Chapter  4 - Living in Water

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4.1 THE AQUATIC ENVIRONMENT

1. Water versus Air [Box 4-1]
   1. Density
   2. Buoyancy
   3. Viscosity
   4. Oxygen content
   5. Heat Capacity and Heat Conductivity
   6. Electrical conductivity

ORGANISMIC RESPIRATION - GAS EXCHANGE IN ANIMALS

ANIMAL RESPIRATORY SYSTEMS 

• diffusion: process of gas exchange: O₂ in and CO₂ out.
• specialized structures required for larger animals; gas exchange region must be kept moist - O₂ and CO₂ dissolve in H₂O, thin, and be large relative to body size.
• respiration: gas exchange between organism and environment; organismic respiration - O₂ taken up and delivered to individual cells
• ventilation: active movement in and out

Obtaining Oxygen in Water

1. small: use body surface - e.g. hydras and planarians
2. gills: thin moist folds of tissue; gas exchange with water; large surface area
   1. gills in invertebrates
      1. outgrowths of body surface
      2. mollusk gills: ciliated for ventilation
   2. gills in craniates
      1. internal (usually) gill [external in larval amphibians] 
      2. gill filaments with secondary laminae [fig. 4-1, 4-2]
      3. pharyngeal (extension of the digestive tract)
   3. gills in bony fish
      1. located inside the pharyngeal cavity
      2. covered by the operculum.
         1. flap of tissue which covers and protects the gills. [fig 4-1]
      3. gill filaments with secondary laminae
         1. filaments are covered with a thin epidermal membrane that is folded repeatedly into platelike gill lamellae
         2. tissue richly supplied with blood vessels for gas exchange.
      4. counter-current exchange system [fig 4-2]
      5. buccal pumping of mouth and opercular cavitymoves water across gills
      6. ram ventilation in perpetually swimming [sharks, tuna] and filter feeding fishes
3. structures for breathing air in low oxygen conditions
   1. Vascularized mouths, swim bladders, stomachs, head chambers [anabantids]
      1. Facultative in most
      2. Obligatory air breathing in electric eels and anabantids
   2. Lungs
      1. Evaginations of the gut - found in primitive fishes, modified to form swimbladder

• lungs in fishes.  image © BIODIDAC. used with permission

 Terrestrial Animals

1. respiration of air versus water--see chapter 10
2. Body Surface
   1. capillary network in skin for gas exchange
   2. small animals with low metabolic rate e.g. earthworms, lungless salamanders
   3. supplementary structure for some fish, many amphibians
   4. must be kept moist
3. lungs: structures for breathing air
   1. fish: inpocketing of esophagus - modified to form swimbladder
   2. amphibians: simple lungs
   3. amphibians, reptiles, mammals: incomplete (tidal) ventilation
   4. birds
      1. highly modified system for high metabolic rate; complete ventilation; air sacs reach all parts of the body; one-way airflow thru parabronchi - countercurrent exchange
   5. amphibians: positive pressure
   6. reptiles, birds, mammals: negative pressure;
   7. mammals
      1. diaphragm: muscular floor of pleural cavity
      2. inhalation (inspiration): upward expansion (elevation) of rib cage; depression of diaphragm
      3. exhalation (expiration): reverse; changes pressure in lung cavity
4. hemoglobin: respiratory pigment present in all vertebrates

BUOYANCY

• tissue more dense than seawater
• sharks and some pelagic bony fish (e.g., tunas, mackerel, swordfish) are negatively buoyant and must swim to stay afloat. In sharks, heterocercal tail provides tail lift and pectoral fins act as angled planes to provide head lift.
• bony fish: swim bladders [Fig. 4.3]
  1. provides neutral buoyancy
  2. primitively a lung connected to esophagus by pneumatic duct; Some fish have to "gulp" air from above the water surface to fill the swim bladder. e.g. trout. The gulped air is forced into the swim bladder through a pneumatic duct which connects the swim bladder to the esophagus.
  3. The more advanced fishes have a gas gland which secretes gas into the swim bladder. It is heavily vascularized, has slow gas exchange with blood, and can not rapidly adjust; At -7000 m, air is compressed to 0.7 g/cc, replaced by equal density fat in abyssalpelagic fish.
  4. mesopelagic fish
     1. rapid expansion of air bladder causes death if rapid ascent; warmer temperatures lethal in slow ascent
  5. not present in actively swimming fish (tuna) or bottom dwellers

4.2 WATER AND THE SENSORY WORLD OF FISHES 

Receptors 

1. Eye
2. Chemoreceptors 
   1. Sharks can detect one part in 10 billion (1 drop in about 79,000 gallons of water). 
3. Mechanical receptors 
   1. Detection of water displacement 
   2. Neuromast organs [figure 4.4] are associated with 
   3. Lateral line system
      1. can detect vibrations of struggling fish in sharks
      2. used for schooling 
4. Electric discharge
   1. Modified muscle cells [fig. 4-6] in electric eels, electric catfish, etc.
   2. stun prey, deter predators, also courtship 
5. Electroreception by elasmobranchs 
   1. Ampullae of Lorenzini [fig. 4-7] electroreception for detection of prey, especially at close range 
   2. possibly for navigation; Electrolocation using the geomagnetic field [fig. 4-8] 
6. Electrolocation by teleosts 
   1. Pulsating electric field to detect objects, other fish [fig. 4-9] 
   2. Phylogenetic distribution of electosensitivity in vertebrates [fig. 4-10]

---

homeostasis:

maintenance of an internal steady state; maintain composition of extracellular fluids, adjust salt balance, etc.

