Salinity and Buoyancy: Fish Swim Bladder

http://www.scienceisart.com/B_SwimBladder/SwimBladder.html

Buoyancy• The relationship between density of a fluid, weight of an object,

and buoyancy is an absolutely basic concept for all students of the sea or fresh waters, because density directly affects everything in the water.

• Scientific understanding of this basic science concept was first elucidated more than 2,000 years ago by Archimedes, a Greek scientist and middle school teacher– Archimedes was interested, among other things, in why things float. – He answered this question by a series of careful experiments in which he

weighed the water displaced by each of the objects he tried to float and then weighed the object.

• Archimedes found that the weight of the water displaced by the floating object is greater than or equal to the weight of the object.

Buoyancy• Buoyancy counteracts the earth’s gravitational force, which pulls the

objects downwards, towards the center of the earth. – Because the density of saltwater is greater than the density of fresh water,

objects in seawater float higher than they do in freshwater. • Buoyancy is why ships float.

– Ships float because the force of gravity pulling the ship down is less than the force of the water pushing the ship up.

• Empty ships ride high in the water. • When loaded with cargo a ship rides lower in the water.

– It is important not to load too much cargo aboard or the ship will take on water and sink.

• A “Plimsoll mark” is found on all large ships. – This mark shows proper loading in fresh or seawater.

• A ship can hold more cargo in seawater because it floats higher.

Bell Work: Fish Buoyancy

1. Why do you think fish don’t float to the top of the water or sink to the bottom in ocean water?

2. Why do different fish swim at different depths in the open ocean?

3. Do you think freshwater fish would have trouble swimming in ocean water and saltwater fish have trouble swimming in freshwater? Explain.

CWM Objectives:

• The student should be able to:– Analyze the relationship between buoyancy and

water displacement– Explain how salinity relates to buoyancy of an

object in water– Identify how and why the fish regulates its

buoyancy in water – Describe the way fish fill their swim bladder– Propose some advantages of a counter-current

exchange system

• Buoyancy Neutral: having same density as the water (you don’t sink or float)

• Density of ocean water varies with location and time do to:– Freshwater inflow from rivers (estuaries)– Evaporation rates (slow or fast)– Amount of rain– Depth of water (layering)

The swim bladder of fish...

• Swim Bladder (red oval) – maintains neutral buoyancy– Contains oxygen gas which can be filled or emptied

to achieve the desired density for the whole fish.– Gills (black crescents) - the functional equivalent of

our lungs; blood adsorbs oxygen from the water and releases carbon dioxide

– FYI - Swim bladder in the adult is not connected to the mouth or digestive system. • However, during embryonic development it was formed

from a pouch of the digestive system.

Moving gas into and out of the swim bladder

• Oxygen gas is carried by hemoglobin in the blood of the fish– Hemoglobin enables the fish to fill or empty its swim

bladder– If blood becomes more acidic, the hemoglobin releases

oxygen into the bladder. – If the blood becomes less acidic the oxygen is

reabsorbed by hemoglobin. – Root effect -The hemoglobin of fish with bladders is

extremely sensitive to acidic conditions, releasing about half of bound oxygen even at high oxygen concentrations.

Oxygen Binding Curve: Response of hemoglobin

to acidity

• A binding curve shows how the amount of oxygen bound to hemoglobin increases with an increase in the free oxygen concentration.

• Black curve - shows the binding of oxygen to hemoglobin under normal conditions.

• Blue curve - Root effect- approximately half of the oxygen is released, even if the free oxygen concentration is very high.

• Red curve - Bohr effect - The hemoglobin of most animals is sensitive to acidic conditions, but to a much reduced degree.

The fish uses lactic acid to force oxygen

into the bladder• Cells on the wall of the swim bladder convert glucose to

lactic acid. – The lactic acid diffuses into blood circulating over the outer

surface of the bladder.• Due to the Root effect, oxygen is released from

hemoglobin and then diffuses into the bladder– expands and gives the fish greater buoyancy.

• The acidic blood and high oxygen concentration is shown as red in the Figure. – mainly restricted to vessels on the surface of the bladder.

• How does the fish do it?1. Blood entering and leaving the bladder surface

pass through the rete mirable (Latin for wondrous network).

2. In this network of capillaries the two blood flows are separated by only thin walls of capillaries.

3. This is a counter-current exchange system, in which acid and free oxygen in the blood leaving the bladder diffuses back into the less acidic blood entering the bladder.

The fish uses lactic acid to force oxygen

into the bladder• The advantage of this localization is three fold: 1. Acidic blood flowing into the main circulation of the fish

would depress the performance of the swimming muscles – (remember how your muscles ache after a hard race).

2. Keeping the acid in the region of the bladder means the acid cells of the bladder don't have to produce as much lactic acid.

3. Recycling the free oxygen back to the bladder increases its concentration and forces it into the bladder.

• Bohr effect (decrease in oxygen affinity in acidic

conditions) appears to have been present some 500 million years ago (when fish first appeared in the oceans).

• Root effect (release of oxygen in acidic conditions)

evolved 350 million years ago.• However, evolution of a swim bladder rete

occurred independently at least four times (but shown only twice in the simplified time line on the left).

• In recent times (if you call the last 100 millions years recent) the swim bladder rete, the choroid rete (for the eye), and even the Root effect has been lost several times by various groups of fish as they evolved.

Lab Quiz1. Turbidity is a measure of _____________, specifically, of how much the solid

matter suspended in water decreases the passage of light through the water. 2. Turbidity impacts aquatic life in the following ways:

– Turbidity interferes with the penetration of __________ needed for the growth of large plants.

– Suspended particles can transport heavy _______ and other toxic substances into the aquatic habit.

– Fish cannot see very well in turbid water and may have difficulty finding __________.

3. What is the order of the solutions that results with three distinct layers?(ocean, brackish, freshwater)

4. Why did this result happen in that order? 5. In an estuary where the river meets the sea, where would the salinity of the

water be the greatest?6. What was your hypothesis for the “Pirate Booty” lab? 7. Does changing the salinity of the water affect the buoyant properties of a “ship”? 8. Explain why saltwater can hold up ships better than freshwater? (include the

relationship between water displacement and mass of saltwater vs freshwater)