Precious Corals

CHAPTER 11

PRECIOUS CORALS

Background

Precious corals have been highly valued as raw material for making jewelry and various art objects since antiquity. They consist of a diverse assemblage of coelenterates belonging primarily to three orders of the class Anthozoa: Gorgonacea, Zoanthidae, and Antipatharia. The most valuable of the precious corals are species of the genus Corallium in the order Gorgonacea. The historically famous red coral of commerce from the Mediterranean Sea, Corallium rubrum, belongs to this genus. Other highly valued species of Corallium include C. japonicum, C. elatius, and C. konojoi from the far western Pacific between latitudes of 19oN and 35oN and C. secumdum and C. sp. nov. from the Hawaiian Archipelago and Emperor Seamount complex. Other gorgonians that are considered valuable include several gold corals in the family Primnoidae (Primnoa resedaeformis and P. willeyi) from Alaska and the bamboo corals in the families Isisiidae (Acanella spp.) and Lepidisisidae (Lepidisis olapa) in the western Pacific. Another gold coral found in Hawaiian waters, Gerardia (= Parazoanthus) sp., belongs to the order Zoanthidae. The last major group of precious corals are the black corals in the order Antipatharia. Of the 200 species of black corals known in the world’s oceans, about ten (mostly in the genus Antipathes) are used for the commercial production of jewelry.

Semi-precious corals include an even more diverse grouping of coelenterates. They include mainly the stylasterine corals and Allopora in the class Hydrozoa, the blue corals (Heliopora) in the Anthozoan order Coenothecalia, the organ pipe corals (Tubipora) in the Antohzoan order Stolonifera, and several gorgonians in the family Melitodiidae, order Gorgonacea. Like precious corals, semi-precious species are also used primarily for jewelry. However, because of their abundance and high porosity, they are not as highly valued. Stony corals (scleractinians) are even more porous than semi-precious varieties and are almost never used for jewelry. Stony corals are, however, sold as curios or as decorations in many part of the world. The value of stony corals imported to the United States averaged $1.0 million annually from 1975 to 1980 (Wells, 1981a). In contrast, the value of the precious coral industry in Taiwan and Japan (major production centers) in 1981 was about $50 million (Grigg, 1982a).

Ecology of Precious Corals

Depth is perhaps the most convenient parameter with which to distinguish the ecological requirements of various groups of precious corals. None of the precious corals is a reef building species per se. All are ahermatypic species that lack zooxanthellae. The shallowest species are commercial varieties of black coral that almost all occur within SCUBA diving depths. Except for the Mediterranean Sea population of Corallium rubrum, which has an extremely broad depth range (10-250 meters), all other precious corals occur at various depths below the euphotic zone. Patterns of distribution and depth zonation of all significant species of precious corals are summarized in Table 11.1. Excluding Alaska, two rich depth zones exist in the Pacific Ocean, one between 100 and 400 meters and the other between 1.0 and 1.5 km. The former zone includes the most valuable species of Corallium, the Hawaiian gold coral (Geradia) and the bamboo corals. The 1.0-1.5 km zone is confined to the Emperor Seamonts in the north Pacific, where the majority of the world’s production of Corallium (sp. nov.) is now harvested (Grigg, 1982b). In Alaska the primnoid gold corals have the broadest depth range of all, generally between 50 and 800 meters (Cimberg, et al., 1981).

It is evident from Table 1 that for at least Corallium spp. (the most valuable group of precious coral) all known commercial concentrations are found north of 19oN latitude. Corallium is known to exist in the southern hemisphere but not in commercial quantities. A survey by CCOP/SOPAC (Cooperative Committee for Offshore Prospecting in the South Pacific-United Nations Development Program) determined the presence of Corallium in the Solomon Islands, Vanuatu, Fiji, Tonga, Samoa, and the Cook Islands (Grigg and Eade, 1981). Of these areas, the Solomon Islands hold the most promise. Corallium is also known to occur in Indonesia, particularly the northeastern islands (Talaud and Sangi) but to date only small colonies have been recovered (Bayer, 1950).