Water Balance

EXCRETORY SYSTEM 

[link to the Online Biology Textbook by M. J. Farabee if you need some background]

kidneys: [link to Gondar Design Science]

main organ for disposal of nitrogenous wastes, water, salts; nonselective filtration and selective resorption by nephron of small molecules & water

1. osmoregulation: maintenance of water and salt balance by excretory system; retention & excretion
2. excretion: disposal of nitrogenous wastes.

nephron: functional unit of the kidney - produces urine

glomerulus (Fig. 4-11): site of blood filtration

convoluted tubules (Figs. 4-12): resorption, secretion, and excretion.

OSMOTIC PROCESSES

osmosis: movement of H₂O across semipermeable membranes

Osmoregulation of Ions and Body Fluids (Table 4.2)

1. isosmolal
   1. fluids are in equilibrium with sea water
   2. 1000 millimoles/kg of water
   3. most many invertebrates
   4. hagfish and probably ostracoderms
2. hyposmolal
   1. body fluids are less concentrated than sea water (water loss problem)
   2. 350-450 millimoles/kg of water
   3. lampreys, marine teleosts
3. slightly hyperosmal
   1. marine sharks and rays and the coelacanth
4. hyperosmal
   1. body fluids are more concentrated than fresh water (water gain problem)
   2. 200-300 millimoles/kg of water
   3. freshwater fish

salinity and distribution

1. stenohaline:
   1. most fish
   2. either inhabit salt or fresh water
2. euryhaline: ability to adapt to varying salinities
   1. estuarine (within limits)
   2. anadromous [salmon]
   3. catadromous [eels]; spawn in Sargasso Sea

Invasion of Fresh Water

freshwater bony fish (mostly teleosts) [Fig. 4-13 left]

1. ancestors of freshwater bony fish invaded fresh water by the Devonian
2. hypertonic (hyperosmotic) - water gain problem.
3. relatively impermeable scales and mucus covered integument
4. gills absorb water by osmosis and lose ions by diffusion
5. do not drink
6. large glomeruli excrete copious hypotonic urine
7. chlorine glands on gills absorb salt by active transport

amphibians

1. highly permeable skin
2. excretion by glomerular kidneys of copious dilute urine
3. skin actively transports NaCl in;

Acidity inhibits active transport of Na and Cl.  Acid precipitation causes death of amphibians and fresh water fish

Marine Fishes

marine bony fishes (teleosts) [Fig. 4-13 right]

1. ancestors evolved in freshwater
2. returned to sea in Triassic
3. hypotonic (hypososmotic) - water loss by osmosis problem
4. highly impermeable integument resists water loss
5. drink seawater
6. chloride cells in gills excrete NaCl, lose MgSO₄ in feces & urine
7. scant isotonic urine

cartilaginous fish (chondrichthyes) and coelacanth:

• shark osmoregulation [by Tim Cullinan & Jillian Lawson--University of New Brunswick Physiology of Marine Vertebrates]
• John Hayden's shark physiology web site: osmoregulation

1. salt concentration of blood similar to bony fish, but retain urea to increase osmolality
2. slightly hypertonic
   1. gain water- do not need to drink
3. copious hypotonic urine
4. Na content is hypotonic relative to seawater; absorb Na via gills
   1. chondrichthyans excrete Na from rectal gland
5. urea contributes to the buoyancy of sharks
6. most sharks also have large, oil filled livers to promote buoyancy

Salt and Water Balance in Terrestrial Animals

• marine & terrestrial mammals: concentrated hypertonic urine
• marine birds and reptiles: salt gland near eye [Salt Secretion Glands in Marine Birds -- Jennifer Cool & Dina Gagnon--University of New Brunswick] (Table 12.2)

EXCRETION OF NITROGENOUS WASTES (Fig. 4-14)

deamination of amino acids that are not immediately needed --> NH₂

Ammonia

in aquatic organisms (bony fish, aquatic invertebrates, larval amphibians); little energy to produce, toxic, but highly soluble

Urea:

terrestrial amphibians and mammals; less toxic; can be excreted in moderately concentrated forms; energy expensive to produce

Uric Acid

insects, reptiles, birds, dalmatians; not very toxic, low solubility; can be concentrated; reptiles and birds absorb water in cloaca, probably evolved for waste storage in allantois of amniotic egg; most energy intensive to produce;

TEMPERATURE

• critical regulator of distribution and activity

Definitions

1. poikilothermic
   1. variable body temperature --- "cold blooded"
   2. body temperature = ambient temperature
2. homeothermic: constant body temperature --- "warm blooded"
3. ectothermic
   1. environment controls T_body
4. endothermic
   1. metabolic regulation controls T_body:  cellular metabolism, muscle contraction
   2. adaptation to nocturnal environment, cold environment
   3. constant high activity level; requires high caloric intake
5. stenothermal
6. eurythermal

1. temperature dependent activity (species specific)
2. Q^{10---metabolic} rates double to triple with each 10° C rise in temperature
3. ectothermic polar species grow more slowly; live longer; reach greater size; reproduce less frequently than tropical organisms

1. endothermic: mammals and sea birds
2. regional heterothermy 
   1. body size & countercurrent exchange in large tuna, sharks [Fig. 4-16]

Temperature Regulation in Ectotherms

1. behavioral modification: seek favorable environments to regulate T _body [Figs 11-15, 11-16]
2. temperature compensation (biochemical alterations): adjust metabolic rates to maintain level of activity independent of temperature (e.g. different temperature sensitive enzymes)

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