Vertebrate Life PDF - Page 79-99

Pore in skin surface Canal Lateral line Neuromast organ Pores in skin surface Canal Neuromast organ Nerve (a) Cupula Hair-cell receptor potential Hyperpolarization Depolarization Kinocilia Excitation Recording sites Static discharge Inhibition Nerve impulse discharge (b) Figure 4–4 Lateral line syst...

Pore in skin surface Canal Lateral line Neuromast organ Pores in skin surface Canal Neuromast organ Nerve (a) Cupula Hair-cell receptor potential Hyperpolarization Depolarization Kinocilia Excitation Recording sites Static discharge Inhibition Nerve impulse discharge (b) Figure 4–4 Lateral line systems. (a) Semidiagrammatic representations of the two configurations of lateral line organs in fishes. (b) Hair-cell deformations and their effect on hair-cell transmembrane potential (receptor potential) and afferent nerve-cell discharge rates. Deflection of the kinocilium (dark line) in one direction (the right in this diagram) depolarizes the cell and increases the discharge rate (excitation). Deflection of the kinocilium in the opposite direction (to the left in the diagram) hyperpolarizes the cell and reduces the discharge rate (inhibition). inhibits the neuromast’s nerve discharge. Each haircell pair, therefore, signals the direction of cupula displacement. The excitatory output of each pair has a maximum sensitivity to displacement along the line joining the kinocilia, and falling off in other directions. The net effect of cupula displacement is to increase the firing rate in one afferent nerve and to decrease it in the other nerve. These changes in lateral line nerve firing rates thus inform a fish of the direction of water currents on different surfaces of its body. Several surface-feeding fishes and African clawed frogs provide vivid examples of how the lateral line Water and the Sensory World of Fishes 79 organs act under natural conditions. These animals find insects on the water surface by detecting surface waves created by their prey’s movements. Each neuromast group on the head of the killifish, Aplocheilus lineatus, provides information about surface waves coming from a different direction (Figure 4–5). The groups of neuromasts have overlapping stimulus fields, allowing the fish to determine the precise (a) Freestanding organs Nasal organs Supraorbital canal Postorbital canal (b) Left nasal field Left orbital field Right supraorbital field Figure 4–5 Distribution of the lateral line canal organs. (a) The dorsal surface of the head of the killifish Fundulus notatus. (b) The sensory fields of the head canal organs in a different species of killifish, Aplocheilus lineatus. The wedgeshaped areas indicate the fields of view for each group of canal organs. Note that fields overlap on opposite sides as well as on the same side of the body, allowing the lateral line system to localize the source of a water movement. 80 CHAPTER 4 Living in Water location of the insect. Removing a neuromast group from one side of the head disturbs the directional response to stimuli, showing that a fish combines information from groups on both sides of the head to interpret water movements. The large numbers of neuromasts on the heads of some fishes might be important for sensing vortex trails in the wakes of adjacent fishes in a school. Many of the fishes that form extremely dense schools (herrings, atherinids, mullets) lack lateral line organs along the flanks and retain canal organs only on the head. These well-developed cephalic canal organs concentrate sensitivity to water motion in the head region, where it is needed to sense the turbulence into which the fish is swimming, and the reduction of flank lateral line elements would reduce noise from turbulence beside the fish. Electrical Discharge Unlike air, water conducts electricity, and the torpedo ray of the Mediterranean, the electric catfish from the Nile River, and the electric eel of South America can discharge enough electricity to stun prey animals and deter predators. The weakly electric knifefishes (Gymnotidae) of South America and the elephant fishes (Mormyridae) of Africa use electrical signals for courtship and territorial defense. All of these electric fishes use modified muscle tissue to produce the electrical discharge. The cells of such modified muscles, called electrocytes, are muscle cells that have lost the capacity to contract and are specialized for generating an ion current flow (Figure 4–6). When at rest, the membranes of muscle cells and nerve cells are electrically charged, with the intracellular fluids about 84 millivolts more negative than the extracellular fluids. The imbalance is primarily due to sodium ion exclusion. When the cell is stimulated, sodium ions flow rapidly across the smooth surface, sending its potential to a positive 67 millivolts. Only the smooth surface depolarizes; the rough surface remains at –84 millivolts, so the potential difference across the cell is 151 millivolts (from –84 to +67 millivolts). Because electrocytes are arranged in stacks like the batteries in a flashlight, the potentials of many layers of cells combine to produce high voltages. The South American electric eel has up to 10,000 layers of cells and can generate potentials in excess of 600 