Coralline algae are

red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

in the order Corallinales. They are characterized by a

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms were ...

that is hard because of

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

deposits contained within the cell walls. The colors of these

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...

are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps w ...

of coral reefs.

Sea urchins Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells ( tests) ...

parrot fish Parrotfishes are a group of about 90 fish species regarded as a family (Scaridae), or a subfamily (Scarinae) of the wrasses. With about 95 species, this group's largest species richness is in the Indo-Pacific. They are found in coral reefs, ro ...

, and

limpets Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indepe ...

and

chitons Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail s ...

(both mollusks) feed on coralline algae. In the temperate

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...

, coralline algae are the main builders of a typical algal reef, the ''Coralligène'' ("coralligenous"). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens (

maerl Maerl (also rhodolith) is a collective name for non-geniculate coralline red algae with a certain growth habit. Maerl grows at a rate of c. 1 mm per year. It accumulates as unattached particles and forms extensive beds in suitable sublitto ...

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 gr ...

s) may form relatively smooth compact balls to warty or fruticose thalli. A close look at almost any

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species of ...

rocky shore A rocky shore is an intertidal area of seacoasts where solid rock predominates. Rocky shores are biologically rich environments, and are a useful "natural laboratory" for studying intertidal ecology and other biological processes. Due to their ...

or coral reef will reveal an abundance of pink to pinkish-grey patches, distributed throughout the rock surfaces. These patches of pink "paint" are actually living crustose coralline red algae. The red algae belong to the division Rhodophyta, within which the coralline algae form the order

Corallinales Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of re ...

. There are over 1600 described species of nongeniculate coralline algae. The corallines are presently grouped into two families on the basis of their reproductive structures.

Distribution

Coralline algae are widespread in all of the world's oceans, where they often cover close to 100% of rocky substrata. Only one species, '' Pneophyllum cetinaensis'', is found in freshwater. Its ancestor lived in brackish water, and was already adapted to osmotic stress and rapid changes in water salinity and temperature. Many are

epiphytic An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

(grow on other algae or marine angiosperms), or

epizoic An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...

(grow on animals), and some are even parasitic on other corallines. Despite their ubiquity, the coralline algae are poorly known by

ecologists This is a list of notable ecologists. A-D * John Aber (USA) * Aziz Ab'Saber (Brazil) * Charles Christopher Adams (USA) * Warder Clyde Allee (USA) * Herbert G. Andrewartha (Australia) * Sarah Martha Baker ( UK) * Fakhri A. Bazzaz (USA) * ...

, and even by specialist

phycologists Phycology () is the scientific study of algae. Also known as algology, phycology is a branch of life science. Algae are important as primary producers in aquatic ecosystems. Most algae are eukaryotic, photosynthetic organisms that live in ...

(people who study algae). For example, a recent book on the seaweeds of Hawaii does not include any crustose coralline algae, even though corallines are quite well studied there and dominate many marine areas.

Forms

Corallines have been divided into two groups, although this division does not constitute a taxonomic grouping: * the geniculate (articulated) corallines; * the nongeniculate (nonarticulated) corallines. Geniculate corallines are branching, tree-like organisms which are attached to the substratum by crustose or calcified, root-like holdfasts. The organisms are made flexible by having noncalcified sections (genicula) separating longer calcified sections (intergenicula). Nongeniculate corallines range from a few micrometres to several centimetres thick crusts. They are often very slow growing, and may occur on rock, coral skeletons, shells, other algae or seagrasses. Crusts may be thin and leafy to thick and strongly adherent. Some are parasitic or partly endophytic on other corallines. Many coralline crusts produce knobby protuberances ranging from a millimetre to several centimetres high. Some are free-living as

s (rounded, free-living specimens). The morphological complexity of rhodoliths enhances species diversity, and can be used as a non-taxonomic descriptor for monitoring. Thalli can be divided into three layers: the hypothallus,

perithallus {{Short pages monitor