxygen they get from their gills with additional oxygen obtained from the air via lungs or accessory air respiratory structures. The accessory surfaces used to take up oxygen from air include enlarged lips that are extended just above the water surface and a variety of internal structures into which air is gulped. The anabantid ﬁshes of tropical Asia (including the bettas and gouramies seen in pet stores) have vascularized chambers in the rear of the head, called labyrinths. Air is sucked into the mouth and transferred to the labyrinth, where gas exchange takes place. Many of these ﬁshes are facultative air breathers; that is, they switch oxygen uptake from their gills to accessory respiratory structures when the The Aquatic Environment 73 LATERAL VIEW OF HEAD Gill filaments Water flow Gill arch Efferent artery (to dorsal aorta) Mouth Secondary lamellae Buccal cavity Gill arch Gill filaments (primary lamellae) Gill skeleton Gill slit Gill filament ORAL CAVITY Operculum Water flow Opercular cavity DIAGRAMMATIC HORIZONTAL SECTION THROUGH HEAD (a) Gill arch Opercular cavity Afferent artery (from ventral aorta) DETAIL OF GILL FILAMENTS (b) Figure 4–1 Anatomy of bony fish gills. (a) Position of gills in head and general ﬂow of water; (b) countercurrent ﬂow of water (colored arrows) and blood (black arrows) through the gills. level of oxygen in the water becomes low. Others, like the electric eel and some of the snakeheads, are obligatory air breathers. The gills alone cannot meet the respiratory needs of these ﬁshes, even if the water is saturated with oxygen, and they drown if they cannot reach the surface to breathe air. We think of lungs as being the respiratory structures used by terrestrial vertebrates, as indeed they are, but lungs ﬁrst appeared in ﬁshes and preceded the evolution of tetrapods by millions of years. Lungs develop embryonically as outpocketings (evaginations) of the pharyngeal region of the digestive tract, originating from its ventral or dorsal surface. The lungs of bichirs (a group of air-breathing ﬁshes from Africa), lungﬁshes, and tetrapods originate from the ventral surface of the gut, whereas the lungs of gars (a group of primitive bony ﬁshes) and the lungs of the derived bony ﬁshes known as teleosts originate embryonically from its dorsal surface. 74 CHAPTER 4 Living in Water Lungs used for gas exchange need a large surface area, which is provided by ridges or pockets in the wall. This structure is known as an alveolar lung, and it is found in gars, lungﬁshes, and tetrapods. Increasing the volume of the lung by adding a second lobe is another way to increase the surface area, and the lungs of lungﬁshes and tetrapods consist of two symmetrical lobes. (Bichirs have non-alveolar lungs with two lobes, but one lobe is much smaller than the other; gars have single-lobed alveolar lungs.) Adjusting Buoyancy Holding a bubble of air inside the body changes the buoyancy of an aquatic vertebrate, and bichirs and teleost ﬁshes use the lungs and swim bladders to regulate their position in the water. Air-breathing aquatic vertebrates (whales, dolphins, seals, and penguins, for example) can adjust their buoyancy by altering the volume of air in their lungs when they dive. Water entering gill (low oxygen) Water leaving gill (low oxygen) Water current Blood leaving gill (high oxygen) Secondary lamellae Cross section of gill filament Blood entering gill (low oxygen) (a) Blood flow through secondary lamellae Water current 15 40 70 100 Percent saturation Diffusion gradient 5 (b) 30 60 90 Percent saturation Blood flow Water current 50 50 65 100 Percent saturation Diffusion gradient 50 (c) 50 35 Percent saturation 5 Blood flow Figure 4–2 Countercurrent exchange in the gills of bony fishes. (a) The direction of water ﬂow across the gill opposes the ﬂow of blood through the secondary lamellae. Blood cells are separated from oxygen-rich water only by the thin epithelial cells of the capillary wall, as shown in the cross section of a secondary lamella. (b) Countercurrent ﬂow maintains a diﬀerence in oxygen concentration (a diﬀusion gradient) between blood and water for the full length of the lamella and results in a high oxygen concentration in the blood leaving the gills. (c) If water and blood ﬂowed in the same direction, the diﬀerence in oxygen concentration and the diﬀusion gradient would be high initially, but would drop to zero as the concentration of oxygen equalized. No further exchange of oxygen would occur, and the blood leaving the gills would have a low oxygen concentration. Bony Fishes Many bony ﬁshes are neutrally buoyant (i.e., have the same density as water). These ﬁshes do not have to swim to maintain their vertical position in the water column. The only movement they make when at rest is backpedaling of the pectoral ﬁns to counteract the forward thrust produced by water as it is ejected from the gills and a gentle undulation of the tail ﬁn to keep them level in the water. Fishes capable of hovering in the water like this usually have welldeveloped swim bladders. The