Osmoregulation-in-Fishes-Balancing-Water-and-Salt.pptx

Osmoregulation in Fishes

1. Osmoregulation in Fishes: Balancing Water and Salt PRESENTED BY: SAIMA SHOWKAT RIYAAN GULZAR RAHILA MANZOOR Presented to: DR ISHRAT MOHAMMAD

2. Introduction to Osmoregulation Osmoregulation is the physiological process by which organisms maintain a stable internal balance of water and dissolved solutes, such as salts, despite fluctuations in their external environment. For fishes, this is a constant challenge due to the osmotic gradient between their bodies and the surrounding water. They face the risk of either excessive water gain or loss, coupled with problematic salt gain or loss, depending on their habitat. The primary organs involved in this delicate balance include: Gills: Key sites for ion exchange. Kidneys: Regulate water and waste excretion. Skin: Acts as a barrier to reduce permeability.

3. Osmoconformers vs. Osmoregulators Fishes employ two fundamental strategies to cope with osmotic challenges: osmoconformity and osmoregulation. Understanding these approaches is crucial to appreciating the diversity of physiological adaptations in aquatic life. Feature Osmoconformers Osmoregulators Internal Osmolality Matches environment (~1000 mOsm) Regulated internally (~300 mOsm) Energy Cost Low High Examples Hagfish, marine elasmobranchs (sharks, rays) Most bony fishes (teleosts) Strategy Tolerate internal change Actively regulate ions/water

4. Osmoconformers (Elasmobranchs) Marine osmoconformers , particularly elasmobranchs like sharks and rays, have evolved a unique strategy to maintain osmotic balance in seawater. Instead of actively expelling large amounts of salt, they elevate their internal osmolality to near that of seawater. Urea/TMAO Retention They retain high concentrations of urea and trimethylamine N-oxide (TMAO) in their blood and tissues, making their internal fluids slightly hyperosmotic to seawater. This minimizes water loss to the environment. Kidney Role Their kidneys play a crucial role in reabsorbing most of the filtered urea, preventing its loss. Urine output is minimal and iso-osmotic to blood plasma. Rectal Gland Despite urea retention, some excess sodium chloride still enters their bodies. The rectal gland, a specialized organ, actively secretes a highly concentrated NaCl solution, effectively removing the excess salt.

5. Osmoregulators: Freshwater Fishes Freshwater fishes live in a hypoosmotic environment, meaning the surrounding water has a lower solute concentration than their internal fluids. This creates two primary challenges: a constant influx of water into their bodies and a continuous loss of essential salts to the environment. Kidneys They possess well-developed glomeruli with a high filtration rate, producing large volumes of very dilute urine to expel excess water while retaining essential salts. Gills Specialized "chloride cells" (also known as mitochondria-rich cells) in their gills actively uptake Na⁺ and Cl⁻ ions from the surrounding water, counteracting salt loss. Skin Their skin is relatively impermeable to water and ions, minimizing unwanted water influx and salt efflux across the body surface.

6. Osmoregulators: Marine Fishes Marine bony fishes (teleosts) face the opposite osmotic challenge compared to their freshwater counterparts. They live in a hyperosmotic environment, where the surrounding seawater has a higher solute concentration than their internal fluids. This leads to constant water loss from their bodies and a gain of excess salts. Drink Seawater To compensate for continuous water loss via osmosis, marine bony fishes actively drink large quantities of seawater. This ingested water is then processed to extract usable water and excrete excess salts. Gills for Excretion Their chloride cells in the gills reverse their function from freshwater fish, actively excreting excess Na⁺ and Cl⁻ ions back into the surrounding seawater. This is a highly energy-intensive process. Kidney Function The kidneys of marine bony fishes produce small volumes of isotonic (iso-osmotic to blood) urine. Their primary role is to excrete divalent ions such as magnesium (Mg²⁺) and sulfate (SO₄²⁻), which cannot be effectively removed by the gills.

