In an answer to [this question](https://worldbuilding.stackexchange.com/questions/30738/how-do-mermaids-perform-major-surgical-procedure-underwater) [Cyrus](https://worldbuilding.stackexchange.com/users/14892/cyrus) suggested that pre-technology merpeople might use shrimp or prawns in lue of maggots to eat away rotting flesh. 

Similar to how plants and (in some cases) animals were used to heal ancient people in our own world; Opium poppies (from which comes morphine), Honey (which helps sooth sore throats), marigold (I think this helps heal infections, if I'm wrong let me know), leeches (blood letting, not successful but still used) and maggots. There are freshwater and salt water variants of the merpeople, so answer involving salt and fresh water life would be helpful. My merpeople live in medium and shallow levels of water but can go to deeper areas if they need to. So for example if they were bitten by a shark, 'stepped' on a sea urchin, or cut on coral, what underwater life would be used to aid the injured in a merperson culture?