In nature this occurs at the mouths of rivers and swamps near the sea. Some fish live in salt water but are spawned in brackish or fresh water and vice versa. There are several brackish species available in the aquarium hobby, see Freshwater Fish for descriptions. Sometimes permanent although more usually temporary attachment of tough organic threads secreted from a gland in the foot of the clam. See anthostele .
Sharks are a good example.
Turtles, crabs, and boxfish are good examples. The single fin mounted vertically at the rear of the fish. The tail fin. Consists of mesoglea and may have sand imbedded in it. Fish will scratch themselves against rocks and breathing may become rapid if gills are affected.