What projectile weapons could a civilization of merfolk build without the use of magic.
The problem with most undersea projectiles is that water resistance makes them fairly range-limited - unless, like a torpedo, they have built-in propulsion. Other than that, compressed air or springs can power harpoons (with or without exploding tips) without too much technical know-how. Heck, even an old school crossbow can do the job over a short range.
They could extract weapons/projectiles from these sea creatures :
1 - Poison projectiles :
2 - Electrical projectiles :
3 - Ink projectiles :
4 - Sharp spears :