Precious coral

Precious coral, or red coral, is the common name given to a genus of marine corals, Corallium. The distinguishing characteristic of precious corals is their durable and intensely colored red or pink-orange skeleton, which is used for making jewelry.

Precious coral

Scientific classification

Domain: Eukaryota

Kingdom: Animalia

Phylum: Cnidaria

Subphylum: Anthozoa

Class: Octocorallia

Order: Alcyonacea

Family: Coralliidae

Genus: Corallium

Cuvier, 1798

Description

The colonial multicellular animals (eumetazoans) that compose it, come from an embryo with 2 layers, the ectoderm and the endoderm, which will give on the one hand the external wall and on the other hand the internal wall with digestive function of the body in the form of a "double-walled bag" (former phylum of the coelenterates). It forms tree-like colonies, branched in all planes, of variable size. The colonies generally measure from 5 to 20 cm. Formerly, it was possible to find colonies reaching up to 1 meter in length but they disappeared due to overexploitation. The growth of red coral is 2 to 8 mm per year. Generally red in color, there are pink or even white colonies. The polyps are white.

The colony is supported by a hard central skeletal axis, or polyp, made of calcium carbonate in the form of calcite colored bright red by a pigment from the carotenoid family, canthaxanthin. This skeleton results from the welding of spicules or sclerites secreted by the colony and embedded in a calcareous cement. This is the red coral, highly sought after by jewelers and for cult objects in Italy, on the coasts of Algeria and Tunisia. The polyp is covered with a fleshy, vermilion-red living crust, crossed by canals. The individuals are transparent white polyps with biradial or bilateral symmetry in the shape of a double-walled, hollow column, surmounted by a buccal disc surrounded by 8 hollow tentacles with small extensions or pinnules. The central digestive cavity and the pharynx are partitioned by symmetrical vertical membranes. The pharynx is crossed by a ciliated gutter or siphonoglyph which ensures the entry of water into the gastric cavity.

Features

Like almost all octocorals, the red coral forms colonies consisting of many individual polyps. The colonies grow to between 5 and 30 cm in size and are irregular and sparsely branched; in continuous strong currents the coral also forms crusts. Colonies over one meter in size and weighing up to 30 kilograms have disappeared due to excessive exploitation for jewelry production. The ends of the branches are 3 to 5 mm thick; the base of large colonies can be up to 3 cm thick. The calcareous branches of the colony are formed from fused sclerites. The living coenenchyma is vermillion, dark red to bright red, and more rarely pink or white, and contrasts markedly with the white, more or less transparent polyps, which have eight feathery tentacles. Red coral colonies grow at only 2 to 8 mm per year.

The red coral can be confused with the encrusting leather coral (Parerythropodium coralloides) and some bryozoans, such as the false coral (Myriapora truncata) or the elkhorn bryozoan (Schizotheca serratimargo). The former, however, does not have its own calcareous skeleton, but covers dead gorgonians; the latter are orange in color and have branch tips that appear to have been cut off.

Habitat

Red corals grow on rocky seabottom with low sedimentation, typically in dark environments—either in the depths or in dark caverns or crevices. Red coral lives permanently attached (known as a " sessile " species) to dark rocky bottoms (benthic species) and the walls of semi-dark caves in the circalittoral zone, as well as on deeper rocky cliffs. However, it has been observed that well-lit colonies are often luxuriant. Red coral also requires clear, agitated waters with an average temperature of 15 °C; therefore, its distribution in the Mediterranean is quite limited.

The red corals is light-shy and grows primarily below a depth of 40 meters, down to depths of over 100 meters (maximum 280 meters), less frequently in shallower water protected by caves. Free-standing, in dim light, without the protection of overhangs or crevices, it only occurs below a depth of 80 meters. The precious coral feeds on zooplankton. Precious corals are colonized by various drilling sponges.

Red corals colonies are usually sexually distinct. The eggs are fertilized in the female polyp, develop there into planula larvae, and are then expelled. They live planktonic lives for a few days until they attach to a solid substrate. The planula larva then transforms into the primary polyp, which continues to reproduce by budding and forms a new colony.

