The calyx . Sessile means the organism is attached to the substrate. Pelagic refers to living in the water of the ocean above the bottom. Pelagic organisms usually have some ability to move around. 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.