king crab
鱟 refers specifically to the king crab, a marine arthropod. The meaning is narrow and concrete, without broader abstract extensions.
鱟 is a phono-semantic compound: the bottom component 魚 (fish) indicates the aquatic animal, while the top component 學 (abbreviated) likely provides the sound. The exact historical development is uncertain.
The bottom is 魚 (fish), and the top looks like a complex spiky shell. Picture a king crab with a fish-like tail and a crown of spikes.
For コウ, imagine a king crab wearing a tiny crown: crown -> コウ, and the crab rules the ocean floor.