7. Role of Kidneys in Osmoregulation The kidneys of fishes exhibit remarkable adaptations to support their osmoregulatory strategies in different environments. Their structure and function are finely tuned to manage water and solute balance. Freshwater Fish Kidneys The kidneys of freshwater fishes are typically large and possess a high number of well-developed glomeruli. These structures are crucial for filtering large volumes of blood. Type: Primarily mesonephric, considered more primitive in terms of kidney evolution but highly effective for their environment. Function: They exhibit a high glomerular filtration rate (GFR), producing copious amounts of very dilute urine. This helps excrete the constant influx of water while essential ions are selectively reabsorbed from the filtrate. Marine Fish Kidneys Marine fish kidneys, particularly in bony fishes, are adapted to conserve water and excrete specific ions. Type: Often have reduced or even absent glomeruli (aglomerular kidneys), especially in some marine teleosts. Function: They exhibit a very low GFR, conserving water. Their primary role shifts to the excretion of divalent ions (Mg²⁺, SO₄²⁻) that are absorbed from ingested seawater and cannot be efficiently removed by gills.

8. Role of Gills in Ion Transport The gills are the primary site for active ion transport in fishes, responsible for 80-90% of overall osmoregulation. This crucial function is largely carried out by specialized cells known as chloride cells, or mitochondria-rich cells, which are strategically located within the gill epithelium. Chloride Cells in Freshwater Fish In freshwater environments, chloride cells are adapted for ion uptake to compensate for constant salt loss. Mechanism: They actively absorb Na⁺ from the water, often via Na⁺/H⁺ exchange, and Cl⁻ via Cl⁻/HCO₃⁻ exchange. These processes are driven by an electrochemical gradient maintained by Na⁺/K⁺-ATPase. Chloride Cells in Saltwater Fish In saltwater, chloride cells reverse their function to excrete excess ions absorbed from drinking seawater. Mechanism: They actively excrete Na⁺ and Cl⁻ into the surrounding water. This typically involves Na⁺-K⁺-2Cl⁻ cotransporters on the basolateral membrane, followed by Cl⁻ efflux through channels on the apical membrane and Na⁺ efflux via Na⁺/K⁺-ATPase.

9. Species-Specific Strategies Beyond the broad categories of osmoconformers and osmoregulators, various fish species exhibit highly specialized osmoregulatory adaptations tailored to their unique ecological niches. These strategies highlight the diverse evolutionary pathways fishes have taken to survive in challenging aquatic environments. Euryhaline Fish (e.g., Salmon) These incredible migrators can move between freshwater and saltwater, completely reversing their gill and kidney functions to adapt to the changing salinity. Hagfish As true osmoconformers , hagfish have internal fluid osmolality essentially identical to that of seawater, representing a very primitive osmoregulatory strategy. Sharks While largely osmoconformers due to urea retention, sharks also exhibit a form of weak osmoregulation through their rectal gland, which actively secretes excess NaCl. Mudskippers These amphibious fish can tolerate extended periods out of water, relying on their skin and specialized mucous layers to reduce water loss and facilitate cutaneous respiration.

10. Hormonal Regulation of Osmoregulation Fish osmoregulation is not merely a function of organs; it is a complex process intricately controlled by various hormones, ensuring rapid and precise responses to changes in environmental salinity. These hormones orchestrate the activities of gills, kidneys, and other tissues to maintain internal homeostasis. 1 Antidiuretic Hormone (ADH) Primarily in saltwater fish, ADH (or its fish equivalent, arginine vasotocin) acts on the kidneys to reduce water loss, promoting water reabsorption and decreasing urine volume. 2 Cortisol This corticosteroid is crucial for saltwater adaptation, promoting morphological changes in gill chloride cells and increasing the activity of Na⁺/K⁺-ATPase, facilitating salt excretion. 3 Prolactin Essential for freshwater adaptation, prolactin reduces water permeability of the body surface and enhances active ion uptake by the gills, helping to retain salts and prevent water influx. 4 Stanniocalcin Produced by the corpuscles of Stannius, stanniocalcin is involved in calcium homeostasis. In freshwater fish, it inhibits excessive calcium uptake by the gills, preventing hypercalcemia.