BIOLOGY 457/657
PHYSIOLOGY OF MARINE & ESTUARINE ANIMALS
February 18, 2004
Osmoregulation in Marine Vertebrates
A COMPARISON OF VERTEBRATE &
INVERTEBRATE OSMOREGULATION
VERTEBRATE OSMOREGULATION:
OVERVIEW
VERTEBRATE OSMOREGULATION
Two Major Vertebrate Groups:
• Fish & Amphibians (evolved in water)
• Reptiles, Birds, & Mammals (evolved in air – to these animals,
seawater is a “desert”)
It is thought that the first modern vertebrates evolved in freshwater,
because their solute levels are low. However, some primitive
fishes have solute concentrations very similar to seawater.
Modern fishes must have returned to the ocean from freshwater.
Some fish alternate between habitats:
Anadromous – breed in FW, live as adults in SW
Catadromous – breed in SW, live as adults in estuaries
OSMOREGULATION IN FISHES
FISHES I: CYCLOSTOMES
Cyclostomes (Class: Agnatha) are jawless, primitive fish.
Hagfishes (marine) are isosmotic, using FAA to adjust Π.
Lampreys (freshwater or anadromous)
are like teleosts, with excellent Na+
transport systems Kt (Na+) = 0.13
mM in gill tissue.
FISHES II: ELASMOBRANCHS
Elasmobranchs (Class: Condrichthyes; sharks & rays)
are generally marine, with a few freshwater species
(e.g. Amazon ray, bull sharks of Lake Nicaragua)
Their inorganic solute
concentrations are similar
to those of teleosts (bony
fishes). The deficit is
made up using organic solutes.
ELASMOBRANCHS (2)
The shark kidney actually reabsorbs urea, back into the blood. Shark proteins
are highly tolerant of elevated [urea].
Total osmoticity is usually slightly > 1000 mOsm, so water continually enters
by osmosis (for metabolism and urine formation)
Freshwater elasmobranchs excrete urea, reduce drinking, and increase salt
uptake (perhaps by the gills or digestive tract).
CLASS: CROSSOPTERYGII
The lobe-fin fishes, represented today only by the coelacanth (genus
Latimeria), are ancestral to all terrestrial vertebrates.
Physiologically (with regard to osmoregulation), they are like the
elasmobranchs.
CLASS: TELEOSTII (Bony Fishes)
All teleosts have similar blood osmolality, about 25% to 40% of
seawater. Many species occupy both fresh and salt water.
The Osmotic Problem:
MARINE FISHES
Are hyposmotic:
LOSE water
through the gills,
body surface, and
in wastes.
DRINK seawater
(a flounder drinks
about 1% of its
mass per hour)
Are hypoionic:
GAIN salts across gills, body surface, and in food.
EXCRETE salts (Na+, Cl-) across gills. Other ions (K+, Ca++, Mg++, SO4=) are
excreted by the kidney in the hypotonic urine, by tubular secretion.
MARINE FISHES
Chloride Cells: Sites of ion
transport in fish gills.
Cations (Na+, K+) are
cotransported with Clions at the basal surface.
Cl- passive passes out of
anion channels at the
apical surface, and Na
follows through leaky
tight junctions between
cells.
FRESHWATER FISHES
Are hyperosmotic, and gain water in food, across the gill,
and across the body surface.
Excrete a dilute, copious urine (up to 400 ml/kg/day).
Are hyperionic, and lose salts in urine and by diffusion.
Import salts in diet and by active transport at the gill.
EURYHALINE FISHES
Example: Xiphister (blenny)
(Must shift between modes)
CLASS AMPHIBIA
Most amphibians are freshwater or terrestrial, but a few are
euryhaline. Most are ammonicotelic, but some euryhaline
species are ureotelic. Euryhaline species actually
osmoconform at elevated salinities, using ions or urea.
CLASS REPTILIA
Reptiles are essentially terrestrial, but some species live in
freshwater, marine, or estuarine habitats. All are uricotelic.
However, the estuarine diamondback terrapin (Malaclemys)
actually uses urea as an osmotic effector!
Marine species (turtles, crocodiles, snakes, iguana) produce an
isotonic urine and excrete salt using extrarenal salt glands.
CLASS REPTILIA
SALT GLANDS
Marine Iguana – nasal
gland, sneezes
Snakes, Turtles supraorbital gland,
drain secretions into
the orbit (turtles) or
oral cavity (snakes)
Crocodiles – sublingual
salt glands, drain into
oral cavity
CLASS AVES
Like reptiles, birds are essentially terrestrial animals, but have
many successful marine species (and are more diverse than
any other air-breathing vertebrate). All reproduce on land.
Urine is hypotonic or isotonic. All are uricotelic.
Marine birds have a large extrarenal
pair of salt glands external to the
skull, producing a concentrated solution of NaCl.
CLASS AVES
Salt-gland secretion by a herring gull. The gland uses a
countercurrent mechanism to concentrate salt in the
secretion, and can transport salt at up to 20x the rate (per
gram of tissue) as a human kidney.
CLASS MAMMALIA
All marine mammals are secondarily evolved from terrestrial
ancestors. They form a diverse group of animals: pinnipeds,
sirenians, otters, and cetaceans.
Many marine mammals gain water from their food (e.g. other
vertebrates, teleosts). Their kidneys can produce urine that
is hypertonic NOT ONLY to the blood, BUT ALSO to
seawater.
CLASS MAMMALIA
CLASS MAMMALIA