Rhodoliths

Rhodoliths are free-living nodules formed by coralline red algae (Rhodophyta) that grow on the seafloor without attaching to a solid substratum. They typically take spherical or ellipsoidal shapes and can occur individually or in large aggregations on shallow marine bottoms.

They are produced by crustose coralline algae that secrete calcareous skeletons, which build up around a nucleus or merge with other nodules to form a cohesive, carbonate framework. Growth occurs by gradual accretion and lateral expansion, while currents and bottom movement help roll the nodules, giving them their characteristic rounded forms.

Rhodoliths occur worldwide in a range of shallow to somewhat deeper, well-lit environments, from tropical to temperate regions. In favorable areas, extensive rhodolith beds develop, creating complex three-dimensional habitats that host various invertebrates and small fishes and contribute to local carbonate sediment production. These beds can influence local hydrodynamics and provide surfaces for attachment and foraging.

Ecology and significance: Rhodoliths contribute to biodiversity by offering shelter and feeding surfaces and can serve as archives of past marine conditions, as their skeletons preserve records of water chemistry and climate. Growth rates are generally slow, making rhodolith beds vulnerable to disturbance and environmental change.

Threats and conservation: Physical disturbances such as bottom trawling, dredging, and anchoring, along with climate-related stressors like ocean warming and acidification, threaten rhodolith beds by causing physical damage and reducing calcification.

Distribution and paleontology: Rhodoliths are present in many oceans and have a long fossil record, spanning several geological periods and reflecting historical shifts in sea level and sedimentation.