1. Biology 2672a: Comparative Animal Physiology
Osmoregulation in fishes

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

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

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

5. 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

6. 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

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

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

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

10. 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

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

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

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

14. 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

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

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

17. Effects of solute concentration on enzyme function
Urea Km
Concentration

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

19. Counteracting Solutes
Fig 26.10

20. 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

21. 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?

22. The situation for a marine teleost
Fig 27.7b

23. 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

24. 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:

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

26. Export of Chloride
Box 27.2

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

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

29. 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

30. The situation for a freshwater teleost
Fig. 27.7a

31. 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

32. Chloride cells again
Figs 27.3 & 27.4

33. Exchange of CO2 wastes for NaCl
Fig. 26.2

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

35. Cl- uptake

36. 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

37. 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)

38. Reading for Thursday
Water balance in terrestrial organisms pp