Here's why:

* They are filter feeders. Brain corals obtain their food by filtering tiny organisms, like plankton and algae, from the water.

* They have symbiotic algae. They also have a symbiotic relationship with zooxanthellae, microscopic algae that live within their tissues. These algae provide the coral with food through photosynthesis.

So, while brain corals do consume some small organisms, their primary food source comes from the algae they host, making them more akin to heterotrophs than any specific dietary category.