WFSC 448 – Fish Ecophysiology(Week 5 – 22 Sep 2019)

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—gills as problem and solution

The goal in ion regulation, physiologically, is to …1. Maintain osmotic homeostasis at set point2. Specific ionic balance3. Excretion of ammonia (NH4)

Why would a stable water-solute balance be important?Maintains relatively stable environment for animal cells.

When hyper-osmotic, cells may not function and may even rupture Variations in ion concentrations may affect structure and function of

macromolecules. Cells exposed to osmotic gradients can shrink or swell. Changes in cell volume can damage cells directly

o cells may burst when transitioning from hypo-osmotic states.

Homeostasis is often not cheap. Energy is frequently necessary for physiological regulation, as in fish osmoregulation.

Osmoconformers body fluid is iso-osmotic with surrounding water only in saltwater hagfish, skates, sharks high osmolarity maintained by high balance of organic solutes

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 animal2. Size of gradient3. 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).

Kinne (1940). Classic paper is aquatic physiology. Carp placed into brackish water suffer severe respiratory difficulty and irreversible damage—those allowed to acclimate over several days have no troubles. For lobster…

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 - 40o 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-

Salt Water scenarioFunctions 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 (ion exchange), and the potassium diffuses back to body.

Chloride builds up so much inside cells it also diffuses 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 crypt, 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?

Another facultative (phenotypically plastic) function of chloride cells:

In mammals, o nitrogenous waste in the form of urea is removed by kidney

In fish, o nitrogenous waste in the form of ammonia is removed by the gillso kidneys are more about osmotic and ionic balance

Functions of Excretory Systems:1. Adjust the quantity of water and various plasma constituents to be

conserved by the body or eliminated in the urine2. 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. Glumeruli2. Bowman’s capsule3. Nephric (or convoluted) tubule4. Longitudinal collecting duct

The collecting ducts empty into the bladder.

Nephron

Why filter from arteries instead of veins? (ROATP)

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 Thomas Conena video as a good explanation of kidney function

for water retention: http://www.youtube.com/watch?

v=VpNegk0KXysNote 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:

One can reverse this trend by applying pressure on the ‘salt side’ of the membrane. Pressure essentially presses, or forces, water through, even against a concentration gradient.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?