The original species, C. rubrum (formerly Gorgonia nobilis), is found mainly in the Mediterranean Sea. It grows at depths from 10 to 300 meters below sea level, although the shallower of these habitats have been largely depleted by harvesting. In the underwater caves of Alghero, Sardinia (the "Coral Riviera"), it grows at depth from 4 to 35 meters. The same species is also found at Atlantic sites near the Strait of Gibraltar, at the Cape Verde Islands and off the coast of southern Portugal. Other Corallium species are native to the western Pacific, notably around Japan and Chinese Taiwan; these occur at depths of 350 to 1500 meters below sea level in areas with strong currents.

Food

It is a predatory zoophagous microphagous consumer (= carnivore) that captures small planktonic prey using the tentacles of the polyps which constitute a fine network that filters seawater and retains eggs, larvae, copepod crustaceans... but also inert organic particles; it is therefore considered a passive filter feeder (= suspension feeder). The internal cavities of the polyps communicate with each other, the products of digestion benefit the rest of the colony. The tentacles of gorgonians, unlike hydroids, jellyfish or sea anemones are poor in stinging cells.

Reproduction

Red coral reproduces sexually, with colonies being hermaphroditic. Individuals are either male or female, but they can only be distinguished by dissecting them. In some places in the genome, females carry identical variants on both copies of their genes and males carry different variants, and in other places it is the opposite. Sex determination is therefore similar to the XX/XY system but more complex.

Fertilization is external and the egg gives rise to a ciliated larva or planula which attaches itself to a hard substrate and gives rise to a small polyp which forms a colony by budding. There is therefore no medusa stage in the development of this animal. (See Anthozoan development cycle).

Red coral also propagates asexually, by budding. Growth is slow.

Anatomy

In common with other Alcyonacea, red corals have the shape of small leafless bushes and grow up to a meter in height. Their valuable skeleton is composed of intermeshed spicules of hard calcium carbonate, colored in shades of red by carotenoid pigments. In living specimens, the skeletal branches are overlaid with soft bright red integument, from which numerous retractable white polyps protrude. The polyps exhibit octameric radial symmetry.

Species

The following are known species in the genus:

Corallium abyssale Bayer, 1956

Corallium bathyrubrum Simpson & Watling, 2011

Corallium bayeri Simpson & Watling, 2011

Corallium borneanse Bayer

Corallium boshuense Kishinouye, 1903

Corallium carusrubrum Tu, Dai & Jeng, 2012

Corallium ducale Bayer

Corallium elatius Ridley, 1882

Corallium gotoense Nonaka, Muzik & Iwasaki, 2012

Corallium halmaheirense Hickson, 1907

Corallium imperiale Bayer

Corallium japonicum Kishinouye, 1903

Corallium johnsoni Gray, 1860

Corallium kishinouyei Bayer, 1996

Corallium konojoi Kishinouye, 1903

Corallium laauense Bayer, 1956

Corallium maderense (Johnson, 1899)

Corallium medea Bayer, 1964

Corallium niobe Bayer, 1964

Corallium niveum Bayer, 1956

Corallium occultum Tzu-Hsuan Tu et al., 2015

Corallium porcellanum Pasternak, 1981

Corallium pusillum Kishinouye, 1903

Corallium regale Bayer, 1956

Corallium rubrum (Linnaeus, 1758)

Corallium secundum Dana, 1846

Corallium sulcatum Kishinouye, 1903

Corallium taiwanicum Tu, Dai & Jeng, 2012

Corallium tricolor (Johnson, 1899)

Corallium uchidai Nonaka, Muzik & Iwasaki, 2012

Corallium vanderbilti Boone, 1933

Corallium variabile (Thomson & Henderson, 1906)

As a gemstone

The hard skeleton of red coral branches is naturally matte, but can be polished to a glassy shine. It exhibits a range of warm reddish pink colors from pale pink to deep red; the word coral is also used to name such colors. Owing to its intense and permanent coloration and glossiness, precious coral skeletons have been harvested since antiquity for decorative use. Coral jewellery has been found in ancient Egyptian and prehistoric European burials, and continues to be made to the present day. It was especially popular during the Victorian age.

The calcareous axis of the red coral is used for jewelry making. Precious coral has hardness 3.5 on the Mohs scale. It can be worked like a hard stone, unlike corals, which are full of pores and do not allow the creation of sculptures. Due to its softness and opacity, coral is usually cut en cabochon, or used to make beads. The center of jewelry production is Torre del Greco near Naples. 

