WFSC 448 – Fish Ecophysiology
(Week 4 – 21 Sep 2015)
Major point: Physical and chemical properties of water dominate lives of
aquatic organisms in ways largely alien to terrestrial vertebrates
Osmoregulation—what goes in, what comes out
The goal is homeostasis
Why would this be important for water balance? Maintaining relative stable
environment for animal cells. Cells explode when overfilled, can not function
biochemically and suffer damage when underfilled with water.
Challenge to homeostasis depends on:
 steady state concentration of solutes in the body fluids and tissues as well as
 concentration of solutes in the external environment
o marine systems: environment concentration = 34 - 36 parts per
thousand salinity
o freshwater systems: environment concentration < 3 ppt
The rate of transfer for water and salts from an environment depends on:
1. Surface area of animal
2. Size of gradient
3. Permeability of the surface
Note that the consequences of change in these parameters differ depending on
rate of change and species natural history (hence, adaptations).
From Kinne: carp placed into brackish water suffer sevear respiratory difficulty and
irreversible damage—those allowed to acclimate over several days have no
troubles.
 Cells exposed to osmotic gradients can shrink or swell.
o Changes in cell volume can damage cells directly.
Each species has a range of environmental osmotic conditions in which it can
function:
 stenohaline - tolerate a narrow range of salinities in external environment either marine or freshwater ranges
 euryhaline - tolerate a wide range of salinities in external environment - fresh
to saline:
o short term changes: estuarine - 10 - 32 ppt, intertidal - 25 - 40
o long term changes: diadromous fishes
How do animals meet the ionic and osmotic challenges of osmotic gradients in a
wide variety of environments?
Osmoconformers—animals that maintain an internal osmolarity similar to the
osmolarity of the surrounding medium.
 E.g. hagfishes—blood/plasma salinity same as sea water
 E.g. elasmobranchs—maintain internal salt concentration 1/3 seawater,
remaining 2/3 is urea and trimethylamine oxide (TMAO), so total internal
osmotic concentration equal to seawater. Also their gill membrane has low
permeability to urea so it is retained within the fish. Because internal
inorganic and organic salt concentrations mimic that of their environment,
passive water influx or efflux is minimized
Osmoregulators—animals that maintain an internal osmolarity different from the
surrounding medium.
Marine teleosts
 ionic conc. approx. 1/3 of seawater
 drink copiously to gain water
 kidneys eliminate Mg++ and SO4=
 chloride cells eliminate Na+ and Cl-
Functions of chloride cells
Chloride cells are salt secreting cells in marine teleosts; salt acquiring in freshwater
teleosts.
 CCells are in gill tissue
 CCells are mitochondrion rich (what does that imply?)
 Chloride and sodium enter tissue from external environment and GI tract.
 Both ions are carried into CCells passively but via a well designed gateway for
passage (passage facilitated by carrier proteins in cell membrane).
 Sodium pumped out of cells at high concentration into a channel where it
can eek back to sea. This uses energy! Potassium is pumped in to facilitate
Na removal, and the potassium eeks back to body.
 Chloride builds up so much inside cells it eeks back to sea via a portal from
the cell directly to sea.
Freshwater teleosts
 Ionic conc. approx. 1/3 of seawater
 These fish don’t purposely drink
 Chloride cells fewer, work in reverse
 Kidneys eliminate excess water; ion loss occurs in process
 Ammonia & bicarbonate ion exchange mechanisms
Salt
Fresh
FIG. 1. Morphology and transport mechanisms of gill chloride cells in seawater and fresh water. See text
for details of transport mechanisms. Chloride cells are characterized by numerous mitochondria and an
extensive tubular system that is continuous with the basolateral membrane. In seawater, chloride cells are
generally larger and contain a deep apical cyrpt, whereas in freshwater the apical surface is broad and
contains numerous microvilli. In some species, such as tilapia the H+-ATPase and apical sodium channel
may be present in pavement cells rather than chloride cells. Recent evidence suggests that individual
chloride cells can move between these two mophological states (Hiroi et al., 1999), and also arise from
undifferentiated stem cells (Wong and Chan, 1999). Growth hormone and cortisol can individually promote
the differentiation of the seawater chloride cell, and also interact positively to control epithelial transport
capacity. Prolactin inhibits the formation of seawater chloride cells and promotes the development of fresh
water chloride cells. Cortisol also promotes acclimation to fresh water by maintaining ion transporters and
chloride cells, and by interacting to some degree with prolactin. PVC = pavement cell. [figure and legend
from: McCormick (2001) Endocrine control of osmoregulation in teleost fish. Amer. Zool. 41:781-94]
Note also that these systems are up- or down-regulated via the endocrine system
Discuss: How and why would fathead minnow males guarding nests be
hyperosmotic?
Functions of Excretory Systems:
1. Adjust the quantity of water and various plasma constituents to be
conserved by the body or eliminated in the urine
2. Eliminate toxic metabolic waste and foreign compounds from an animal's
body.
Obviously in fresh water the kidneys remove a lot of water from plasma and retain
salts. Thus FW fish urine is voluminous and dilute. In marine fish it is opposite.
KIDNEY TUBULE FUNCTION
The vertebrate kidney is made up of thousands of tubular structures called
nephrons. Nephrons are composed of:
1. Glumeruli
2. Bowman’s capsule
3. Nephric (or convoluted) tubule
4. Longitudinal collecting duct
The collecting ducts empty into the bladder.
Nephron
Why filter from arteries instead of veins?
Freshwater
Saltwater
Since marine fish need to retain their water, as do terrestrial vertebrates, the
kidney model is highly similar. Water is a key element to track as one thinks about
the function of kidneys, so I recommend the following video as a good explanation
of kidney function for water retention:
http://www.youtube.com/watch?v=VpNegk0KXys
Note especially the multiplier effect due to countercurrent exchange.
Keep in mind in freshwater the system works largely opposite this one, and keep in
mind that similar mechanisms are at play for all the molecules passing through the
nephritic tubule walls.
FYI: Handy view of osmosis and ion exchange across membrane:
So now let’s think through the ecology. Connect osmoregulation to natural
history, environmental changes, community ecology, etc. [Brainstorm] Lateral
thinking practice—Why are estuaries preferred nursery grounds for marine fishes?