Rhodolith

Rhodolith refers to free-living, calcareous nodules formed mainly by crustose coralline algae. These nodules are often spherical or irregular in shape and can range from a few centimeters to several decimeters in diameter. The skeletons are built by calcium carbonate deposited by red algae in the order Corallinales (genera such as Lithothamnion and Sporolithon), creating a durable, interwoven composite that can persist as a distinct habitat on soft sediments.

Rhodoliths grow as unattached thalli on sediment surfaces and are shaped by currents that roll or transport them, preventing attachment to a fixed substrate. Over time they accumulate into beds known as rhodolith or maërl beds, which are widely distributed in both temperate and tropical marine environments. These beds often occur in shallow to moderately deep waters, from the intertidal zone to depths of hundreds of meters, depending on light availability and water clarity.

Ecologically, rhodolith beds provide structural habitat that supports a diverse assemblage of invertebrates, fishes, and epifaunal organisms. They influence local hydrodynamics and sediment stabilization and serve as nursery grounds for various species. Their fossilizable skeletons make rhodoliths important for paleoceanographic studies, offering records of past ocean chemistry and temperature.

Threats to rhodolith beds include physical disturbance from bottom-contact fishing practices, dredging, and coastal development. Ocean acidification, warming, and increasing turbidity also threaten calcifying algae, potentially reducing bed extent and ecological function in affected regions.