Any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals WordNet 3.0 & Open English WordNet

An integrative science that studies the basic structural and functional relationships within and among living populations and their physical-chemical environments in marine ecosystems. Marine ecology focuses on specific organisms as well as on particular environments or physical settings. OmegaWiki

A branch of biology that deals with those living organisms which inhabit the sea. OmegaWiki