ray, skate
鱏 centers on flat-bodied cartilaginous fish: rays and skates. The two meanings are closely related, both referring to similar bottom-dwelling sea creatures.
鱏 combines 魚 (fish) with 覃, which likely contributes the sound. The character was created to name specific types of rays and skates.
The fish radical 魚 on the left tells you it's a fish. The right side 覃 looks like a wide, flat creature spreading out—just like a ray or skate gliding along the ocean floor.
For シン, imagine a ray gliding through the deep blue shin (shin as in the front of your leg, but here think of the smooth, flat surface of the sea creature). The ray's body is as flat and smooth as a shin, and the sound シン helps you remember the reading.