Water Balance in Aquatic Organisms

Question 1

Passive diffusion

Answer 1

• Passive movement of material from an area of high concentration to an area of low concentration of the same substance (along a concentration gradient)

Question 2

Osmoregulation/Osmotic cocentration/Osmolarity

Answer 2

• Quantity of solute particles per litre of solution
• Measured in osmoles of solute per litre (osmol/L ir mosmol/L)
• Seawater: 1000 mosm/L
• Freshwater: ~0.5-10mOsmol/L
• Sharks: 1000-1050 mosm/L
• Human blood: 300 mosm/L

Question 3

Osmosis

Answer 3

• Passive diffusion of water across a semi-permeable membrane

Question 4

Isotonic

Answer 4

• environment has same water concentration as cell

Question 5

Hypotonic

Answer 5

• too much water - cells gain too much water -explode

Question 6

Hypertonic

Answer 6

• too little water - cells lose water ad shrivel up

Question 7

Isosmotic

Answer 7

• internal concentrations of water and salt equal concentrations in the environment
• Salt and water diffuse at approximately equal rates in and out

Question 8

Hyperosmotic

Answer 8

• Lower internal concentration of water and higher internal concentration of salt compared to environment
• Salts diffuse out at higher rate
• Water diffuses in at higher rate
• Higher osmolarity inside because more solutes than outside

Question 9

Hypoosmotic

Answer 9

• Higher internal concentration of water, lower internal concentration of salt compared to environment
• salts diffuse in at higher rate
• water diffuses out at a higher rate
• Lower osmolarity inside because there are less solutes inside

Question 10

Osmoregulation

Answer 10

• Regulation of water and solute concentrations

Question 11

Challenges for marine, freshwater and terrestrial species in osmoregulation

Answer 11

• Marine: how to not become dehydrated (limit water loss)
• Freshwater: how to not become overhydrated (limit water gain)
• Terrestrial: how to not become dehydrated

Question 12

Why is water necessary?

Answer 12

Transport
Metabolism
Excretion

Question 13

Excretion

Answer 13

• Eliminating waste
• Must be considered when discussing osmoregulation because water and solutes are eliminated
• Some occurs through gills in aquatic organisms
• most occurs through the excretory system

Question 14

Three different nitrogenous bi-products

Answer 14

• Ammonia; very toxic, freshwater and some marine organisms which excrete is across gills and urine
• Urea; produced in liver and kidneys from ammonia, less toxic, terrestrial vertebrates except birds and reptiles, cartilaginous fish
• Uric Acid; excreted as paste (guano in birds), insects, birds and diapsids (crocodiles, snakes, turtles, lizards)

Question 15

Kidneys

Answer 15

• Made up of nephrons
• Filtrates water and solutes (nitrogenous waste), pushed by blood pressure through a filter called glomerulus
• Reabsorbs water and solutes needed passively or actively
• Whatever left is excreted

Question 16

Why is salt necessary?

Answer 16

• Sodium-Potassium pump
• Active transport by Na/K ATPase pump, an enzyme in the plasma membrane that pumps Na out and K into the cell, moving ions across a steep concentration gradient
• Powers cellular activities like signaling in neurons

Question 17

Stenohaline vs. Euryhaline

Answer 17

o Stenohaline; function in a small range of osmolarity
o Euryhaline; survive much larger fluctuations in external osmolarity (they can adjust themselves)
- Ex: barnacles, muscles, bass and some salmon

Question 18

Time frame of adaptations to different salt concentrations

Answer 18

o Genetic adaptation: conditions change over evolutionary time
- Ex: freshwater fish evolve from marine fish
o Phenotypic plasticity: conditions change from one generation to the next
- Ex: freshwater stickleback fish grown in salt water have longer jaws and tend to be thinner
o Acclimatization/acclimation: conditions change at least once in lifetimes
- Ex: salmon and other diadromous fish
o Regulation: conditions fluctuate regularly
- Ex: intertidal zones

Question 19

Osmoconformer

Answer 19

• Isoosmotic with environment; same internal conditions as it’s environment
• Generally marine animals (intertidal)
• Constant exchange, but no net gain or lost
• Tend to live in environment where osmolarity is fairly constant
• Excretion of salt ions and water is done in a minimal way; small amount of urine form kidney

Question 20

Osmoregulators

Answer 20

• Controls independent osmolarity to it’s environment
• Freshwater, marine or terrestrial usually
• Survive changes in external osmolarity
• In freshwater, deal with an inflow of water from environment and in salt water resists loss of water from tissue
• Excretion of salt ions and large amounts of water in dilute urine from kidneys (freshwater)

Question 21

Marine: Algae and invertebrates

Answer 21

o Isosmotic
o Conformers
o Environment: constant, with reasonable osmotic concentration (OC)
o Ex: hagfish, slime eel (skull but not vertebral column or jaw)

