Brain Coral

The Purple Brain Coral is a type of coral. It produces bubbles that refill 10 units of oxygen. It is found in many biomes, namely Safe Shallows and their caves. The fact that they spawn in caves is very helpful and in some cases life-saving, because they spawn about three oxygen bubbles at a time, filling up the player's oxygen level by a total of 30 points.

Purple Brain Corals spawn bubbles day and night. They do not react to being attacked, and they are completely ignored by other life forms.

Speculation
An artwork by the developers in which the Purple Brain Coral appears suggests that the Purple Brain Coral might be inhabited by some sort of worm-like creatures in future builds.