Biology 2672a: Comparative Animal Physiology Osmoregulation in fishes

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

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

Terrestrial fish Inside: Outside: Wet Dry High Na+ & Cl- 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) “Go with the flow”: No energy expended trying to stay different from environment 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 [Na+] Internal [Urea] Internal Osmolarity External Osmolarity

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

Ureo-osmoconformer Internal Osmolarity Internal [Na+] Internal [Urea] 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 H3C N+ CH3 O-

Counteracting Solutes Fig 26.10

Inside Outside 930 mOsm 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

Salt Water Fresh Water Drinking Lots Little Urine Little, concentrated Copious, dilute Ion flux Passive into fish; active out of fish Na+,K+-ATPase Na+ into bloodstream Tight junctions Yes Cl- Transcellular transport driven by Na+ gradient Transcellular via HCO3- antiporter (driven by H+ pump) Na+ Paracellular driven by electochemical gradient Transcellular driven by electrochemical gradient (set up by H+ pump and Na+,K+-ATPase)

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