Terrestrial Animals and Their Evolution

Terrestrial Animals

• Animals originated in the oceans
• Terrestrial animals evolved from ancestors that independently left the aquatic environment
• Earliest recorded terrestrial animal (myriapod) was approximately 428 million years old, but the initial transition likely occurred earlier
• Relatively few animal taxa are purely terrestrial
• Examples include gastropods, arthropods (insects, arachnids, myriapods, crustaceans), nematodes, annelids, and amniote vertebrates
• Terrestrial environments provided many evolutionary opportunities
• Velvet worms and onychophorans are the only completely terrestrial animal phyla.

Conditions on Land

• Terrestrial environments present different demands compared to aquatic environments
• Gravity necessitates posture maintenance and locomotion
• Air is less dense than water, requiring adaptations for oxygen acquisition and locomotion
• Involves considerations of sensory modes, thermal properties, and the necessity of water as a limited resource
• Animals must be able to cope with exposure to UV radiation

Life on Land

• Desiccation must be avoided
• Animals must have mechanisms to counteract water loss and replace lost water from various loss mechanisms
• Examples include tolerance, excretion mechanisms, gas exchange adaptations, such as specialized internal organs (e.g., lungs, book lungs, tracheal systems)

Desiccation and the Environment - Terrestrial Animals

• Terrestrial animals are affected by the availability of water
• This influences their anatomy, behavior, and physiology
• Water loss occurs by evaporation, urine, and feces
• Animals may use thermoregulation to reduce water loss (e.g., sweating, panting)

Nitrogenous Wastes

• Ammonia, a toxic byproduct of amino acid and nucleic acid catabolism, requires removal or conversion
• Reptiles, birds, and insects convert ammonia into uric acid (semi-solid) for excretion, maximizing water conservation
• Mammals convert ammonia to urea for excretion, but this process necessitates water loss in urine

Desiccation and The Environment - Terrestrial Animals (Insects)

• Small size necessitates adaptations to counteract the higher surface area to volume ratio
• Waxy cuticles and spiracles (opening for air intake) minimize water loss.

Desiccation Tolerance

• Terrestrial tardigrades form a resistant state (tun) to endure harsh conditions (dehydration, temperature extremes)

Sexual Reproduction

• Sexual reproduction can result in genetic variability in organisms.

Aestivation

• Aestivation functions as a coping mechanism against harsh environmental factors (i.e. lack of water and high temperatures)
• Desert-dwelling species of toads and frogs exhibit this behavioral response.

More Requirements for a Terrestrial Life

• Protecting gametes and embryos from desiccation – this often requires internal fertilization

Gas Exchange with Air

• Animals utilize internalized gas exchange organs (lungs, book lungs, tracheal systems) to minimize water loss during gas exchange

Disadvantages of breathing air

• CO₂ diffuses into air less readily than water
• Respiratory surfaces must stay wet

Advantages of breathing air

• Air contains greater oxygen concentration than water; diffusion rate and process are faster than in water
• Less muscular effort is required to move air than water

Hard Skeletons - Endoskeletons

• Bones are composed of collagen that are mineralized by calcium phosphate crystals, highly vascularized and support weight
• Arthropod exoskeletons are composed of chitin, often impregnated with calcium carbonate, and comprised of plates (tergites) with joints to allow motion.

Hydrostatic Skeletons

• Fluid-filled compartments under pressure from muscles support the body shape

Aquatic Animals

• Water support reduces weight, rendering adaptations for support unnecessary.
• Desiccation is a less pressing issue under aquatic conditions.

Salt and Water Balance

• Marine tetrapods excrete excess salt through specialized salt glands

Homeostasis and Thirst and Survival

• Maintenance of a stable internal environment is critical for the survival of most organisms.
• Significant variables to regulate include temperature, pH, concentration of fluids and solutes, and nutrients.

Homeostasis - Negative Feedback Loops

• Homeostasis is maintained by regularly regulating physiological variables using a reference point (setpoint).

Body Weight (Allometry)

• Body mass grows disproportionately faster than the cross-sectional area of limbs in larger animals
• Skeletons are also affected by allometric growth - larger animals need proportionately stronger and larger limbs to support their increased size.

