Final answer:
Estuaries favor halophytic plants and animals like mussels and clams that have adaptations to cope with the fluctuating salinity levels.
Step-by-step explanation:
The physical characteristics of estuaries tend to favor halophytic plants and animals with specialized adaptations such as mussels and clams. Estuaries present a unique challenge due to the rapid and short-term variation in salinity from the mixing of fresh water and salt water. Halophytic plants are capable of tolerating salty conditions, with some having filters in their roots to remove salt, and they can also pump oxygen into their roots. Animals in estuaries have developed behavioral adaptations to cope with this environment; for instance, mussels and clams can switch from aerobic respiration to anaerobic respiration when the salinity levels drop and revert to aerobic respiration when salinity increases.