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 the Safe Shallows and their caves. The fact that the Purple Brain Coral spawns in caves is very helpful and in some cases life-saving, as it spawns 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. If attacked by the player with a knife, the Purple Brain Coral will turn white after some hits and stop producing bubbles. While this suggests that the Purple Brain Coral is dead, it will continue to pulse.

Speculation
A piece of 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.