Coralline algae

Coralline algae are red algae in the Family Corallinaceae of the order Corallinales characterized by a thallus that is hard as a result of calcareous deposits contained within the cell walls. Unattached specimens (maerl, rhodoliths) may form relatively smooth compact balls to warty or fruticose thalli. Many are typically encrusting and rock-like, found in tropical marine waters all over the world. They play an important role in the ecology of coral reefs. Colors are most typically pink or some other shade of red, but may be purple, yellow, blue, white or gray-green. Sea urchins, parrot fish, limpets (molluscs) and chitons molluscs feed on coralline algae.

A close look at almost any intertidal rocky shore or coral reef will reveal an abundance of pink to pinkish-grey patches, splashed as though by a mad painter over rock surfaces. These patches of pink paint are actually living algae: crustose coralline red algae. The red algae belong to the division Rhodophyta, within which the coralline algae form a distinct, exclusively marine order, the Corallinales. Coralline algae are widespread in all of the world's oceans, where they often cover close to 100% of rocky substrata. Many are epiphytic (grow on other algae or marine angiosperms), or epizoic (grow on animals), and some are even parasitic on other corallines. Despite their ubiquity, the coralline algae are poorly known by ecologists, and even by specialist phycologists (people who study algae). For example, a recent book on the seaweeds of Hawai'i does not include any crustose coralline algae even though corallines are quite well studied there and dominate many marine areas.

Traditionally, corallines have been divided into two groups, although this division does not constitute a taxonomic grouping: Geniculate corallines are branching, tree-like plants which are attached to the substratum by crustose or calcified, root-like holdfasts. The plants are made flexible by having non-calcified 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 rhodoliths (rounded, free-living specimens). There are over 1600 described species of nongeniculate coralline algae.
 * the geniculate (articulated) corallines;
 * the non-geniculate (non-articulated) corallines.

History
The first coralline alga recognized as a living organism was probably Corallina in the 1st century AD. In 1837 Philippi recognized that coralline algae were not animals and he proposed the two generic names Lithophyllum and Lithothamnion as Lithothamnium. For many years they were included in the order Cryptonemiales as the family Coallinaceae until in 1986 they raised to the order Corallinales

Corallines in community ecology
Many corallines produce chemicals which promote the settlement of the larvae of certain herbivorous invertebrates, particularly abalone. This is adaptive for the corallines as the herbivores then remove epiphytes which might otherwise smother the crusts and pre-empt available light. This is also important for abalone aquaculture, as corallines appear to enhance larval metamorphosis and the survival of larvae through the critical settlement period. It also has significance at the community level, as the presence of herbivores associated with corallines can generate patchiness in the survival of young stages of dominant seaweeds. This has been seen this in eastern Canada, and it is suspected the same phenomenon occurs on Indo-Pacific coral reefs, yet nothing is known about the herbivore enhancement role of Indo-Pacific corallines, or whether this phenomenon is important in coral reef communities.

Some corallines slough off a surface layer of epithallial cells, which in a few cases may be an anti-fouling mechanism which serves the same function as enhancing herbivore recruitment. This also affects the community, as many algae recruit on the surface of a sloughing coralline, and are then lost with the surface layer of cells. This can also generate patchiness within the community. The common Indo-Pacific corallines Neogoniolithon fosliei and Sporolithon ptychoides slough epithallial cells in continuous sheets which often lie on the surface of the plants looking so much like wet tissue paper.

Not all sloughing serves an anti-fouling function. Epithallial shedding in most corallines is probably simply a means of getting rid of damaged cells whose metabolic function has become impaired. My students and I have studied sloughing in the South African intertidal coralline algae, Spongites yendoi, a species which sloughs up to 50% of its thickness twice a year. This deep-layer sloughing, which is energetically costly, does not have any effect on seaweed recruitment when herbivores are removed. The surface of these plants is usually kept clean by herbivores, particularly the pear limpet, Patella cochlear. Sloughing in this case is probably a means of getting rid of old reproductive structures and grazer-damaged surface cells, and reducing the likelihood of surface penetration by burrowing organisms.