swim bladder is located between the peritoneal cavity and the vertebral column (Figure 4–3). The bladder occupies about 5 percent of the body volume of marine teleosts and 7 percent of the volume of freshwater teleosts. The diﬀerence in volume corresponds to the diﬀerence in density of salt water and freshwater—salt water is denser, so a smaller swim bladder is suﬃcient. The swim bladder wall, which has smooth walls composed of interwoven collagen ﬁbers without blood vessels, is virtually impermeable to gas. Neutral buoyancy produced by a swim bladder works as long as a ﬁsh remains at one depth, but if a ﬁsh swims vertically up or down, the hydrostatic pressure that the surrounding water exerts on the bladder changes, which in turn changes the volume of the bladder. For example, when a ﬁsh swims deeper, the additional pressure of the water column above it compresses the gas in its swim bladder, making the bladder smaller and reducing the buoyancy of the ﬁsh. When the ﬁsh swims toward the surface, water pressure decreases, the swim bladder expands, and the ﬁsh becomes more buoyant. To maintain neutral buoyancy, a ﬁsh must adjust the volume of gas in its swim bladder as it changes depth. A bony ﬁsh regulates the volume of its swim bladder by secreting gas into the bladder to counteract the increased external water pressure when it swims down and removing gas when it swims up. Primitive teleosts— such as bony tongues, eels, herrings, anchovies, salmon, and minnows—retain a connection, the pneumatic duct, between the gut and swim bladder (see Figure 4–3a). These ﬁshes are called physostomous (Greek phys = bladder and stom = mouth), and goldﬁsh are a familiar example of this group. Because they have a connection between the gut and the swim bladder, they can gulp air at the surface to ﬁll the bladder and can burp gas out to reduce its volume. The pneumatic duct is absent in adult teleosts from more derived clades, a condition termed physoclistous (Greek clist = closed). Physoclists regulate the volume of the swim bladder by secreting gas from the blood into the bladder. Both physostomes and physoclists have a gas gland, which is located in the anterior ventral ﬂoor of the swim bladder (see Figure 4–3b). Underlying the gas gland is an area with many capillaries arranged to give countercurrent ﬂow of blood entering The Aquatic Environment 75 Table 4–2 Anatomical and physiological characteristics of three types of fishes Oxygen Consumption (ml O2. g–1. h–1) Gill Area (mm2. g body mass–1) Oxygen Capacity (ml O2. 100 ml blood–1) High, swims continuously 0.73 1160 14.8 Porgy (Stenotomus) Intermediate 0.17 506 7.3 Toadfish (Opsanus) Sluggish, bottom dweller 0.11 197 6.2 Species of Fishes Activity Mackerel (Scomber) and leaving the area. This structure, which is known as a rete mirabile (“wonderful net,” plural retia mirabilia), moves gas (especially oxygen) from the blood to the gas bladder. It is remarkably eﬀective at extracting oxygen from the blood and releasing it into the swim bladder, even when the pressure of oxygen in the bladder is many times higher than its pressure in blood. Gas secretion occurs in many deep-sea ﬁshes despite the hundreds of atmospheres of gas pressure within the bladder. The gas gland secretes oxygen by releasing lactic acid and carbon dioxide, which acidify the blood in the rete mirabile. Acidiﬁcation causes hemoglobin to release oxygen into solution (the Bohr and Root eﬀects). Because of the anatomical relations of the rete mirabile, which folds back upon itself in a countercurrent multiplier arrangement, oxygen released from the hemoglobin accumulates and is retained within the rete until its pressure exceeds the oxygen pressure in Swim bladder Hemoglobin saturation Normal Stomach Pneumatic duct Intestines Dorsal aorta To heart (c) Liv e r Constrictor muscles Gas gland (b) Oxygen pressure Ovale Rete mirabile with parallel vessels dissected apart Hemoglobin saturation (a) Bohr effect Normal (d) Figure 4–3 Swim bladder of bony fishes. (a) The swim bladder is in the coelomic cavity just beneath the vertebral column. This is a physostomous ﬁsh, in which the swim bladder retains its ancestral connection to the gut via the pneumatic duct. (b) The vascular connections of a physoclistous swim bladder, which has lost its connection to the gut. (c) The Bohr eﬀect is a reduction in the aﬃnity of hemoglobin for oxygen in the presence of acid. By creating a Bohr eﬀect, the gas gland causes hemoglobin to release oxygen (i.e., to bind less oxygen). (d) The Root eﬀect is a reduction in the maximum amount of oxygen that hemoglobin can bind. By creating a Root eﬀect, the gas gland prevents oxygen in the gland from binding to hemoglobin in the blood. As a result, the oxygen pressure in the gas gland rises, and oxygen is released into the swim bladder. 