Which marine animal is the largest member of the phylum Porifera?

The giant barrel sponge is the largest member of the phylum Porifera.

The giant barrel sponge, Xestospongia muta, is a massive reef sponge found in the tropical western Atlantic and Caribbean. It commonly grows into a large barrel-shaped form, with a wide cavity that can provide shelter for small reef organisms.

Like all sponges, it is a stationary filter feeder rather than a plant. Water enters through many tiny pores, and specialized cells capture food particles and move water through the body. The filtered water leaves through the large opening at the top, called the osculum.

Giant barrel sponges can live for many decades and may continue growing throughout life. Their surfaces often host microbes and small animals. Although they look simple, sponges are animals with specialized cells and internal water-flow systems, not algae, corals, or rocks.