Since mankind, scientists are struggling with the definition of life.
Before something is called a living thing it should meet a number of life characteristics such as breathing, move, grow ... and reproduce.
When the life characteristics are not present, it is dead or inanimate. Living things are called organisms.
All organisms are studied in biology, literally translated as ‘the study of life’.'.
Marine biology is the part of biology that studies all life in water.
Because there are millions of organisms, organisms are divided into domains. Each domain is, or could be, divided into one or more kingdoms. In biology, classification of organisms is called taxonomy. Taxonomy is a dynamic system that –unfortunately still today- is sometimes being adjusted after new discoveries and researches.
However all organisms are hierarchically classified according to these main taxonomic ranks: