A flat red sponge, sometimes across a meter of dead coral, and it is worth stopping at because of the white specks scattered over it.
They are jellyfish. Each speck is the polyp stage of Nausithoe punctata, a crown jellyfish that spends that part of its life embedded in sponge tissue rather than on rock. The sponge pumps water and the polyps sit in the flow taking what comes past, and the arrangement is common enough in this species that the sponge is named for it.
The sponge itself is doing something more aggressive than it looks. It grows over dead coral, and where it meets a living colony it does not stop politely at the edge: Mycale can overgrow live coral tissue, which makes it one of the organisms that takes the ground after a reef has been damaged and does not readily give it back.
Everything about it runs on water. There is no mouth and no gut, only a system of channels lined with beating cells that drive a current through the body and strain out bacteria and particles too small for anything else here to bother with.
- bright red encrusting sponge with raised chimney-like openings
- scattered tiny white spots over the surface, which are not part of the sponge
- grows over dead coral and pushes onto living coral
- up to 40 cm across