General ecological requirements of all precious corals include the following: the presence of a firm substratum, relatively strong bottom currents, and the absence of significant sources of sediment. There is an interaction between all three of these variables. Strong bottom currents tend to prevent sediments from accumulating, thereby exposing rocky substrata. Because of the longevity of precious corals, which is on the order of 75 years, the stability of the habitat is as important as its suitability (see Grigg, 1975). For example, the only areas where

Table 11.1. Distribution of major species of precious corals
Species / Common name / Where found / Depth range (m) / Reference
Corallium rubrum / Red coral of commerce / Mediterranean Sea, primarily coasts of Sardinia, Corsica, southern Italy, Sicily and northern Tunisia / 10-250 / Belloc (1950), Marchetti (1965a), Lacaze-Duthiers (1864)
Corallium secundum / Angel skin or pelle d’ange / Hawaiian archipelago from Hawaii (20oN) to the Milwaukee Banks (36oN) / 350-475 / Grigg (1974)
Corallium sp. nov. / Midway deep-sea coral / MidwayIsland to Emperor Seamounts, 28o-36oN / 1,000-1,500 / Grigg (1982a)
Corallium japonicum / Aka-sango / Japan, Okinawa & Bonin Islands, 26o-36oN / 100-300 / Grigg (1982a) and Kitahara (1902)
Corallium konojoi / Shiro-sango / Japan to northern Philippines, 19o-36oN / 50-150 / Grigg (1982a) and Kitahara (1902)
Corallium elatius / Momoiro-sango / Northern Philippines to Japan, 19o-36oN / 150-330 / Grigg (1982a) and Kitahara (1902)
Primnoa resedaeformis, Primnoa willeyi / Alaskan gold coral / Southeastern Alaska (Dixon Entrance) to Amchitaka, Aleutian Islands / 10-800 / Cimberg, et al.(1981)
Gerardia sp. (= Parazoanthus) / Hawaiian gold coral / Hawaiian archipelago & Emperor seamounts / 300-400 / Grigg (1974)
Antipathes dichotoma / black coral / Main Hawaiian Islands, Indo-West Pacific region / 30-100 / Grigg (1976) and observations from the Starr II submersible, which supercede depth ranges given in Grigg (1974)
Antipathes grandis / black coral, pine or umimatsu / Main Hawaiian Islands (Hawaii to Niihau) / 45-100 / Grigg (1976) and observations from the Starr II submersible, which supercede depth ranges given in Grigg (1974)
Antipathes spp. / black coral / Caribbean Sea / Taxonomy, patterns of distribution & abundance yet to be adequately described.
Antipathes spp., Cirrhipathes sp. / Philippine Sea / Taxonomy, patterns of distribution & abundance yet to be described.

large beds of C. secundum have been located in Hawaii are in environments where sediments virtually never accumulate. Between 1971 and 1975 a large-scale survey using the submersible Star II was used to investigate all potential sites for precious coral in the major Hawaiian Islands. Thirty-one dives to 400 meters were completed. These surveys showed conclusively that most shelf areas near 400 meters are periodically covered by shallow lenses of sand and silt. Only in habitats always free of sediment were large and abundant stands of Corallium found.

In habitats well removed from terrestrial sources of sediment such as seamounts, sedimentation rates would be expected to be much lower. In such cases the strength and consistency of bottom currents may be of less importance. The Emperor Seamounts may provide one such example (Grigg, 1974). Even so, the large scale correlation between the position of the Kuroshio Current in the western Pacific and the location of rich beds of Corallium emphasizes the overall importance of bottom currents.

Bottom currents are also an important ecological factor in terms of transporting food and carrying away metabolic wastes. Many species of both precious and non-precious deep sea corals exhibit various adaptations in skeletal morphology, branching and orientation apparently to maximize the exposure of feeding surfaces to water-borne food particles (Grigg, 1965, 1972, 1976; Wainwright and Dillon, 1969; Warner, 1981). Many species of pink, black, and gold coral form fan-shaped colonies that orient at right angles to the prevailing current. The bamboo coral, Lepidisis olapa, is unbranched but forms long coils that trail in the current, a shape that presumably increases the feeding efficiency of polyps. The feeding habits of most precious corals are unknown, although it has been shown that several species of black coral are largely planktivores (Grigg, 1965; Lewis, 1978; Warner, 1981).

