BIOLOGY 457/657
PHYSIOLOGY OF MARINE & ESTUARINE ANIMALS
February 16, 2004
Water Balance in Aquatic Animals
Osmoregulation in Invertebrates
BIOLOGICAL PROPERTIES OF WATER
• Is usually a liquid at the temperatures on
Earth’s surface.
• Has a high specific heat.
• Has a high latent heat of vaporization
• Is denser in the liquid state than the solid
state.
• Is a powerful solvent.
• Is a polar compound.
COLLIGATIVE PROPERTIES OF
AQUEOUS SOLUTIONS
• Reduction in vapor pressure:
= original v.p. x (nsolvent)/(nsolvent + nsolute)
• Elevation in boiling point:
= +0.52°C per mole solute per liter H2O
• Reduction in freezing point:
= -1.86°C per mole solute per liter H2O
• Production of osmotic pressure:
V = nRT
= n/V x RT
(in atm) = 22.4 x C (moles/liter H2O)
TERMINOLOGY
Mole = gram molecular weight
Molar = moles per liter of solution
Molal = moles per liter (1000g) of solvent
(Note that this ratio determines colligative properties.)
Osmole = grams of solute (per kg H2O) required
to produce = 22.4 atm
Osmolal = number of osmoles per liter (kg) of H2O
(For instance, in seawater 1000 mOsm/kg is equivalent
to 1129 mM/kg. The osmotic concentration of a
solution is its osmoticity)
OSMOSIS
Definition: the movement of solvent through a semipermeable
membrane due to its concentration gradient.
(Note that no biological membrane is truly semipermeable.)
Biological Significance of Osmosis:
The actual pressure of osmosis applies to plants, not animals.
Water movements that do occur quickly alter concentration
gradients.
In animals, it is often more useful to think of osmotic
concentrations.
It is important to distinguish between osmoticity and tonicity.
OSMOTIC COMPARTMENTS OF ANIMALS:
Directions of Movement of Water & Solutes
Internal
Compartments
Intracellular
compartment
Extracellular
compartments
Hemolymph (blood)
Coelomic fluid
IONIC CONCENTRATIONS IN LIVING
CELLS
ION REGULATION
• Even when tissues are isosmotic, ion compositions
differ between tissues and water
• The details of ion regulation vary among taxa
• Within taxa, similar systems of regulation exist
Ions frequently reduced in concentration: SO4=, Mg2+
Ions frequently increased in concentration: K+, Ca2+
Na+ and Cl- tend to be similar to seawater concentrations
ION
REGULATION
(2)
Solute
concentrations
in seawater,
cells, and
extracellular
fluids.
ION REGULATION (3)
Where are ions regulated?
Cells - across the cell membrane
Blood - across the gills,
digestive tract, and extretory
membranes
Mechanism: ion transporters
(ATPases)
ION REGULATION (4)
Data:
Adaptation in blue
crabs (Callinectes
sapidus) to
changes in the
osmolality of
their tank (in the
laboratory)
Top: osmolality
Bottom: ATPase
activity
VOLUME REGULATION
Whenever an animal moves
into water of altered
osmotic concentration, it
will tend to gain or lose
water. The process of
controlling this expansion
or shrinkage is called
volume regulation.
VOLUME REGULATION (2)
Water movement in the
spider crab, Maja sp. , upon
transfer to 58% seawater
(~580 mOsm) from 100%
seawater. The crab rapidly
gained weight at first, and
also quickly lost salts to the
more dilute medium to
which it was transferred.
VOLUME REGULATION (3)
How can a marine osmoconformer cope with
this volume change?
(1) Lowered salinity: Animal is hyperosmotic
Animal tends to gain water and lose solutes
* Reduce permeability
* Produce a copious, dilute urine
(2) Increased salinity: Animal is hyposmotic
Animal tends to lose water and gain salts
* Reduce permeability
* Actively excrete salts
INTRACELLULAR ISOSMOTIC
REGULATION
Remember that animal cells must be in osmotic
equilibrium with the extracellular fluids that
surround them. Therefore, the cells must change
in concert with these fluids.
