Final answer:
Estuarine animals and plants have evolved adaptations to cope with changing salinity levels, with plants often having specialized root systems and animals displaying behavioral changes like switching respiration methods to adjust to the brackish water environment.
Step-by-step explanation:
Animals and plants that live in estuaries must be able to adapt to changing salinity. This fluctuation in salt concentration is due to the mixing of freshwater from rivers and saltwater from the ocean, creating a brackish water environment. Certain plant species in estuaries, known as halophytes, are specially adapted to handle these salty conditions. Some possess filters within their roots to extract salt from the water they absorb, while others can transport oxygen to their roots to cope with the salty environment.
Similarly, animal residents of estuaries, such as mussels and clams from the phylum Mollusca, have evolved behavioral adaptations to varying salinity levels. During periods of low salinity, these animals cease feeding, close up their shells, and switch from aerobic respiration, which relies on oxygen from the water absorbed through their gills, to anaerobic respiration, which does not require oxygen. These changes are energy-intensive but crucial for survival in the dynamic estuarine environment. Once the salinity and oxygen levels increase again with the incoming tide, these animals resume feeding and revert to aerobic respiration.