Some coralline algae develop into thick crusts which provide microhabitat for many invertebrates. For example, off eastern Canada, I found that juvenile sea urchins, chitons, and limpets suffer nearly 100% mortality due to fish predation unless they are protected by knobby and under-cut coralline algae. This is probably an important factor affecting the distribution and grazing effects of herbivores within marine communities. Nothing is known about the microhabitat role of Indo-Pacific corallines. However, the most common species in the region, Hydrolithon onkodes, often forms an intimate relationship with the chiton Cryptoplax larvaeformis. The chiton lives in burrows that it makes in H. onkodes plants, and comes out at night to graze on the surface of the coralline. This combination of grazing and burrowing results in a peculiar growth form (called castles) in H. onkodes in which the coralline produces nearly vertical, irregularly curved lamellae.

Non-geniculate corallines are of particular significance in the ecology of coral reefs, where they provide calcareous material to the structure of the reef, help cement the reef together, and are important sources of primary production. Coralline algae are especially important in reef construction, as they lay down calcium carbonate as calcite. Although they contribute considerable bulk to the calcium carbonate structure of coral reefs, their more important rôle in most areas of the reef, is in acting as the cement which binds the reef materials together into a solid and sturdy structure.

An area where corallines are particularly important in constructing reef framework is in the algal ridge that characterizes surf-pounded reefs in both the Atlantic and Indo-Pacfic regions. Algal ridges are carbonate frameworks that are constructed mainly by nongeniculate coralline algae (after Adey 1978). They require high and persistent wave action to form, so are best developed on the windward reefs in areas where there is little or no seasonal change in wind direction. Algal ridges are one of the main reef structures that prevent oceanic waves from striking adjacent coastlines, and they thus help to prevent coastal erosion.

Economic importance
Despite their hard, calcified nature, coralline algae have a number of economic uses. One use dates back to the 18th century, and involves the collection of unattached corallines (maërl) for use as soil conditioners. This is particularly significant in Britain and France, where more 300 000 tonnes of Phymatolithon calcareum (Pallas) Adey & McKinnin and Lithothamnion corallioides are dredged annually. Several thousand kilometres of maërl beds, composed of as-yet undetermined species belonging to the genera Lithothamnion and Lithophyllum, exist off the coast of Brazil, and have been subjected to a low level of commercial exploitation. Maërl is also used as a food additive for cattle and pigs, as well as in the filtration of acidic drinking water.

Corallines are also used in medicine, where the earliest use involved the preparation of a vermifuge from ground geniculate corallines of the genera Corallina and Jania. This use stopped towards the end of the 18th century. Modern medical science has found a more high-tech use for corallines in the preparation of dental bone implants. Apparently, the cell fusions provide an ideal matrix for the regeneration of bone tissue.

Since coralline algae contain calcium carbonate, they fossilize fairly well. They are useful as stratigraphic markers of particular significance in petroleum geology. Coralline rock has also been used as building stones, with the best examples being in Vienna, Austria.

As a colorful component of live rock sold in the marine aquarium trade, coralline algae are desired in home aquariums for their aesthetic qualities.

Further references

 * Morton, O. and Chamberlain, Y.M. 1985. Records of some epiphytic coralline algae in the nortth-east of Ireland. Ir. Nat. J. 21: 436 - 440.
 * Morton, O. and Chamberlain, Y.M. 1989. Further records of encrusting coralline algae on the north-east coast of Ireland. Ir. Nat. J. 23: 102 - 106.
 * Suneson, S. 1943. The structure, life-history, and taxonomy of the Swedish Corallinaceae. Lunds Universitets Arsskrift, N.F., Avd.2, 39(9): 1 - 66.
 * Woelkerling, W.J. 1993. Type collections of Corallinales (Rhodophyta) in the Foslie Herbarium (TRH). Gunneria 67 1 - 289.
 * ITIS Report for Corallinaceae