Light may be the most important factor in terms of setting the upper depth limit for a number of precious corals. Grigg (1965) has presented strong evidence that the larvae of the black corals Antipathes grandis and A. dichotoma are negatively phototaxic. This behavior has the advantage of concentrating settlement at depths below the wave base where wave induced abrasion is minimal or absent. The pattern of distribution of Corallium rubrum in the Mediterranean Sea, where in water less than 30 meters deep colonies are found only in dimly lit caves, suggests a similar larval behavior (Marchetti, 1965b). Furthermore, the relative shallow occurrence of species of Corallium in the region of the Kuroshio Current may be due to the high productivity and turbidity of the water. Kuroshio translates into English as black current.

In general temperature does not appear to play a strong role in the ecology of most species of precious coral except possibly in setting lower depth limits. Species of Corallium in the Pacific Ocean are found in waters that range between 8o and 20oC. Similarly, C. rubrum in the Mediterranean Sea occurs at depths between 5 and 300 meters, which may involve as much as a 10oC difference in temperature (Barletta, et al., 1968). Evidence that temperature may set the lower depth limit for two species of black coral is the correspondence between their lower depth ranges and the top of the thermocline in the major Hawaiian Islands(100 m, see Sechel, 1962).

The growth rate of all precious corals is relatively slow. For species for which data exist there appears to be an approximate relationship between maximum size and growth rate. Species that form large colonies tend to grow faster (Table 11.2). If this is true generally, one consequence would be that many species would be characterized by roughly the same longevity. However, to be cautious the data must be said to be too meager to generalize at this point. In fact they only represent commercial grade pink and black corals and may only reflect differences between these two groups.

Table 11.2. Growth rates of precious corals
Species / Maximum height (cm) / Growth rate
(cm y-1) / Location / Reference
Corallium secundum / 75 / 1.0 / Hawaii / Grigg (1976)
Corallium rubrum / 45 / 0.5-2.0 / Mediterranean / Bauer (1909)
Antipathes dichotoma / 250 / 6.4 / Hawaii / Grigg (1976)
Antipathes grandis / 300 / 6.1 / Hawaii / Grigg (1976)
Antipathes salix / 250 / 4.5 / Caribbean / Olsen and Wood (1980)

The growth rates of black corals in Table 11.2 are based on direct measurements in the field. For C. secundum the estimate is based on an assumption that concentric growth rings that are visible in all sections (Brown, 1976) are annual. The rate of C. rubrum is inferred from rates of recovery of harvested grounds off Algeria in the Mediterranean Sea (Bauer, 1909).

All species of precious coral except one develop distinct non-coalescing colonies. Branches grow together only within colonies(Grigg, 1965 for Corallium and Antipathes spp.; Kishinouye, 1904 for Corallium, Grigg, personal communication for Lepidsis and Acanella). The exception to this rule is Gerardia sp., which in Hawaii is always found in association with Acanella as a parasitic overgrowth. This behavior for Hawaiian specimens has previously been noted by Brown (1976). Gerardia is therefore only found in areas where colonies of Acanella previously exist.

What is known of the reproductive biology of precious corals suggests that within most species sexes are separate. This behavior applies to C. rubrum(Vighi, 1970), C. secundum and A. dicohtoma(Grigg, 1976). For these species the reproductive cycles are annual, and spawning occurs during summer months. For C. rubrum gametogenesis is affected by temperature, and shallow colonies mature earlier in the year (Vighi, 1970), as does Muricea californica off California (Grigg, 1977a). Age at reproductive maturity in C. secundum and A. dichotoma is similar (~12 years old), and the two species have similar life spans (~75 years). Fertilization in C. rubrum is internal, and larvae are released as fully mature planulae (Lacaze-Duthiers, 1864). As mentioned above, patterns of settlement suggest that C. rubrum larvae are negatively phototaxic. The same appears true for A. dichotoma and A. grandis.

