A cuttlefish normally regulates buoyancy with its cuttlebone, a chambered internal shell whose gas-to-liquid ratio it adjusts. In Metasepia the cuttlebone is small relative to the body, and sustained swimming is costly. The animal instead moves along the sediment using the tips of its third arm pair and two flaps of the ventral mantle, in a rocking gait.
Walking on open sand leaves it exposed, and the species does not respond by hiding. Waves of colour sweep across the body, most vividly when the animal is disturbed. Bold, high-contrast display in a slow-moving animal is the pattern biologists call aposematism, or warning colouration.
The claim usually attached is that its flesh is uniquely toxic. That originates in a preliminary report of acutely toxic muscle tissue presented at a 2010 meeting, which has not been followed by a published characterisation of the compound. So the display is real and the walking gait is real, while the toxicity is a plausible but poorly documented inference. Its close relative Metasepia tullbergi shows a similar gait.