Final answer:
Amoebas are called "shape-shifters" due to their capacity to actively change their shape via pseudopodia for movement and feeding, owed to their lack of a rigid cell wall.
Step-by-step explanation:
Amoebas are called "shape-shifters" because of their unique ability to change their shape actively through the formation of pseudopodia. These are temporary projections of the cell's cytoplasm used for movement and feeding. The supergroup Amoebozoa, to which amoebas belong, is characterized by these amoeboid forms of movement. The process involves actin microfilaments that produce pseudopodia, allowing the remainder of the protoplasm to flow into these extensions, thereby propelling the organism forward.
The cause of this shape-shifting ability is primarily due to the amoeba's lack of a rigid cell wall, which grants them the flexibility to change form. This cellular flexibility also contributes to their predatory habits, as it allows them to encase and digest food items. Amoebas like Amoeba proteus are well-known examples of this group, showing vivid shape-shifting behaviors as they move about their environment.