Corallinales

Lore & Background

Corallines have been divided into two groups based on growth forms: geniculate (articulated) and nongeniculate (nonarticulated). Geniculate corallines are branching, tree-like organisms with non-calcified sections (genicula) separating calcified sections (intergenicula). Nongeniculate corallines range from thin crusts to several centimeters thick, and may be free-living as rhodoliths. All corallines begin with a crustose stage; some later become frondose. Their thalli consist of three layers: hypothallus, perithallus, and epithallus, with the epithallus periodically shed.

The fossil record of corallines is complete and continuous. Stem-group corallines are reported from the Ediacaran Doushantuo formation, while true (crown group) corallines appear in rocks of Jurassic age onwards. The crown group has an excellent fossil record from the Early Cretaceous, consistent with molecular clocks. Nonarticulated forms have a better fossil record because the unmineralized geniculae of articulated forms break down quickly. The earliest known coralline deposits date from the Ordovician, though modern forms radiated in the Cretaceous.

Coralline algae are widespread in all of the world's oceans, often covering close to 100% of rocky substrata. They live from intertidal zones to depths of 270 meters. Some species tolerate brackish or hypersaline waters; only one strictly freshwater species exists: Pneophyllum cetinaensis. They are more common in higher latitudes and the Mediterranean, and their ability to calcify in low light makes them some of the deepest photosynthetic multicellular organisms in the ocean.

Reader's Guide

Coralline algae play an important ecological role in coral reefs. Sea urchins, parrot fish, limpets, and chitons feed on them. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef called the Coralligène. Many corallines produce chemicals that promote the settlement of larvae of certain herbivorous invertebrates, particularly abalone, which is adaptive because the herbivores remove epiphytes that might otherwise smother the crusts. This settlement is also important for abalone aquaculture, as corallines enhance larval metamorphosis and survival through the critical settlement period. At the community level, the presence of herbivores associated with corallines can generate patchiness in the survival of young stages of dominant seaweeds.

Because coralline algae contain calcium carbonate, they fossilize well and are significant as stratigraphic markers in petroleum geology. Coralline rock was used as building stone since ancient Greece. The calcite crystals in their cell walls normally contain magnesium, and higher magnesium content makes deposits more soluble in ocean water, particularly in colder waters, making some deposits more vulnerable to ocean acidification. The group's internal taxonomy is in flux; molecular studies are proving more reliable than morphological methods, though recent advances in skeletal ultrastructure show promise. The diversity of corallines has closely tracked the efficiency of grazing herbivores; for instance, the appearance of parrotfish in the Eocene marked a spike in coralline diversity and the extinction of many delicately branched forms.

Did You Know?

• Coralline algae can grow as deep as 268 meters, making them some of the deepest photosynthetic multicellular organisms in the ocean.
• Only one strictly freshwater coralline species exists: Pneophyllum cetinaensis.
• The fossil record of corallines is complete and continuous, with stem-group forms reported from the Ediacaran Doushantuo formation.