You are a quick little reef fish. On the right, an armored urchin core hides behind a curved wall of coral. Swim into coral cells to eat them, or shoot pellets to chip them away one cell at a time.
Eating two cells, or bumping the core itself, charges the harpoon cannon, which then appears on the left edge and follows your height. Press fire to launch it straight across the water, and get out of its way: the harpoon will spear you too. Coral stops it, so open a gap in the wall first. A hit on the core clears the level, and hitting it while it is lunging is worth much more.
A drifting jelly hunts you and the core sometimes winds up, glows and lunges at where you were. The golden kelp current in the middle hides you from both, but you cannot fire from inside it, and the harpoon still reaches you there. Swim off the top or bottom to come back on the other side.
Odd levels have a fixed arch of coral; even levels have coral that rolls down through the arch like a conveyor. Each level is a little faster. You have three lives.