lecture 8 Osmoregulation and Environmental Challenges in Fish

ICHPTHYOLOGY

Why is osmoregulation important for fish?

Fish live in environments with different salinities than their internal fluids, requiring active regulation to maintain homeostasis.

What is homeostasis?

Maintaining a steady internal equilibrium through hormones, enzymes, and active processes like osmoregulation.

What is the average salinity of seawater?

Approximately 30 parts per thousand (ppt).

Define isosmotic

Isosmotic: equal osmotic pressure

Hyperosmotic:

saltier than environment (freshwater fish)

Hyposmotic

less salty than environment (marine fish)

What organ primarily regulates osmoregulation in fish?

Gills, with kidneys playing a key supporting role.

What strategy do hagfish use for osmoregulation?

Osmoconformers (isosmotic) with no active osmoregulatory strategy.

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How do elasmobranchs (e.g., sharks) manage osmoregulation?

They are hyperosmotic—retain urea and TMAO to match seawater osmolarity but actively excrete salts via rectal glands.

Why doesn't high urea denature shark proteins?

TMAO stabilizes proteins against the effects of urea and hydrostatic pressure.

What challenges do marine teleosts face?

Being hyposmotic—they lose water and gain salts; must drink seawater and excrete excess salts.

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How do marine teleosts remove excess salt?

Active secretion of monovalent ions via chloride cells; divalent ions are removed via kidneys and feces.

What is solute-linked water transport?

Water absorption in the gut is coupled with ion uptake, allowing hydration despite drinking salty water.

Describe the function of a chloride cell in marine teleosts.

Sodium and chloride enter passively, sodium is pumped out via ATPase, chloride exits passively along an electrochemical gradient.

How do freshwater fish regulate osmoregulation?

Being hyperosmotic, they lose salts and gain water. They actively uptake ions and excrete dilute urine via large kidneys.

What ion exchange mechanisms do freshwater fish use?

Swap ammonium or protons for sodium, and bicarbonate for chloride across gill epithelium.

What adaptations reduce water uptake in freshwater fish?

Tough skin, thin blood-gill barrier, low drinking rate, large Bowman’s capsule, and dilute hypertonic urine.

What are euryhaline fish?

Fish that can live in a range of salinities (e.g., intertidal or estuarine zones).

What are diadromous fish?

Fish that migrate between fresh and saltwater:

• Anadromous: live in sea, spawn in freshwater (e.g., salmon)

• Catadromous: live in freshwater, spawn in sea (e.g., eel)

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What changes occur when euryhaline fish migrate?

Hormonal and physiological changes (metamorphosis) switch their osmoregulatory systems.

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What challenges do marine teleosts face in freezing environments?

Being hypotonic, they are at risk of freezing. They produce antifreeze compounds to prevent ice crystal formation.

How does stress affect fish osmoregulation?

: Stress (e.g., from tanks or humans) triggers adrenaline, increasing gill permeability and disrupting osmoregulation.

How can stress-related osmoregulatory issues in transported fish be reduced?

By adjusting water salinity—either diluting or adding salts to balance osmotic pressure.

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What are the main osmoregulatory differences between freshwater and marine teleosts

• Freshwater: gain water, lose salts, excrete dilute urine, uptake salts actively.

• Marine: lose water, gain salts, drink seawater, excrete salts actively via chloride cells.