barracuda
魳 refers specifically to a barracuda, a predatory fish. The meaning is narrow and concrete, without broader abstract extensions.
魳 combines the fish radical 魚 with the phonetic component 帀, which likely indicates the sound. The exact historical development is uncertain.
The fish radical 魚 on the left tells you it's a fish, and the right side 帀 looks like a barracuda's sharp teeth and streamlined body. Picture a barracuda swimming with its mouth open, showing those teeth.
For シ, imagine a barracuda swimming silently through the sea: the 'shi' sound of the water rushing past its body cues シ.