76 CHAPTER 4 Living in Water Root effect Oxygen pressure the swim bladder. At this point oxygen diﬀuses into the bladder, increasing its volume. The maximum multiplication of gas pressure that can be achieved is proportional to the length of the capillaries of the rete mirabile, and deep-sea ﬁshes have very long retia. A large Root eﬀect is characteristic only of the blood of ray-ﬁnned ﬁshes, and it is essential for the function of the gas gland. Physoclistous ﬁshes have no connection between the swim bladder and the gut, so they cannot burp to release excess gas from the bladder. Instead, physoclists open a muscular valve, called the ovale, located in the posterior dorsal region of the bladder adjacent to a capillary bed. The high internal pressure of oxygen in the bladder causes it to diffuse into the blood of this capillary bed when the ovale sphincter is opened. Cartilaginous Fishes Sharks, rays, and ratﬁshes do not have swim bladders. Instead, these ﬁshes use the liver to create neutral buoyancy. The average tissue densities of sharks with their livers removed are heavier than water—1.06 to 1.09 grams per milliliter compared to about 1.025 grams per milliliter for seawater. The liver of a shark, however, is well known for its high oil content (shark-liver oil). Shark-liver tissue has a density of only 0.95 gram per milliliter, which is lighter than water, and the liver may contribute as much as 25 percent of the body mass. A 4-meter tiger shark (Galeocerdo cuvieri) weighing 460 kilograms on land may weigh as little as 3.5 kilograms in the sea. Not surprisingly, bottom-dwelling sharks, such as nurse sharks, have livers with fewer and smaller oil vacuoles in their cells, and these sharks are negatively buoyant. Nitrogen-containing compounds in the blood of cartilaginous ﬁshes also contribute to their buoyancy. Urea and trimethylamine oxide in the blood and muscle tissue provide positive buoyancy because they are less dense than an equal volume of water. Chloride ions, too, are lighter than water and provide positive buoyancy, whereas sodium ions and protein molecules are denser than water and are negatively buoyant. Overall these solutes provide positive buoyancy. Deep-Sea Fishes Many deep-sea ﬁshes have deposits of light oil or fat in the gas bladder, and others have reduced or lost the gas bladder entirely and have lipids distributed throughout the body. These lipids provide static lift, just like the oil in shark livers. Because a smaller volume of the bladder contains gas, the amount of secretion required for a given vertical descent is less. Nevertheless, a long rete mirabile is needed to secrete oxygen at high pressures, and the gas gland in deepsea ﬁshes is very large. Fishes that migrate over large vertical distances depend more on lipids such as wax esters than on gas for buoyancy, whereas their close relatives that do not undertake such extensive vertical movements depend more on gas for buoyancy. Air-Breathing Divers Air in the lungs of air-breathing aquatic vertebrates reduces their density. Unlike most ﬁshes, air-breathing vertebrates must return to the surface at intervals, so they do not hover at one depth in the water column. Deep-diving animals, such as elephant seals and some whales and porpoises, face a different problem, however. These animals dive to depths of 1000 meters or more and are subjected to pressures more than 100 times higher than at the surface. Under those conditions, nitrogen would be forced from the air in the lungs into solution in the blood and carried to the tissues at high pressure. When the animal rose toward the surface, the nitrogen would be released from solution. If the animal moved upward too fast, the nitrogen would form bubbles in the tissues—this is what happens when human deep-sea divers get “the bends” (decompression sickness). Specialized diving mammals avoid the problem by allowing the thoracic cavity to collapse as external pressure rises. Air is forced out of the lungs as they collapse, reducing the amount of nitrogen that diﬀuses into the blood. Even these specialized divers would have problems if they made repeated deep dives, however; a deep dive is normally followed by a period during which the animal remains near the surface and makes only shallow dives until the nitrogen level in its blood has equilibrated with the atmosphere. Despite these specializations, the bones of sperm whales, which dive to depths of 2000 m, contain areas of dead tissue caused by the bends. 4.2 Water and the Sensory World of Fishes Water has properties that inﬂuence the behaviors of ﬁshes and other aquatic vertebrates. Light is absorbed by water molecules and scattered by suspended particles. Objects become invisible at a distance of a few hundred meters even in the very clearest water, whereas distance vision is virtually unlimited in clear air. Fishes supplement vision with other senses, some of which can operate only in water. The most important of these aquatic senses is mechanical and consists of detecting water movement via the lateral line system. Small currents of water can stimulate the sensory organs of the lateral line because water is dense and Water and the Sensory World of Fishes 77 viscous. Electrical sensitivity is another sensory mode that depends