Asexual reproduction by fragmentation is common in many species of coral (Highsmith, 1982), including some precious corals. However, in contrast to reef building species whose fragments frequently regenerate, reattachment of fragments is uncommon for species of both Corallium(Kishinouye, 1904) and Antipathes (Grigg, pers. comm.). On the other hand, colonies of Antipathes have been successfully transplanted if firmly secured by an artificial base (Grigg, 1965).

Recruitment and mortality rates appear to be quite low for most precious corals. This is an obvious corollary to their longevity. A notable exception is Corallium rubrum, for which heavy settlement has been frequently observed in shallow caves in the Mediterranean Sea. Interestingly, C. rubrum is the smallest and probably the shortest lived of all the commercial species of Coralllium. Mortality rates have been measured for two species of black coral and one species of Corallium in the field (Table 11.3). The best estimates vary between 4% and 7% per year. This means that turnover of a population occurs about once every 15-25 years.

In favorable environments for C. secundum and A. dichotoma in Hawaii, the age frequency distribution for both species has been found to be relatively stable (Grigg, 1976), suggesting that recruitment and mortality are approximately in steady state. In contrast, precious coral populations surveyed near unsuitable habitats in Hawaii all exhibit truncated or highly uneven age-frequency distributions for both species (Grigg, 1984). Hence the shape of the age-frequency distribution (smooth versus uneven) of precious corals might serve as an index of habitat stability.

Table 11.3. Mortality rates of precious corals.
Species / Locality / Mortality (% per year) / Reference
A. dichotoma / Hawaii / 7 ± 2 / (Grigg, 1976)
A. salix / Caribbean / 4 / (Olsen and Wood, 1980)
C. secundum / Hawaii / 6.6 / (Grigg, 1976)

The most common cause of mortality in marginal habitats for Corallium in Hawaii is smothering by movement of sand along the bottom. Steep slopes in close proximity to shallow water are often unsuitable because of downslope transport of sand and other debris. Sand deposits have also been observed to completely obliterate exposed limestone terraces that would otherwise serve as ideal substrata for black corals in Hawaii. Differences in the retention of sand on the insular shelves of the Hawaiian Islands is probably the main factor that accounts for variability in the abundance of black coral around the Hawaiian Islands.

In environments essentially free of heavy sedimentation, the most common source of mortality for both Corallium and Antipathes in Hawaii is toppling caused by organisms that bore into and weaken the site of basal attachment. Encrustation is a secondary cause of mortality to the black corals, particularly at shallow depths.

Nature of Skeleton and Criteria for Evaluation of Raw Material

The skeletons of precious corals provide the raw material for the industry. Red and pink corals consist of a very hard, high magnesium calcium carbonate. Black and gold corals of the genus Gerardia have entirely protein skeletons, while the bamboo corals consist of skeletons that have alternating sections of calcium carbonate and protein. Finally, the gold corals in the genus Primnoa have skeletons of protein that are abundantly infused with calcite (CaCO3) spicules.

The value of the raw material of any species of precious coral depends primarily on its size, color, abundance, and condition (whether collected live or dead). Precious corals less than 1 cm in diameter directly above the base rarely have any commercial value. Rich beds generally consist of much larger colonies (3-10 cm in basal diameter). Color varies according to species and locality where collected. The color of at least some species of Corallium is due to the presence of organic matter in the skeleton (Kishinouye, 1904). The organic substance is actually a matrix of spicule sacs that are cemented together to form the skeleton. As for the effect of color on value, fashion trends change. During the 1980s the red varieties of Corallium were considered the most valuable. During the 1970s angel-skin varieties were preferred (Grigg, 1984).

Abundance is also an important factor that affects value. As a general rule, the greater the abundance, the lower the price.

Condition refers to the state of the precious coral when it was collected. For species of Corallium, Japanese fishermen have four terms to distinguish condition: ikiki = alive, tachigareki = dead but still attached, ochii = dead but fallen, and mushikui = dead, fallen, and “worm” eaten. For black, gold, and bamboo corals, the same general criteria apply, except that for black coral an additional factor is the degree to which dead portions of the skeleton are encrusted.