volts. Most electric fishes are found in tropical freshwaters of Africa and South America. Few marine forms can generate specialized electrical discharges—among marine cartilaginous fishes, only the torpedo ray (Torpedo), the ray genus Narcine, and some skates are electric; and +− −+ +− −+ + +K − −+ − 84 mV +− − + − 84 mV Na+ high +− −+ outside cell Resting (a) electrocyte −+ Na+ K+ high inside cell −+ −+ −+ − 84 mV −+ −+ −+ K+ −+ + 67 mV −+ Na+ − + Active electrocyte (b) (c) At the peak of nerve stimulation At rest Tail +− +− +− +− −+ + −+ −+ −+ +− +− +− +− −+ −+ −+ −+ −+ +− +− +− +− −+ −+ −+ −+ −+ Space outside cells Electrocyte Resting electrocytes (d) −+ −+ −+ −+ −+ −+ −+ −+ −+ −+ −+ −+ −+ − − − − − + + + + + Active electrocytes −+ −+ −+ −+ Spinal motor neurons + − Head −+ −+ −+ −+ Positive current flow during discharge © 1994 American Institute of Physics Figure 4–6 Weakly electric fishes. Some fishes use transmembrane potentials of modified muscle cells to produce a discharge. In this diagram the smooth surface is on the left and the rough surface on the right. Only the smooth surface is innervated. (a) At rest, K+ (potassium ion) is maintained at a high internal concentration and Na+ (sodium ion) at a low internal concentration by the action of a Na+/K+ cell-membrane pump. Permeability of the membrane to K+ exceeds the permeability to Na+. As a result, K+ diffuses outward faster than Na+ diffuses inward (arrow) and sets up the –84-mV resting potential. (b) When the smooth surface of the cell is stimulated by the discharge of the nerve, Na+ diffuses into the cell and K+ diffuses out of the smooth surface, changing the net potential to +67 mV. The rough surface does not depolarize and retains a –84-mV potential, creating a potential difference of 151 mV across the cell. (c) A weakly electric South American gymnotid, showing the location of electrocytes along the sides of the body. (d) By arranging electrocytes in series so that the potentials of individual cells are summed, some electric fishes can generate very high voltages. Electric eels, for example, have 10,000 electrocytes in series and produce potentials in excess of 600 volts. among marine teleosts, only the stargazers (family Uranoscopidae) produce specialized discharges. Electroreception by Sharks and Rays The high conductivity of seawater makes it possible for sharks to detect the electrical activity that accompanies muscle contractions of their prey. Sharks have structures known as the ampullae of Lorenzini on their heads, and rays have them on the pectoral fins as well. The ampullae are sensitive electroreceptors (Figure 4–7). The canal connecting the receptor to the surface pore is filled with an electrically conductive gel, and the wall of the canal is nonconductive. Because the canal runs for some distance beneath the epidermis, the sensory cell can detect a difference in electrical potential between the tissue in which it lies (which reflects the adjacent epidermis and environ- ment) and the distant pore opening. Thus, it can detect electric fields, which are changes in electrical potential in space. Electroreceptors of sharks respond to minute changes in the electric field surrounding an animal. They act like voltmeters, measuring differences in electrical potentials across the body surface. Ampullary organs are remarkably sensitive, with thresholds lower than 0.01 microvolt per centimeter, a level of detection achieved by only the best voltmeters. Sharks use their electrical sensitivity to detect prey. All muscle activity generates electrical potential: motor nerve cells produce extremely brief changes in electrical potential, and muscular contraction generates changes of longer duration. In addition, a steady potential issues from an aquatic organism as a result of Water and the Sensory World of Fishes 81 livolt per centimeter—well above the level that can be detected by ampullary organs. In addition, ocean currents generate electrical gradients as large as 0.5 millivolt per centimeter as they carry ions through Earth’s magnetic field. Nostril Gelfilled canal Nerves (a) (b) Sensory cell Epidermis Surface pore Figure 4–7 Ampullae of Lorenzini. (a) Distribution of the ampullae on the head of a spiny dogfish, Squalus acanthias. Open circles represent the surface pores; the black dots are positions of the sensory cells. (b) A single ampullary organ consists of a sensory cell connected to the surface by a pore filled with a substance that conducts electricity. Electrolocation by Teleosts Unusual arrangements of electrocytes are present in several species of freshwater fishes that do not produce electric shocks. In these fishes—which include the knifefishes (Gymnotidae) of South America and the elephant fishes (Mormyridae) of Africa—the discharge voltages are too small to be of direct defensive or offensive value. These weakly electric fishes are mostly nocturnal and usually live in turbid waters where vision is limited to short distances even in daylight; they use their discharges for electrolocation and social communications. When a fish discharges its electric organ, it creates an electric field in its immediate (a) the chemical imbalance between the organism and its surroundings. A shark can locate and