Osmoregulatory Systems in
Fishes
Maintaining homeostasis with
respect to solute concentrations
and water content
Homeostasis
Chapter 7
Zoology 1450
Introduction


Homeostasis = maintaining steady state
equilibrium in the internal environment of
an organisms
Much is done involuntarily by action of
hormones, enzymes and osmoregulatory
processes. Although occasionally fish do
just “pick up and move” if environmental
conditions are unfavorable.
Topics
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Osmoregulation
Endocrine system
Thermal regulation
Definitions
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Homeostasis = maintaining steady state
equilibrium in the internal environment of an
organisms
Solute homeostasis = maintaining equilibrium
with respect to solute (ionic and neutral solutes)
concentrations
Water homeostasis = maintaining equilibrium
with respect to the amount of water retained in
the body fluids and tissues
Definitions, continued

Osmotic concentration - Total
concentration of all solutes in an aqueous
solution: measured in units of osmolals = 1
mole of solute/liter of water or
milliosmolals = 1/1000th of one osmolal
Osmoregulation in different environments

Challenge to homeostasis depends on

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steady state concentration of solutes in the
body fluids and tissues as well as
concentration of solutes in the external
environment
marine systems: environment concentration = 34 36 parts per thousand salinity = 1000 mosm/l
 freshwater systems: environment concentration <
3 ppt = 1 - 10 mosm/l

Osmoregulation in different environments

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:
 short term changes: estuarine - 10 - 32 ppt,
intertidal - 25 - 40
 long term changes: diadromous fishes
Four osmoregulatory strategies in fishes
1. Isosmotic (nearly isoionic, osmoconformers)
2. Isosmotic with regulation of specific ions
3. Hyperosmotic (fresh H20 fish)
4. Hyposmotic (salt H2O fish)
Osmoregulation Strategies
Osmoconforming (no strategy) Hagfish internal salt
concentration = seawater. However, since they live IN the
ocean....no regualtion required!
Osmoregulation Strategies
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.

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.
Osmotic regulation by marine teleosts...
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ionic conc. approx 1/3 of seawater
drink copiously to gain water
Chloride cells eliminate Na+ and Clkidneys eliminate Mg++ and SO4=
advantages and disadvantages?
active tran.
Saltwater teleosts:
passive diff.
H2O
drink
Na+, Cl-
Na+,
Cl-
Mg++, SO4=
Na+, Cl-
Mg++, SO4=
chloride cells
kidneys
active
passive
Chloride Cell fig 6.2:
sea water
PC
pavement
cell
ClCl-
Cl-
+
carrier
Na+
Cl-
K+
chloride cell
internal
PC
Na+
Na+
Na+, Cl-
gut
accessory
cell
Na+
pump
Na+
Na+ K+ ATPase
mitochondria
tubular system
Osmotic regulation by FW teleosts
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Ionic conc. Approx 1/3 of seawater
Don’t drink
Chloride cells fewer, work in reverse
Kidneys eliminate excess water; ion loss
Ammonia & bicarbonate ion exchange mechanisms
advantages and disadvantages?
Freshwater teleosts:
don’t
drink
active
passive
H2O
Na+, Cl-
Na+, Cl-
Ion exchange
pumps; beta chloride cells
water
kidneys
Ion Exchange Mechanisms
freshwater
interior
Na+
active
ATP
pump
NH4+ or H+
Clactive
ATP
pump
HCO3-
gill membrane
Freezing Resistance:

What fishes might face freezing?
hagfishes?
isotonic
marine elasmobranchs?
isotonic
freshwater teleosts?
hypertonic
marine teleosts?
hypotonic
Solution for Antarctic fish

Macromolecular antifreeze compounds
peptides (protein)
glycopeptides
(carbohydrate/protein)
rich in alanine
{
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molecules adsorb (attach) to ice crystal
surface
interfere with ice crystal growth
(disrupt matrix)
Why is this important???
ice ruptures cells; hinders osmoregulation
What about rapid ion flux?
Euryhaline
 Short-term fluctuations in osmotic state of
environment, e.g. in intertidal zone or in
estuaries where salinity can range from 10
to 34 ppt with the daily tidal cycle:

these fish have both kinds of chloride cells
when salinity is low, operate more like FW fishes
 when salinity is high, operate like marine fishes
 kidneys function only under low salinity conditions
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Euryhaline
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Diadromous fishes (spend part of life in salt
water, part in freshwater – catadromous
(migrate seaward) or anadromous (migrate
up river)
hormone-mediated changes associated
with metamorphosis - convert from FW
adaptations to SW or vice versa, depending
on direction of migration
What about stress??
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Stressors (handling, sustained exercise such as
escape from predator pursuit) cause release of
adrenaline (epinephrine) - for mediating escape,
etc.
Adrenaline causes diffusivity of gill epithelium to
increase (become “leaky” of water & ions)
This accentuates the normal osmoregulatory
challenge for FW or marine fishes
How to reduce stress in stressed fishes?