Question 22

Marine: Cartilaginous fish, Crab, eating frog, Coelacanth

Answer 22

o Slightly hyperosmotic (internal environment has a higher concentration of solutes BUT less salty – i.e salt isn’t the solute that is higher)
o Conformers (generally)
o Fish: environment constant with reasonable OC
o Crab-eating frog: the only frog that can tolerate salt water: lives in freshwater but hunts in brackish or salt water
o How do these animals maintain such high solute concentrations?
- Counteracting solute strategy
• Retain urea in blood to increase Osmotic Concentration (urea is a waste product of metabolism; most animals excrete urea through urine and across gills)
• Urea is toxic, it degrades proteins
• To counteract urea, increase TMAO, a protein stabilizer that counteracts the effects of urea
o Shark: slightly higher osmolarity than seawater so water diffuses into the shark; BUT high osmolarity due to urea, sodium concentration is slightly lower in the shark than in the seawater, so sodium diffuses into the shark
o Excretion of nitrogen waste
- Cartilaginous fish
• Tend to gain water
• Absorbs water through osmosis
• Large amounts of dilute urine
• Excrete some urea
• Urea is produced in liver and mostly reabsorbed in kidneys

Question 23

Marine bony fishes (teleost)

Answer 23

o Hypoosmotic to seawater (lower solute concentration to environment)
- Body OC ~1/3 that of seawater
- Tendency to lose water, mostly across the gills
o Osmoregulators; they maintain their osmolarity at 1/3 that of seawater
o How?
- Impermeable skin
- Drink seawater
- Active transport of salt out through gills (chloride cells)
- Urine scant, but isosmotic
• Highly concentrated especially relative to cartilaginous fish
o Excretion of nitrogenous waste
- Bony marine fish:
• Tend to lose water
• Drinks water
• Little urine, less concentrated than body fluid
• Some ammonia or urea
o Marine bony fish kidneys
- Reduced or no glomerulus to minimize water loss
- Rely on tubular secretion
• Active transport of waste into tubules for excretion

Question 24

Marine and intertidal plants

Answer 24

o Seagrasses, generally near-shore 
o Osmoregulators 
o Special glands to excrete salts in the leaves 
o Accumulation of salts in roots 
- Lower osmotic pressure draws water in 
o Large vacuoles store Na+

Question 25

Halophiles

Answer 25

o Primarily archaea
o Some bacteria, some eukaryotes (fungi, algae)
o Found in hyper saline environments (~5X greater OC than normal ocean)
- Dead sea, great salt lake, evaporation ponds
o Isosmotic, osmoconformers
o How?
- Increase intracellular solute concentrations using amino acids and sugars
- Selective influx of K
• Involves covering internal surfaces (ex mitochrondria) with charged amino acids, allowing retention of water

Question 26

Osmoregulation in Marine Mammals

Answer 26

o Osmoregulators
o Impermeable skin
o May drink seawater but mostly get salts and water from food
o Relatively large amounts of high concentration urine, concentration depending on diet and seawater drinking
o Excrete urea

Question 27

Freshwater fish

Answer 27

o Osmoregulators
o Adaptations
- Impermeable skin, unless involved in gas exchange (amphibians)
o Lower osmolarity than marine species
o Urine hypoosmotic to blood
- More dilute, lower solute concentration
o Active transport of salt inwards through gills
o Drink almost no water and excrete large amounts of very dilute urine from kidneys
o Excretion of nitrogen waste
- Freshwater fish
- Tend to gain water, do not drink, large amounts of urine, urine mostly water, some ammonia
o Freshwater aquatic kidneys
- Nitrogenous waste largely excreted through ills so kidney is mostly to get rid of extra water

Question 28

Air breathing freshwater vertebrates

Answer 28

o Similar concerns to terrestrial vertebrates
o Ex: lungfish, freshwater seals, amphibians
o Osmoregulators
o Relatively large amounts of highly concentrate urine (depending on diet and water drinking)
o Impermeable skin – some
o Excrete urea
o Obtains salts from food
o Freshwater aquatic kidney (frog)
- Nitrogenous waste largely excreted through skin, so kidney is mostly to get rid of extra water. Bladder stores extra water when in water to be reabsorbed when on land

Question 29

Osmoregulation in brackish water

Answer 29

• Brackish water has more salinity than fresh, but less than marine
• Under relatively high salinity (Ex; seawater), conformers using compatible solutes (amino acids)
• Regulate at low salinity
  o Ex: escape by closing shells
• High salinity = conform
• Low salinity = regulate
• Osmoregulate over some range but osmoconform at either ends of that range
• Intertidal organisms are euryhalines, may vary somewhat in osmolarity (contain more water when in less saline environment)

Question 30

Osmoregulation in Diadromous fish

Answer 30

Migrate between seawater and freshwater
– Anadromous: migrate from the sea up
– Catadromous: migrate from freshwater down
• Regulate in both fresh and salt water
• Mediated by photoperiod in salmonids
• Changes are hormonal and begin occurring
prior to encountering seawater