Cellular solutes:
(1) Inorganic ions
(2) α-amino acids
(3) other small organic molecules; e.g. trimethyl
amine oxide (TMAO), glucose, glycerol
INTRACELLULAR ISOSMOTIC
REGULATION (2)
Note the relatively
serious effects on
enzyme function of
using inorganic
salts or some
charged amino
acids. Neutral
amino acids have
almost no effect of
PEP substrate
binding.
INTRACELLULAR ISOSMOTIC
REGULATION (3)
Advantages of using amino acids as osmotic effectors:
(1) No changes in electrical potential at neutral pH
(2) Reduced direct effects on enzyme function
INTRACELLULAR ISOSMOTIC
REGULATION (4)
INTRACELLULAR ISOSMOTIC
REGULATION (5)
Adjustment to hypotonic media:
(1) Efflux of amino acids.
(2) Incorporation of amino acids into proteins.
(3) Deamination of amino acids and metabolic disposal
of their products, with the production of NH3.
Adjustment to hypertonic media:
(1) Protein hydrolysis to release free amino acids.
(2) Uptake of amino acids in solution in blood.
(3) Intracellular synthesis of new free amino acids.
INTRACELLULAR ISOSMOTIC
REGULATION (6)
PATTERNS OF OSMOREGULATION
(A) Freshwater
hyperosmoregulators
(B) Euryhaline hyper-
and hypo- regulators,
with reduced internal
osmolality
(C) Euryhaline
hyperosmoregulators
(D) Marine
osmoconformers
(E) Marine hyper- and
hypo- regulators
OSMOCONFORMERS
The relationship between
medium concentration and
blood concentration in
several euryhaline
invertebrates
• Callianassa californiensis
• Buccinum undatum
• Eupagurus bernhardus
• Porcellana platycheles
• P. longicornis
• Maia squinado
• Mercierella enigmatica
OSMOCONFORMERS (2)
Found in echinoderms, cephalopods, sipunculids,
ascidians, coelenterates, most marine molluscs,
marine polychaetes, most marine crustaceans
Only intracellular solutes (e.g. FAA) must be
regulated
May be associated with euryhalinity (e.g. molluscs)
In intertidal habitats, animals often escape into tubes,
burrows, or shells
Can be associated with “apparent osmoregulation”,
or osmotic buffering
OSMOCONFORMERS (3)
Tidal buffering in the
Chesapeake Bay oyster,
Crassostrea virginica
HYPEROSMOTIC REGULATORS
Limited hyperregulators
(e.g. annelids, molluscs,
some crustaceans)
• Reduced permeability to water
and ions
• Intake of ions from food,
active transport (gills?)
• Form a dilute urine
Excellent hyperregulators
(e.g. many crustaceans)
• Same mechanisms, but with
more powerful physiological
action
HYPER/HYPOSMOTIC REGULATORS
Found in a few crustaceans;
rare elsewhere
Involves powerful,
bidirectional ion
transport systems
NO invertebrate can
produce a hypertonic
urine for H2O
conservation
LIFE IN TERRESTRIAL
ENVIRONMENTS
Life on land is analogous to life in
hyperosmotic, aquatic
environments.
(1) Animals reduce water loss by
reducing permeability, enclosing
respiratory surfaces, and using
ureotelic or uricotelic excretion
(2) Animals increase water gain by
moving to seawater, to damp
locations, or by special physiological
adapatations (next page).
THE LAND
CRABS
Terrestrial and semiterrestrial
crabs, with varying ability to
tolerate dessicating environments.
Top, Ocypode quadrata: Ghost crabs
extract soil interstitial water within
their burrows, and are thus able to
tolerate the extreme heat and dry
conditions of open beaches.
Middle, Cardisoma sp. These land
crabs use water within the burrow,
which may be required because their
diet is extremely low-quality.
Bottom, Gecarcinus lateralis. These
crabs occupy dry burrows which they
use as a refuge except when aerial
humidity is high. Thus, they feed
only intermittently, on low-quality
plant food, and are slow-growing.