Minimize the osmotic challenge by placing
fish in conditions that are isosmotic
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add salt to freshwater, e.g. in transporting fish
or when exposing them to some other shortterm challenge
dilute saltwater for same situation with marine
species
Thermoregulation in Fishes
Temperature effects on fish
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Temperature exhibits the greatest influence on
fish’s lives!
Affects metabolism
Affects digestion
Signals reproductive maturation and behavior
Fish are conformers (well, sort of...)
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Body temperature is that of the environment
Each species has particular range of
temperatures that they can tolerate and that
are optimal
Big difference between what you can tolerate
and what you thrive in...
Behavioral Thermoregulation in Fishes
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Although fish are ectotherms, they can
alter their body temperature by moving to
habitats with optimal temperature
Hot Fishes
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Some fish can maintain body temperature
greater than ambient - tunas, billfishes, relatives
(nearly endothermic)
Use retia (similar to rete mirable) in swimming
muscles to conserve heat, exchange O2, etc.
Red muscle is medial rather than distal
Billfishes have warm brains - heat organ from
muscles around eye
Practical application

You’re management decisions and actions
must account for fish responses to
temperature gradients and limitations
Endocrine Systems of Fishes
Pituitary Gland - Master Gland
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Linked with hypothalamus of brain
Produces hormones that affect other
endocrine tissues - indirect influence
Produces hormones that affect nonendocrine tissues directly
Pituitary Gland
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Indirect influence
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ACTH - adrenocorticotrophic
hormone
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TH - thyrotrophic hormone
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stimulates interrenal tissue production
of cortisol
stimulate thyroid production of thyroxin
(growth, metamorphosis-i.e. flounder)
GTH- gonadotrophic hormone

stimulates gonads to produce
androgens/estrogens
Pituitary Gland

Effects non-endocrine tissues directly
 pigmentation - melanophore stimulating
hormone (MSH)
 affects long-term control of color
 osmoregulation - prolactin, vasotocin
 controls fresh/saltwater systems
 growth – somatotrophic hormone
 stimulates > length, cell
multiplication
Thyroid Gland
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isolated follicles distributed in connective tissue
along ventral aorta
controls metabolic rate
affects metamorphosis, maturation
facilitates switch between fresh & salt water
Gonads
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gamete and sex hormone production
controls gametes maturation
cause formation of secondary sex
characteristics: color, shape, behavior
in fish, several sex hormones also serve
as pheromones - e.g. goldfish males
respond to hormones released with
ovulation
Other endocrine tissues in fishes

chromaffin tissues-located near kidneys & heart
 produce adrenaline/noradrenaline – “fight or flight”
 increases blood flow through gills, ventilation rate

interrenal (inside kidney) tissues
 produce cortisol, cortisone - stress response
hormones (reduce inflamation)
Other endocrine tissues in fishes
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pancreatic islets
 produce insulin - controls glucose, glycogen
metabolism (glucagon production)

corpuscles of Stannius
 produce stanniocalcin - controls Ca+2 inflow at gills
Immune System
Introduction
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Obviously, the immune system is important in
homeostatic processes.
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Immune systems of fish have two components:
non-specific and specific.
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As we will see, both are involved in protecting
fish from visible as well as invisible disease
causing agents.
Non-specific immunity
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Skin & Scales—specific solid layers of protection
from pathological and chemical stressors.

Mucus secretion—traps microorganisms;
preventing entry into body cavity or circulation
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Macrophages (phagcytes) and cytotoxic cells—
part of the inflamatory response which destroy
pathogens within the body before they can do
harm.
Specific Immune Response
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More of an active response
where an “invader” is detected
and destroyed.
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Primary organs: kidney,
thymus, spleen, intestine.
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Antigens—invading
compounds which provoke an
immune response.
Source: Cancer Research Institute (2002) www.cancerresearch.org/immhow.html
Specific immune response: What if something does get in??
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White blood cells called B lymphocyte cells (B cells) and
T lymphocyte cells (T cells)—bind to foreign cells and
begin replication and attachement to (sort of markers for
things to come...).

Occasionally, invader actually goes trough a
macrophage first...then B cell responds
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Once B cells replicate, antibodies are produced which
bind specifically to pathogens and tag them for
destruction (eating) by macrophages!
“Looks like meat’s
back on the menu
boys!!!”
Questions???