Animal Invasion of Land - Lecture 3

Question 1

Invasion of aquatic species on land was first accomplished by

Answer 1

Plants, some arthropods, and gastropods

Question 2

First 4 issues to contend with

Answer 2

1. Less water on land so integuments became thicker and less glandular to avoid drying out
2. Oxygen is more abundant in air than water so respiratory system modified (Evolution of lungs)
3. Air provides little buoyancy compared to water so skeletons evolved to support more weight
4. Temperature fluctuations more extreme so adaptations occurred (e.g. insulation and homeothermy

Question 3

Other 4 issues to contend with

Answer 3

5. Sound travels poorly through air relative to water so the inner ear structure was modified
6. Chemicals in air more difficult to detect than in water so a longer snout evolved for olfaction
7. Nitrogenous waste excretion more difficult (comes from metabolism of proteins)
8. Reproduction became more difficult

Question 4

How did nitrogenous waste excretion become more difficult

Answer 4

a. Direct excretion of toxic ammonia is efficient but requires lots of water (i.e. must be very dilute)
b. Less water is lost by concentrating the nitrogenous waste in urine
c. Higher concentrations (i.e. less water loss) are achieved through proliferation of kidney tubules
d. But, concentrating and retaining highly toxic ammonia within the body doesn’t work, so ammonia is converted to other less harmful products in terrestrial vertebrates (mammals convert to urea; birds/reptiles to uric acid)

Question 5

How did reproduction become more difficult

Answer 5

a. Most aquatic (and semi-aquatic) vertebrates are oviparous, laying unshelled gelatinous eggs
b. Gelatinous eggs dry out on land, blocking exchange of water and gases
c. Shelled amniotic egg solved this problem and allowed for oviparous reproduction on land
d. Shelled amniotic egg serves a self-contained little “pond“ for the developing embryo
e. The outer shell protects the contents from desiccation, mechanical injury and pathogens
f. The outer shell is porous so gasses can diffuse in and out
g. Inside shell are four sacs

Question 6

What are the 4 sacs

Answer 6

i. The outer chorion serves as a surface for gas exchange
ii. The amnion contains the fluid that surrounds the embryo
iii. The allantois stores wastes produced by the embryo
iv. The yolk sac contains nutrients for the embryo

Question 7

Hydrophiles

Answer 7

Are life forms that thrive in high water conditions or concentrations

Question 8

What is water intoxication

Answer 8

• Too much water causes the relative concentration of water in extracellular fluids to increase
• This causes osmotic pressure to increase, driving water into cells and causing them to swell
• Swelling of nervous tissue causes symptoms that resemble those of alcohol intoxication and may lead to death

Question 9

What happens when nervous tissues swell

Answer 9

Symptoms that resemble those of alcohol intoxication and may lead to death

Normal electrolyte levels are also impacted – sodium is particularly important

Sodium concentrations are actively maintained so that levels are high outside the cell

Increasing water in the extracellular fluid dilutes serum sodium levels and results in hyponatremia and additional intoxication-like symptoms

Question 10

What specific problems do hydrophiles face

Answer 10

1. Possible respiratory or photosynthetic challenges since … dissolved gas levels in air and water are different
2. Possible osmoregulatory challenges due to osmotic movement of water (in or out)
3. Possible locomotory challenges due to water density and viscosity
4. Possible thermoregulatory challenges due to thermal stratification

Question 11

Respiration issues for unicellular aquatic animals

Answer 11

Respiration issues are not that “big” for unicellular (or small) aquatic animals

Gasses diffuse easily between water and cells

Metabolic demands are minimal

Question 12

Photosynthetic Issues for big photosynthetic plants

Answer 12

Photosynthetic issues are not that “big” for aquatic plants

o Gasses can diffuse relatively easily in and out

Question 13

Respiration issues for large aquatic animals are more serious

Answer 13

Deep cells cannot rely on oxygen diffusing from the environment

Metabolic demands tend to be high

There is much less dissolved oxygen in water than in air

Aquatic animals must have very efficient systems to extract sufficient oxygen

Question 14

Specific adaptations to aquatic animals

Answer 14

Gill epithelium is very thin (generally a single cell layer thick)

Use counter-current exchange systems to maintain gas gradients for diffusion

Ventilation rates are relatively high

Respiratory pigments are adapted to carry a lot of oxygen

Question 15

Many animals and plants that live in water face osmoregulatory challenges

Answer 15

Osmoregulatory problems due to movement of water (in or out)

Specific issue (and mitigative strategy) depends on whether the organism lives in fresh or salt water

Question 16

Organisms that live in freshwater

Answer 16

Have internal fluids and cells with relatively low water concentrations

This sets up an osmotic gradient that favours movement of water into the organism

In freshwater plants, incoming water increases internal pressure (turgor)

Rigid cell walls in most species keep them from bursting

Many freshwater algae and protists eliminate excess water via contractile vacuole

Water flows through the cell wall into the cytoplasm and then into the vacuole

Question 17

How does water enter the vacuole

Answer 17

The walls of the contractile vacuole contain pumps that actively move ions (often with other molecules) inside

This sets up an osmotic gradient that favors water flow into the contractile vacuole

Vacuole then contracts and water is expelled through a pore that is continuous with the plasma membrane

Contraction mechanism is unknown but probably involves an myosin complex

Question 18

Multicellular freshwater animals require a more complex, and efficient system

Answer 18

Release excess fluid by constantly releasing dilute urine

This is achieved using flame cell complexes (invertebrates) and kidney systems (vertebrates)

As large amounts of urine are released, important ions and salts are lost

Question 19

How are salt/ion levels maintained

Answer 19

The diet

Active uptake by the gills via chloride cells (chloride and calcium ions)

Active uptake by the gills via pavement cells (sodium ions)

Question 20

Organisms that live in seawater

Answer 20

Have internal fluids and cells with relatively high water concentrations

This sets up an osmotic gradient that favours movement of water out to the environment

This means that marine organisms must cope with a desiccating environment

Question 21

Many marine plants, invertebrates, and some fish are osmoconformers

Answer 21

Conformers are not regulators. Regulators regulate the water into their bodies

Conformers = Osmoregularity of body doesn’t change whether or not the environment changes

Marine osmoconformers maintain high internal solute concentrations which allows them to become iso-osmotic to their surroundings