Pineapple sea cucumber
Guardian: Aimee Devoil
If you boiled up the pineapple sea cucumber the juice would kill a fish but if you drink some, it is a medicine. It is said to be good for your blood, giving strength to your organs. The Chinese have been eating sea cucumber for health reasons since before the Ming Dynasty (4000 years ago), 3g will ease arthritic pain and the Japanese use it to treat HIV.
So, if you are trapped on a coral island after civilisation has collapsed and there is no modern medicine, lick this sea cucumber (or suck on one of its nodules) and it will nourish your blood.