attack a hidden fish by relying only on this electrical activity (Figure 4–8). Sharks may use electroreception for navigation as well as for locating prey. The electromagnetic field at Earth’s surface produces tiny voltage gradients, and a swimming shark could encounter gradients as large as 0.4 mil- Figure 4–8 Electrolocation capacity of sharks. (a) A shark can locate a live fish concealed from sight beneath the sand. (b) The shark can still detect the fish when it is covered by an agar shield that blocks olfactory cues but allows the electrical signal to pass. (c) The shark follows the olfactory cues (displaced by the agar shield) when the live fish is replaced by chopped bait that produces no electrical signal. (d) The shark is unable to detect a live fish when it is covered by a shield that blocks both olfactory cues and the electrical signal. (e) The shark attacks electrodes that give off an electrical signal duplicating a live fish without producing olfactory cues. These experiments indicate that when the shark was able to detect an electrical signal, it used that to locate the fish—and it was also capable of homing in on a chemical signal when no electrical signal was present. This dual system allows sharks to find both living and dead food items. 82 CHAPTER 4 Living in Water Point of attack Live flounder (b) Agar shield No vertical olfactory cue (c) Chopped bait No electrical cues Displaced olfactory cue (d) Electrical insulation No attack (e) Live electrodes Each accurately attacked vicinity (Figure 4–9). Because of the high energy costs of maintaining a continuous discharge, electric fishes produce a pulsating discharge. Most weakly electric teleost fishes pulse at rates between 50 and 300 cycles per second, but the knifefishes of South America reach 1700 cycles per second, which is the most rapid continuous firing rate known for any vertebrate muscle or nerve. The electric field from even weak discharges may extend outward for a considerable distance in freshwater because electrical conductivity is relatively low. The electric field the fish creates will be distorted by the presence of electrically conductive and resistant objects. Rocks are highly resistive, whereas other fishes, (a) (b) © 2006 The Company of Biologists Ltd. invertebrates, and plants are conductive. Distortions of the field cause a change in the distribution of electrical potential across the fish’s body surface. An electric fish detects the presence, position, and movement of objects by sensing where on its body maximum distortion of its electric field occurs. The skin of weakly electric teleosts contains special sensory receptors: ampullary organs and tuberous organs. These organs detect tonic (steady) and phasic (rapidly changing) discharges, respectively. Electroreceptors of teleosts are modified lateral line neuromast receptors. Like lateral line receptors, they have double innervation—an afferent channel that sends impulses to the brain and an efferent channel that causes inhibition of the receptors. During each electric organ discharge, an inhibitory command is sent to the electroreceptors, and the fish is rendered insensitive to its own discharge. Between pulses, electroreceptors report distortion in the electric field or the presence of a foreign electric field to the brain. Electric organ discharges vary with habits and habitat. Species that form groups or live in shallow, narrow streams generally have discharges with high frequency and short duration. These characteristics reduce the chances of interference from the discharges of neighbors. Territorial species, in contrast, have long electric organ discharges. Electric organ discharges vary from species to species. In fact, some species of electric fishes were first identified by their electric organ discharges, which were recorded by placing electrodes in water that was too murky for any fishes to be visible. During the breeding season, electric organ discharges distinguish immature individuals, females with eggs, and sexually active males. Other Electroreceptive Vertebrates Electrogenesis and (c) Figure 4–9 Weakly electric fishes. An electric field surrounds a weakly electric fish. Electroreceptors in the skin allow a fish to detect the presence of nearby objects by sensing distortion of the lines of electrical force. (a) Nonconductive objects, such as rocks, spread the field and diffuse potential differences along the body surface. (b) Conductive objects, such as another fish, concentrate the field on the skin of the fish. (c) When two electric fish swim close enough to each other to create interference between their electric fields, they change the frequencies of their discharges. electroreception are not restricted to a single group of aquatic vertebrates, and monotremes (the platypus and the echidna, early offshoots off the main mammalian lineages that still lay eggs) use electroreception to detect prey. Electrosensitivity was probably an early feature of vertebrate evolution. The brain of the lamprey responds to electric fields, and it seems likely that the earliest vertebrates had electroreceptive capacity. All fishlike vertebrates of lineages that evolved before the neopterygians have electroreceptor cells. These cells, which have a prominent kinocilium, fire when the environment around the kinocilium is negative relative to the cell. Their impulses pass to the midline region of the posterior third of the brain. Electrosensitivity was apparently lost in neopterygians, and teleosts have at least two separate new evolutions of electroreceptors. Electrosensitivity of teleosts is Water and the Sensory World of Fishes 83 distinct from that of other vertebrates: teleost electroreceptors lack a kinocilium and fire when the environment is positive relative to the cell, and nerve impulses are sent to the lateral portions of the brain rather than to the midline. 