Salinity:
* Saltwater animals: Live in environments with high salt concentrations (like oceans and seas).
* Freshwater animals: Live in environments with low salt concentrations (like rivers, lakes, and ponds).
Osmoregulation:
* Saltwater animals: They tend to lose water to their environment due to osmosis and must actively drink seawater and excrete excess salt to maintain a proper balance.
* Freshwater animals: They tend to gain water from their environment and must constantly excrete excess water and absorb salt to maintain balance.
Physiological Adaptations:
* Saltwater animals: Often have special organs like salt glands to excrete excess salt. Their body fluids are typically isotonic or slightly hypertonic to seawater.
* Freshwater animals: Often have a high urine volume and absorb salts through their gills. Their body fluids are typically hypotonic to freshwater.
Examples:
* Saltwater animals: Fish like tuna, sharks, dolphins, marine invertebrates like crabs, jellyfish, and sea stars.
* Freshwater animals: Fish like trout, bass, catfish, amphibians like frogs and salamanders, freshwater invertebrates like crayfish, snails, and insects.
Other Differences:
* Availability of Oxygen: Oxygen levels can be lower in freshwater environments, leading to adaptations in respiration.
* Temperature: Freshwater environments can have greater temperature fluctuations than saltwater environments.
* Habitat: Saltwater environments are vast and varied, while freshwater environments are more localized and diverse.
Interesting Note: Some animals, like salmon, can live in both freshwater and saltwater environments, undergoing physiological changes to adapt as they migrate between the two.
In summary, the differences between saltwater and freshwater animals are primarily driven by the need to maintain a stable internal environment in the face of varying salinity. This leads to adaptations in osmoregulation, physiological processes, and even body shape.