Osmoregulatory Systems in Fishes

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Osmoregulatory Systems in Fishes

• Maintaining homeostasis with respect to solute
  concentrations and water content

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Homeostasis

• Chapter 7
• Zoology 1450

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Introduction

• 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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Topics

• Osmoregulation
• Endocrine system
• Thermal regulation

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Definitions

• 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 lt 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

• Elasmobranchs
• 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 permeability to 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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Saltwater teleosts
kidneys
chloride cells
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Chloride Cell fig 6.2
sea water

Na
Na
Na K ATPase
K
Cl-
mitochondria
internal
tubular system
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Osmotic regulation by FW teleosts

• Ionic conc. Approx 1/3 of seawater
• Dont 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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Freshwater teleosts
Ion exchange pumps beta chloride cells
kidneys
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Ion Exchange Mechanisms
freshwater
interior
active pump
ATP
active pump
ATP
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Freezing Resistance

• What fishes might face freezing?
• hagfishes?
• isotonic
• marine elasmobranchs?
• isotonic
• freshwater teleosts?
• hypertonic
• marine teleosts?
• hypotonic

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Solution for Antarctic fish

• Macromolecular antifreeze compounds
• peptides (protein)
• glycopeptides
• (carbohydrate/protein)

• molecules adsorb (attach) to ice crystal
• surface
• 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?

• Euryhaline
• 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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Euryhaline

• 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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Thermoregulation in Fishes
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Temperature effects on fish

• Temperature exhibits the greatest influence on
  fishs 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

• Youre management decisions and actions must
  account for fish responses to temperature
  gradients and limitations

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Endocrine Systems of Fishes
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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 gt 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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Gonads

• 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 Ca2 inflow at
  gills

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Immune System
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Introduction

• 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 Scalesspecific solid layers of protection
  from pathological and chemical stressors.
• Mucus secretiontraps microorganisms preventing
  entry into body cavity or circulation
• Macrophages (phagcytes) and cytotoxic cellspart
  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.
• Antigensinvading 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!

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Looks like meats back on the menu boys!!!
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Questions???