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Osmoregulatory Systems in Fishes. Maintaining homeostasis with respect to solute concentrations and water content. Homeostasis. Chapter 7 Zoology 1450. Introduction . Homeostasis = maintaining steady state equilibrium in the internal environment of an organisms

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Osmoregulatory systems in fishes l.jpg

Osmoregulatory Systems in Fishes

Maintaining homeostasis with respect to solute concentrations and water content

Homeostasis l.jpg


Chapter 7

Zoology 1450

Introduction l.jpg

  • Homeostasis = maintaining steady state equilibrium in the internal environment of an organisms

  • Much is done involuntarily by action of hormones, enzymes and osmoregulatory processes. Although occasionally fish do just “pick up and move” if environmental conditions are unfavorable.

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  • Osmoregulation

  • Endocrine system

  • Thermal regulation

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  • Homeostasis = maintaining steady state equilibrium in the internal environment of an organisms

  • Solute homeostasis = maintaining equilibrium with respect to solute (ionic and neutral solutes) concentrations

  • Water homeostasis = maintaining equilibrium with respect to the amount of water retained in the body fluids and tissues

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Definitions, continued

  • Osmotic concentration - Total concentration of all solutes in an aqueous solution: measured in units of osmolals = 1 mole of solute/liter of water or milliosmolals = 1/1000th of one osmolal

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Osmoregulation in different environments

  • Challenge to homeostasis depends on

    • steady state concentration of solutes in the body fluids and tissues as well as

    • concentration of solutes in the external environment

      • marine systems: environment concentration = 34 - 36 parts per thousand salinity = 1000 mosm/l

      • freshwater systems: environment concentration < 3 ppt = 1 - 10 mosm/l

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Osmoregulation in different environments

  • Each species has a range of environmental osmotic conditions in which it can function:

    • stenohaline - tolerate a narrow range of salinities in external environment - either marine or freshwater ranges

    • euryhaline - tolerate a wide range of salinities in external environment - fresh to saline:

      • short term changes: estuarine - 10 - 32 ppt, intertidal - 25 - 40

      • long term changes: diadromous fishes

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Four osmoregulatory strategies in fishes

1. Isosmotic (nearly isoionic, osmoconformers)

2. Isosmotic with regulation of specific ions

3. Hyperosmotic (fresh H20 fish)

4. Hyposmotic (salt H2O fish)

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Osmoregulation Strategies

Osmoconforming (no strategy) Hagfish internal salt concentration = seawater. However, since they live IN the ocean....no regualtion required!

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Osmoregulation Strategies


  • maintain internal salt concentration ~ 1/3 seawater, remaining 2/3 is urea and trimethylamine oxide (TMAO). So total internal osmotic concentration equal to seawater.

  • Gill membrane has low permeabilityto urea so it is retained within the fish. Because internal inorganic and organic salt concentrations mimic that of their environment, passive water influx or efflux is minimized.

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Osmotic regulation by marine teleosts...

  • ionic conc. approx 1/3 of seawater

  • drink copiously to gain water

  • Chloride cells eliminate Na+ and Cl-

  • kidneys eliminate Mg++ and SO4=

    advantages and disadvantages?

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active tran.

passive diff.



Na+, Cl-

Na+, Cl-

Mg++, SO4=

Na+, Cl-

Mg++, SO4=

Saltwater teleosts:


chloride cells

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Na+, Cl-




chloride cell

Chloride Cell fig 6.2:

sea water




Na+ K+ ATPase





tubular system

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Osmotic regulation by FW teleosts

  • Ionic conc. Approx 1/3 of seawater

  • Don’t drink

  • Chloride cells fewer, work in reverse

  • Kidneys eliminate excess water; ion loss

  • Ammonia & bicarbonate ion exchange mechanisms

    advantages and disadvantages?

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Na+, Cl-

Na+, Cl-


Freshwater teleosts:

Ion exchange

pumps; beta chloride cells


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NH4+ or H+



gill membrane

Ion Exchange Mechanisms









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Freezing Resistance:

  • What fishes might face freezing?



    marine elasmobranchs?


    freshwater teleosts?


    marine teleosts?


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rich in alanine

Solution for Antarctic fish

  • Macromolecular antifreeze compounds

    peptides (protein)



  • molecules adsorb (attach) to ice crystal


  • interfere with ice crystal growth

    (disrupt matrix)

  • Why is this important???

  • ice ruptures cells; hinders osmoregulation

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What about rapid ion flux?


  • Short-term fluctuations in osmotic state of environment, e.g. in intertidal zone or in estuaries where salinity can range from 10 to 34 ppt with the daily tidal cycle:

    • these fish have both kinds of chloride cells

      • when salinity is low, operate more like FW fishes

      • when salinity is high, operate like marine fishes

      • kidneys function only under low salinity conditions

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  • Diadromous fishes (spend part of life in salt water, part in freshwater – catadromous (migrate seaward) or anadromous (migrate up river)

  • hormone-mediated changes associated with metamorphosis - convert from FW adaptations to SW or vice versa, depending on direction of migration

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What about stress??

