Rhodolith

Rhodoliths (from Greek for red rocks) are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic growth habit has been attained by a number of unrelated coralline red algae,^[1] organisms that deposit calcium carbonate within their cell walls to form hard structures or nodules that resemble beds of coral.

Rhodoliths do not attach themselves to the rocky seabed. Rather, they roll like tumbleweeds along the seafloor until they become too large in size to be mobilised by the prevailing wave and current regime. They may then become incorporated into a semi-continuous algal mat or form an algal build-up.^[2][3] While corals are animals that are both autotrophic (photosynthesize via their symbionts) or heterotrophic (feeding on plankton), rhodoliths produce energy solely through photosynthesis (i.e. they can only grow and survive in the photic zone of the ocean).

Scientists believe rhodoliths have been present in the world's oceans since at least the Eocene epoch, some 55 million years ago.^[4]