Shimshell

It grazes the algae and encrustations of the deep ruins, hauling its heavy shell slowly along the drowned stonework, and it withdraws entirely into that armored spiral at any disturbance, a shining, sealed door against the dark. The faint glow of its shell seems to lure the tiny drifting creatures it filters from the water, a living lantern that fishes while it feeds. It is slow, patient, and long-lived, and the oldest Shimshell wear shells thick with generations of growth, luminous relics hauling themselves through the ruins they have haunted since before the divers came.