Aquatic Endoskeletons

• These are typically loosely associated (in comparison to terrestrial) with girdles and ribcages are not enclosed.

Water Potential

• Tendency of water to move, encompassing osmosis, hydrostatic pressure, gravity, humidity, other factors
• Osmotic potential (solute potential) - difference in solute concentration across a membrane; affects water movement
• Water pressure potential - mechanical component controlling water movement due to pressure (hydrostatic) on the water.

Osmoregulation

• Maintaining the appropriate solute concentrations inside and outside cells, especially in relation to their environment.
• Osmoregulators adjust their internal solute concentrations to match their environment, often with considerable effort
• Osmoconformers adjust their internal solute concentrations to match those of their surrounding environment

Tonicity and the Environment - Water Dwellers

• Marine bony fish have hypoosmotic systems (lower solute concentration than their environment) and thus lose water
• They gain ions through their gills, drink seawater to offset this loss, and produce dilute urine.
• Chloride cells in their gills pump ions out to maintain the precise ionic balance of their bodies.
• Freshwater bony fish have hyperosmotic systems (higher solute concentration than their environment) and gain water frequently
• These fish produce dilute urine and absorb ions to maintain their internal solute balance.

Osmoregulation in Animals

• Active regulation of water loss and gain
• Excretion of metabolic waste (toxins)
• Controlling salt (NaCl) / water / pH balance in extracellular fluids.

Major Actions of Water Loss Regulation

• Diffusion
• Actions of excretory tubules: filtration, secretion, reabsorption.

Osmoregulation in Animals

• Filtration: Water and solutes are moved into the excretory tubules
• Reabsorption: Valuable substances move from the filtrate back to the blood
• Secretion: Substances from blood are moved into the filtrate.

Ammonia (NH3) Excretion

• Ammonia is highly toxic
• Aquatic animals excrete ammonia directly into surrounding water
• Terrestrial animals produce urea or uric acid to minimize water loss

Protonephridium

• Filter fluid, using flame cells, and eliminating waste through channels

Metanephridium

• Filter coelomic fluid using vessels

Blood Vessels and Blood

• Arteries carry blood away from the heart
• Veins carry blood towards the heart
• Capillaries facilitate oxygen and nutrient exchange.
• Blood consists of plasma (extracellular fluid) and cellular components (RBCs, WBCs, and platelets).

Vertebrate Circulatory Systems

• Variations in circulatory systems are linked to the following: Whether gravity impacts blood flow, location of gas exchange (gills, lungs, or both), and thermoregulatory mechanisms (endothermy vs. ectothermy)

Ventilation

• Organisms use bulk flow (breathing) to exchange gases between the internal tissues and their external surroundings (air or water)
• The system will be influenced by the surface area and volume available to the organism

Atmospheric Composition and Pressure

• Atmospheric pressure decreases with altitude, but gas composition remains constant
• Partial pressures of individual gases in the atmosphere contribute to the total pressure exerted.

Partial Pressure Gradients

• Diffusion of gases relies on differences in their partial pressures
• Higher partial pressure gradients result in more rapid gas diffusion or exchange

Gas Exchange with the ECF

• The rate of gas exchange depends on the surface area for gas exchange, and differences in partial pressures
• Water and blood move in opposing directions to maintain a constant partial pressure difference across the exchange surface (countercurrent exchange)

Gas Exchange with the ECF

• Gas exchange surface area is correlated with body mass and metabolic rate.
• Organisms with high metabolic rates typically have larger gas exchange surfaces to compensate for increased oxygen and carbon dioxide consumption.
• Mammals and birds have a highly specialized respiratory system that efficiently distributes the needed materials

pH Regulation and Homeostasis

• Physiological processes are greatly affected by pH, which reflects the abundance of H⁺ ions in a solution
• Organisms have sophisticated mechanisms to maintain the correct pH for proper functionality
• These mechanisms involve the regulation of CO₂ and buffering processes that either conserve H⁺, or remove H⁺.

Explore the fascinating transition of animals from aquatic to terrestrial environments. This quiz delves into the evolution of terrestrial animals, their adaptations to land, and the unique challenges they face. Test your knowledge about the diversity of terrestrial taxa and their evolutionary significance.