The Atlantic longarm octopus (Macrotritopus defilippi) mimics a flounder to escape detection:

Image by Roger Hanlon via LiveScience

The mimic octopus (Thaumoctopus mimicus) also mimics flatfish:

Image via

The wunderpus (Wunderpus photogenicus) is another fascinating mimicking octopus:

Roberta Bonaldo has conducted research on social mimicry between Octopus insularis and Cephalopholis fulva, a species of grouper:

Images via

More on octopus mimicry here and here

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