Several steps are necessary to create jewelry and sculptures. The coral is washed with bleach. Sections are then cut with a circular saw under running water. Their contours are then regularized by grinding. Coral is worked with certain instruments used by dentists such as drills, milling cutters, wet saws, grinders and polishers, as coral cannot be worked dry. All pieces, except the balls, are drilled before being worked by hand. Polishing, finally, gives the coral its full brilliance.

History of trade

At the beginning of the 1st millennium, there was significant trade in coral between the Mediterranean and India, where it was highly prized as a substance believed to be endowed with mysterious sacred properties. Pliny the Elder remarks that, before the great demand from India, the Gauls used it for the ornamentation of their weapons and helmets; but by this period, so great was the Eastern demand, that it was very rarely seen even in the regions which produced it. Among the Romans, branches of coral were hung around children's necks to preserve them from danger from the outside, and the substance had many medicinal virtues attributed to it. The belief in coral's potency as a charm continued throughout the Middle Ages and early in 20th century Italy it was worn as a protection from the evil eye, and by women as a cure for infertility.

From the Middle Ages onward, the securing of the right to the coral fisheries off the African coasts was the object of considerable rivalry among the Mediterranean communities of Europe.

The story of the Torre del Greco is so interwoven with that of the coral so as to constitute an inseparable pair, and is documented as early as the fifteenth century. In 1790 the Royal Society of Coral was established in the town of Torre del Greco, with the idea of working and selling coral fish. This shows that the coral fishing flourished for many years in the city.

It was also enacted December 22, 1789, by Ferdinand IV of Bourbon Code coral (prepared by the Neapolitan jurist Michael Florio), with the intent to regulate the coral fishing in those years starring, in addition to the sailors Torre del Greco, the locals and those in Trapani This regulation did not have the expected success. From 1805, when he founded the first factory for the manufacturing of coral in Torre del Greco (by Paul Bartholomew Martin, but with French Genoese origin), the golden age for the manufacturing of coral in the city situated on the slopes of the Vesuvius started, because working together with the coral fishing was increasingly under the control of Torre del Greco fishermen. Since 1875, the Torre del Greco began working with the Sciacca coral and a school for the manufacturing of coral was built in 1878 in the city (which closed in 1885 to reopen in 1887), with which in 1933 established a museum of the coral. Then came the time of processing of Japanese coral found in the markets of Chennai and Kolkata.

Other story instead a short period the Tunisian fisheries were secured by Charles V for Spain; but the monopoly soon fell into the hands of the French, who held the right until the Revolutionary government in 1793 threw the trade open. For a short period (about 1806) the British government controlled the fisheries, but this later returned to the hands of the French authorities. Before the French Revolution much of the coral trade was centred in Marseille, but then largely moved to Italy, where the procuring of the raw material and the working of it was centring in Naples, Rome and Genoa.

In culture

The origin of coral is explained in Greek mythology by the story of Perseus. Having petrified Cetus, the sea monster threatening Andromeda, Perseus placed Medusa's head on the riverbank while he washed his hands. When he recovered her head, he saw that her blood had turned the seaweed (in some variants the reeds) into red coral. Thus, the Greek word for coral is 'Gorgeia', as Medusa was one of the three Gorgons.

Poseidon resided in a palace made of coral and gems, and Hephaestus first crafted his work from coral.

The Romans believed coral could protect children from harm, as well as cure wounds made by snakes and scorpions and diagnose diseases by changing colour.

In Hindu astrology red coral is associated with the planet Mars or Graha-Mangala and used for pleasing Mars. It should be worn on the ring finger.

A branch of red coral figures prominently in the civic coat of arms of the town of Alghero, Italy.

Amongst the Yoruba and Bini peoples of West Africa, red precious coral jewellery (necklaces, wristlets and anklets most especially) are signifiers of high social rank, and are worn as a result by titled kings and chieftains.

In traditional Dutch culture, notably in fishing communities, red coral necklaces were worn by the female population as an indispensable part of the traditional costumes.

Conservation

Intensive fishing, particularly in shallow waters, has damaged this species along the Mediterranean coastline, where colonies at depths of less than 50 metres are much diminished. Fishing and climate change threaten their persistence. The three oldest Mediterranean marine protected areas—Banyuls, Carry-le-Rouet and Scandola, off the island of Corsica—all host substantial populations of C. rubrum. Since protection was established, colonies have grown in size and number at shallow and deeper depths.

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