4.3 The Internal Environment of Vertebrates Seventy to eighty percent of the body mass of most vertebrates is water, and the chemical reactions that release energy or synthesize new chemical compounds take place in an aqueous environment. The body fluids of vertebrates contain a complex mixture of ions and other solutes. Some ions are cofactors that control the rates of metabolic processes; others are involved in the regulation of pH, the stability of cell membranes, or the electrical activity of nerves. Metabolic substrates and products must diffuse from sites of synthesis to the sites of utilization. Almost everything that happens in the body tissues of vertebrates involves water, and maintaining the concentrations of water and solutes within narrow limits is a vital activity. Water sounds like an ideal place to live for an animal that itself consists mostly of water, but in some ways an aquatic environment can be too much of a good thing. Freshwater vertebrates—especially fishes and amphibians— face the threat of being flooded with water that flows into them from their environment, and saltwater vertebrates must prevent the water in their bodies from being sucked out into the sea. Temperature, too, is a critical factor for living organisms because chemical reactions are temperature sensitive. In general, the rates of chemical reactions increase as temperature increases, but not all reactions have the same sensitivity to temperature. A metabolic pathway is a series of chemical reactions in which the product of one reaction is the substrate for the next, yet each of these reactions may have a different sensitivity to temperature, so a change in temperature can mean that too much or too little substrate is produced to sustain the next reaction in the series. To complicate the process of regulation of substrates and products even more, the chemical reactions take place in a cellular milieu that itself is changed by temperature because the viscosity of plasma membranes is also temperature sensitive. Clearly, the smooth functioning of metabolic pathways is greatly simplified if an organism can limit the range of temperatures its tissues experience. Water temperature is more stable than air temperature because water has a much higher heat capacity 84 CHAPTER 4 Living in Water than air. The stability of water temperature simplifies the task of maintaining a constant body temperature, as long as the body temperature the animal needs to maintain is the same as the temperature of the water around it. An aquatic animal has a hard time maintaining a body temperature different from water temperature, however, because water conducts heat so well. Heat flows out of the body if an animal is warmer than the surrounding water and into the body if the animal is cooler than the water. In the following sections, we discuss in more detail how and why vertebrates regulate their internal environments and the special problems faced by aquatic animals. 4.4 Exchange of Water and Ions An organism can be described as a leaky bag of dirty water. That is not an elegant description, but it accurately identifies the two important characteristics of a living animal—it contains organic and inorganic substances dissolved in water, and this fluid is enclosed by a permeable body surface. Exchange of matter and energy with the environment is essential to the survival of the organism, and much of that exchange is regulated by the body surface. Water molecules and ions pass through the skin quite freely, whereas larger molecules move less readily. The significance of this differential permeability is particularly conspicuous in the case of aquatic vertebrates, but it applies to terrestrial vertebrates as well. Vertebrates use both active and passive exchange to regulate their internal concentrations in the face of varying external conditions. The Vertebrate Kidney An organism can tolerate only a narrow range of concentrations of the body fluids and must eliminate waste products before they reach harmful levels. The molecules of ammonia that result from the breakdown of protein are especially important because they are toxic. Vertebrates have evolved superb capacities for controlling water balance and excreting wastes, and the kidney plays a crucial role in these processes. The adult vertebrate kidney consists of hundreds to millions of tubular nephrons, each of which produces urine. The primary function of a nephron is removing excess water, salts, waste metabolites, and foreign substances from the blood. In this process, the blood is first filtered through the glomerulus, a structure unique to