Alright, so the sea cucumber eats starfish in turn for sand or gravel, useful for flint farming. The black carpet eats... well anything that comes close which includes you. Luckily it's a bottom dweller and if you kill it it drops an edible cut of it's body. Sea spider: Neutral bottom dwellers which attack when provoked. They would drop their legs which you can use as an alternate way of crafting a potion of poison. Hope this helps!