October 6, 2008. Lecture 14. Gills & Respiration. 1. Why do fish need O2? Why do fish have gills? Why don’t they rely on diffusion across their bodies? 2. Respiratory demands in an aquatic environment 3. How gills work 4. Respiration other than gills 5. Counter current exchange system. 6. Active vs. sluggish fish Despite the fact that O2 has slightly lower solubility in saltwater than in freshwater, most of the airbreathers are freshwater species. Why do you think this is?

Why do fish need oxygen? Why is oxygen necessary for aerobic respiration?

Creatures such as bacteria and protists living in aquatic environments rely on diffusion of oxygen for respiration. Why do fish need gills?

1. The Physical Nature of Water Composition of Dry Atmospheric Air Component % Oxygen 20.95 Carbon Dioxide 0.03 Nitrogen 78.09 Argon 0.93 Total 100

Solubility (ml gas/liter freshwater) 1. The Physical Nature of Water Solubility of gases at 15C at 1 atmosphere pressure Component Solubility (ml gas/liter freshwater) Oxygen 34.1 Carbon Dioxide 1,019.0 Nitrogen 16.9

Bony fish gill Shark gill Gnathostomata have gill arches and gill filaments.

Main points: 2 gill filaments for each gill arch; gill raker shark teleost Main points: 2 gill filaments for each gill arch; sharks have a gill septum which reduces water flow

gill filaments gill lamellae

Water flow through gill of teleost

Main point is that the direction of blood flow is opposite the direction of water flow.

Gill surface area goes up with activity level

Lamellae thickness goes down with activity level.

Summary Questions Why do fish have gills? Why don’t they rely on diffusion across their bodies? Why is respiration in an aquatic environment difficult? In what types of environments do you expect to see fish capable of respiration using structures other than gills? Explain the significance of the counter current exchange system. How do gills differ between fish that swim very fast versus those that are sluggish? Despite the fact that O2 has slightly lower solubility in saltwater than in freshwater, most of the airbreathers are freshwater species. Why do you think this is?