Pictured in this installment of the AquaNerd Picture of the Week is a green open brain coral in a mixed reef aquarium. These corals, though inexpensive and readily attainable, can instantly add great diversity and color to your sand bed. They are really intriguing to look at when the lights go off too. They take on water, swelling to large sizes, and extend tentacles surrounding their mouth. When these tentacles grab food, it directs it to its single large mouth.