  • Stressors (handling, sustained exercise such as escape from predator pursuit) cause release of adrenaline (epinephrine) - for mediating escape, etc.

  • Adrenaline causes diffusivity of gill epithelium to increase (become “leaky” of water & ions)

  • This accentuates the normal osmoregulatory challenge for FW or marine fishes

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How to reduce stress in stressed fishes?

  • Minimize the osmotic challenge by placing fish in conditions that are isosmotic

    • add salt to freshwater, e.g. in transporting fish or when exposing them to some other short-term challenge

    • dilute saltwater for same situation with marine species

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Temperature effects on fish

  • Temperature exhibits the greatest influence on fish’s lives!

  • Affects metabolism

  • Affects digestion

  • Signals reproductive maturation and behavior

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Fish are conformers (well, sort of...)

  • Body temperature is that of the environment

  • Each species has particular range of temperatures that they can tolerate and that are optimal

  • Big difference between what you can tolerate and what you thrive in...

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Behavioral Thermoregulation in Fishes

  • Although fish are ectotherms, they can alter their body temperature by moving to habitats with optimal temperature

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Hot Fishes

  • Some fish can maintain body temperature greater than ambient - tunas, billfishes, relatives (nearly endothermic)

  • Use retia (similar to rete mirable) in swimming muscles to conserve heat, exchange O2, etc.

  • Red muscle is medial rather than distal

  • Billfishes have warm brains - heat organ from muscles around eye

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Practical application

  • You’re management decisions and actions must account for fish responses to temperature gradients and limitations

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Pituitary Gland - Master Gland

  • Linked with hypothalamus of brain

  • Produces hormones that affect other endocrine tissues - indirect influence

  • Produces hormones that affect non-endocrine tissues directly

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Pituitary Gland

  • Indirect influence

    • ACTH - adrenocorticotrophic hormone

      • stimulates interrenal tissue production of cortisol

    • TH - thyrotrophic hormone

      • stimulate thyroid production of thyroxin (growth, metamorphosis-i.e. flounder)

    • GTH- gonadotrophic hormone

      • stimulates gonads to produce androgens/estrogens

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Pituitary Gland

  • Effects non-endocrine tissues directly

    • pigmentation - melanophore stimulating hormone (MSH)

      • affects long-term control of color

    • osmoregulation - prolactin, vasotocin

      • controls fresh/saltwater systems

    • growth – somatotrophic hormone

      • stimulates > length, cell multiplication

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Thyroid Gland

  • isolated follicles distributed in connective tissue along ventral aorta

  • controls metabolic rate

  • affects metamorphosis, maturation

  • facilitates switch between fresh & salt water

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  • gamete and sex hormone production

  • controls gametes maturation

  • cause formation of secondary sex characteristics: color, shape, behavior

  • in fish, several sex hormones also serve as pheromones - e.g. goldfish males respond to hormones released with ovulation

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Other endocrine tissues in fishes

  • chromaffin tissues-located near kidneys & heart

    • produce adrenaline/noradrenaline – “fight or flight”

    • increases blood flow through gills, ventilation rate

  • interrenal (inside kidney) tissues

    • produce cortisol, cortisone - stress response hormones (reduce inflamation)

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Other endocrine tissues in fishes

  • pancreatic islets

    • produce insulin - controls glucose, glycogen metabolism (glucagon production)

  • corpuscles of Stannius

    • produce stanniocalcin - controls Ca+2 inflow at gills

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  • Obviously, the immune system is important in homeostatic processes.

  • Immune systems of fish have two components: non-specific and specific.

  • As we will see, both are involved in protecting fish from visible as well as invisible disease causing agents.

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Non-specific immunity

  • Skin & Scales—specific solid layers of protection from pathological and chemical stressors.

  • Mucus secretion—traps microorganisms; preventing entry into body cavity or circulation

  • Macrophages (phagcytes) and cytotoxic cells—part of the inflamatory response which destroy pathogens within the body before they can do harm.

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Specific Immune Response

  • More of an active response where an “invader” is detected and destroyed.

  • Primary organs: kidney, thymus, spleen, intestine.

  • Antigens—invading compounds which provoke an immune response.

Source: Cancer Research Institute (2002) www.cancerresearch.org/immhow.html

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Specific immune response: What if something does get in??

  • White blood cells called B lymphocyte cells (B cells) and T lymphocyte cells (T cells)—bind to foreign cells and begin replication and attachement to (sort of markers for things to come...).

  • Occasionally, invader actually goes trough a macrophage first...then B cell responds

  • Once B cells replicate, antibodies are produced which bind specifically to pathogens and tag them for destruction (eating) by macrophages!