Animals in Freshwater

• Passive water and ion exchanges: Freshwater animals tend to gain water by osmosis and lose major ions by diffusion
• Most types of freshwater animals share similar regulatory mechanisms
• BOX 28.1 Fish Mitochondria-Rich Cells and Their Diversity
• A few types of freshwater animals exhibit exceptional patterns of regulation
• Why do most freshwater animals make dilute urine?

Animals in the Ocean

• Most marine invertebrates are isosmotic to seawater
• Hagfish are the only vertebrates with blood inorganic ion concentrations that make them isosmotic to seawater
• The marine teleost fish are markedly hyposmotic to seawater
• BOX 28.2 Where Were Vertebrates at Their Start?
• BOX 28.3 Epithelial NaCl Secretion in Gills, Salt Glands, and Rectal Glands
• Some arthropods of saline waters are hyposmotic regulators
• Marine reptiles (including birds) and mammals are also hyposmotic regulators
• Marine elasmobranch fish are hyperosmotic but hypoionic to seawater
• BOX 28.4 The Evolution of Urea Synthesis in Vertebrates

Animals That Face Changes in Salinity

• Migratory fish and other euryhaline fish are dramatic and scientifically important examples of hyper-hyposmotic regulators
• Genomic studies point to greater gene-expression changes in crustaceans than fish
• Animals undergo change in all time frames in their relations to ambient salinity

Responses to Drying of the Habitat in Aquatic Animals

• BOX 28.5 Anhydrobiosis: Life as Nothing More than a Morphological State

Animals on Land: Fundamental Physiological Principles

• A low integumentary permeability to water is a key to reducing evaporative water loss on land
• Respiratory evaporative water loss depends on the function of the breathing organs and the rate of metabolism
• An animal’s total rate of evaporative water loss depends on its body size and phylogenetic group
• Excretory water loss depends on the concentrating ability of the excretory organs and the amount of solute that needs to be excreted
• Terrestrial animals sometimes enter dormancy or tolerate wide departures from homeostasis to cope with water stress
• The total rates of water turnover of free-living terrestrial animals follow allometric patterns

Animals on Land: Case Studies

• Amphibians occupy diverse habitats despite their meager physiological abilities to limit water losses
• Xeric invertebrates: Because of exquisite water conservation, some insects and arachnids have only small water needs
• BOX 28.6 The Study of Physiological Evolution by Artificial Selection
• Xeric vertebrates: Studies of lizards and small mammals help clarify the complexities of desert existence
• Xeric vertebrates: Some desert birds have specialized physiological properties

Control of Water and Salt Balance in Terrestrial Animals