on the properties of water and does not operate in air. In this case it is the electrical conductivity of water that is the key. Even vision is diﬀerent in water and air because of the diﬀerent refractive properties of the two media. Vision Vertebrates generally have well-developed eyes, but the way an image is focused on the retina is diﬀerent in terrestrial and aquatic animals. Air has an index of refraction of 1.00, and light rays bend as they pass through a boundary between air and a medium with a diﬀerent refractive index. The amount of bending is proportional to the diﬀerence in indices of refraction. Water has a refractive index of 1.33, and the bending of light as it passes between air and water causes underwater objects to appear closer to an observer in air than they really are. The corneas of the eyes of terrestrial and aquatic vertebrates have an index of refraction of about 1.37, so light is bent as it passes through the air-cornea interface. As a result, the cornea of a terrestrial vertebrate plays a substantial role in focusing an image on the retina. This relationship does not hold in water, however, because the refractive index of the cornea is too close to that of water for the cornea to have much eﬀect in bending light. The lens plays the major role in focusing light on the retina of an aquatic vertebrate, and ﬁshes have spherical lenses with high refractive indices. The entire lens is moved toward or away from the retina to focus images of objects at diﬀerent distances from the ﬁsh. Terrestrial vertebrates have ﬂatter lenses, and muscles in the eye change the shape of the lens to focus images. Aquatic mammals such as whales and porpoises have spherical lenses like those of ﬁshes. Chemosensation: Taste and Odor Fishes have taste-bud organs in the mouth and around the head and anterior ﬁns. In addition, olfactory organs on the snout detect soluble substances. Sharks and salmon can detect odors at concentrations of less than 1 part per billion. Sharks, and perhaps bony ﬁshes, compare the time of arrival of an odor stimulus on the left and right sides of the head to locate the source of the odor. Homeward-migrating salmon are directed to their stream of origin from astonishing distances by a chemical signature from the home stream that was permanently imprinted when they were juveniles. Plugging the nasal olfactory organs of salmon destroys their ability to home. 78 CHAPTER 4 Living in Water Touch Mechanical receptors detect touch, sound, pressure, and motion. Like all vertebrates, ﬁshes have an internal ear (the labyrinth organ, not to be confused with the organ of the same name that assists in respiration in anabantid ﬁshes) that detects changes in speed and direction of motion. Fishes also have gravity detectors at the base of the semicircular canals that allow them to distinguish up from down. Most terrestrial vertebrates also have an auditory region of the inner ear that is sensitive to sound-pressure waves. These diverse functions of the labyrinth depend on basically similar types of sense cells, the hair cells (Figure 4–4). In ﬁshes and aquatic amphibians, clusters of hair cells and associated support cells form neuromast organs that are dispersed over the surface of the head and body. In jawed ﬁshes, neuromast organs are often located in a series of canals on the head, and one or more canals pass along the sides of the body onto the tail. This surface receptor system of ﬁshes and aquatic amphibians is referred to as the lateral line system. Lateral line systems are found only in aquatic vertebrates because air is not dense enough to stimulate the neuromast organs. Amphibian larvae have lateral line systems, and permanently aquatic species of amphibians, such as African clawed frogs and mudpuppies, retain lateral lines throughout their lives. Terrestrial species of amphibians lose their lateral lines when they metamorphose into adults, however, and terrestrial vertebrates that have secondarily returned to the water, such as whales and porpoises, do not have lateral line systems. Detecting Water Displacement Neuromasts of the lateral line system are distributed in two conﬁgurations—within tubular canals or exposed in epidermal depressions. Many kinds of ﬁshes have both arrangements. Hair cells have a kinocilium placed asymmetrically in a cluster of kinocilia. Hair cells are arranged in pairs with the kinocilia positioned on opposite sides of adjacent cells. A neuromast contains many such hair-cell pairs. Each neuromast has two aﬀerent nerves: one transmits impulses from hair cells with kinocilia in one orientation, and the other carries impulses from cells with kinocilia positions reversed by 180 degrees. This arrangement allows a ﬁsh to determine the direction of displacement of the kinocilia. All kinocilia and microvilli are embedded in a gelatinous secretion, the cupula (Latin cupula = a small tub). Displacement of the cupula causes the kinocilia to bend. The resultant deformation either excites or

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