Biology 2672a: Comparative Animal Physiology

Biology 2672a: Comparative Animal Physiology Osmoregulation in fishes

Freshwater fish Water Inside: 300 mOsm High Na+ & Cl- Outside: <5 mOsm Low Na+ & Cl- Salts

Saltwater fish Salts Inside: 300 mOsm Low Na+ & Cl- Outside: 1000 mOsm High Na+ & Cl- Water

Terrestrial fish Inside: Wet High Na+ & Cl- Outside: Dry No Na+ & Cl- Salts Water

Osmoregulation • Maintenance of water and salt balance in the body • Why freshwater fishes don’t explode, saltwater fishes don’t dry up and people don’t desiccate

Osmolarity/Osmolality • The amount of ‘stuff’ in a solution • 1 Mole of solutes = 1 Osmole • Cumulative: 0.2 M of 5 things = 1 Osmole • Osmolality – per kg of solvent • Osmolarity – per litre of solvent

Osmotic pressure • Solutes exert pressure that moves water from place to place • Can be a source of hydrostatic pressure…

Osmosis • Movement of water across a semi-permeable membrane Net movement of water driven by osmotic pressure

Osmosis and hydrostatic pressure Osmotic pressure has caused bulging – hydrostatic pressure

Osmoconformers and Osmoregulators Internal Osmolarity (mOsm) External Osmolarity (mOsm) Fig. 26.3a,b

Many different types and combos of osmoregulatory strategies Fig. 26.3c

Strategy and Tolerance are not identical Euryhaline Stenohaline Osmoconformer Osmoregulator Internal Osmolarity External Osmolarity

Internal [Urea] Internal [Na+] Internal Osmolarity External Osmolarity

Inside Outside Na+ 286 mM Cl- 246 mM Others 135 mM 667 mOsm 930 mOsm Na+ 286 mM Cl- 246 mM Urea 351 mM Others 135 mM 1018 mOsm From Table 26.5

Ureo-osmoconformer Internal [Urea] Internal [Na+] Internal Osmolarity External Osmolarity

But Urea is Bad! • Chaotropic • Binds strongly to proteins, releasing water and disrupts tertiary structure

Effects of solute concentration on enzyme function Urea Km Concentration

Trimethylamine oxide(TMAO) CH3 CH3 H3C N+ O-

Counteracting Solutes Fig 26.10

Inside Outside Na+ 286 mM Cl- 246 mM Urea 351 mM TMAO 71 mM Others 64 mM 1018 mOsm 930 mOsm From Table 26.5

Ureo-Osmoconformation in sharks • Urea is used to make up the ‘osmotic gap’ between internal and external concentration • Requires high protein diet for manufacturing Urea • TMAO acts as a counteracting solute to preserve protein function in high concentrations of urea. • Why would you soak shark prior to cooking it?

The situation for a marine teleost Fig 27.7b

Gills as exchange organs • CO2 & O2 • Used to remove the salts that are ingested with food and water • (and absorbed through gill surfaces) • Major site for this in marine teleosts

How many ions? • Total daily flux estimated for intertidal Xiphister atropurpureus in seawater • ~10-40 g • Na+: 110 mM/kg fish/day • 0.25g for a 10 g fish (2.5% bw) • Cl-: 72 mM / kg fish/day • 0.25 g • Water: 2480 ml/kg fish/day • 24.8 g water for a 10 g fish (!) Evans (1967) J. Exp. Biol. 47: 525-534

Chloride cells Apical (Mucosa) Water Pavement cell Baso-lateral (serosa) Blood Fig. 27.6

Export of Chloride Box 27.2

Export of Chloride is driven by a Na+ gradient Box 27.2

Active removal of Cl- leads to an electrochemical imbalance that drives Na+ out of blood via paracellular channels Box 27.2

Chloride cell summary • Transcellular transport of Cl- • Driven by Na+,K+-ATPase (requires energy) • Paracellular transport of Na+ • Ionoregulation accounts for ~3-5% of resting MR in marine teleosts

The situation for a freshwater teleost Fig. 27.7a

Gills as exchange organs • CO2 & O2 • Used to take up salts from the environment • Not much NaCl in freshwater, but gills process a huge volume

Chloride cells again Figs 27.3 & 27.4

Exchange of CO2 wastes for NaCl Fig. 26.2

Na+ uptake Note tight junction Box 4.1 Fig.A(2)

Cl- uptake

NaCl uptake summary • Exchange for CO2 • Na+ via electrochemical gradient • Cl- via HCO3- antiport • Very dilute urine gets rid of excess water without losing too much salt

Reading for Thursday • Water balance in terrestrial organisms • pp 700-712

Biology 2672a: Comparative Animal Physiology