Billfish

The largest billfish, the Atlantic blue marlin, weighs up to 820 kg (1800 lb) and has been classified as a vulnerable species.[1][2]

The term billfish refers to a group of predatory fish characterised by prominent bills, or rostra, and by their large size; some are longer than 4 m (13 ft). Billfish include sailfish and marlin, which make up the family Istiophoridae, and swordfish, sole member of the family Xiphiidae. They are apex predators which feed on a wide variety of smaller fish, crustaceans, and cephalopods.

Billfish are pelagic and highly migratory. They are found in all oceans,[3] although they usually inhabit tropical and subtropical waters; swordfish are found in temperate waters, as well. Billfish use their long spears or sword-like upper beaks to slash at and stun prey during feeding. Their bills can also be used to spear prey, and have been known to spear boats (probably accidentally), but they are not normally used in that way. They are highly valued as gamefish by sports fishermen.

Species

The term billfish refers to the fishes of the families Xiphiidae and Istiophoridae. These large fishes are "characterized by the prolongation of the upper jaw, much beyond the lower jaw into a long rostrum which is flat and sword-like (swordfish) or rounded and spear-like (sailfishes, spearfishes, and marlins)."[4]

True billfish

The 12 species of true billfish are divided into two families and five genera. One family, Xiphiidae, contains only one species, the swordfish Xiphias gladius, and the other family, Istiophoridae contains 11 species in four genera, including marlin, spearfish, and sailfish.[4][5] Controversy exists about whether the Indo-Pacific blue marlin, Makaira mazara, is the same species as the Atlantic blue marlin, M. nigricans. FishBase follows Nakamura (1985)[4] in recognizing M. mazara as a distinct species, "chiefly because of differences in the pattern of the lateral line system".[6]

This article is
one of a series on
Commercial fish
Large pelagic
billfish, bonito
mackerel, salmon
shark, tuna
Forage
anchovy, herring
menhaden, sardine
shad, sprat
Demersal
cod, eel, flatfish
pollock, ray
Mixed
carp, tilapia
Billfish species
Family Genus Common name Scientific name Maximum
length		 Common
length		 Maximum
weight		 Maximum
age		 Trophic
level		 FishBase FAO IUCN status
Xiphiidae Xiphias Swordfish Xiphias gladius (Linnaeus, 1758) 455 cm 300 cm 650 kg years 4.49 [7] [8] Least concern[9]
Istiophoridae Istiophorus
Sailfish		 Atlantic sailfish Istiophorus albicans (Latreille, 1804) 315 cm cm 58.1 kg 17 years[10] 4.50 [11] Not assessed
Indo-Pacific sailfish Istiophorus platypterus (Shaw, 1792) 340 cm cm 100 kg years 4.50 [12] [13] Least concern[9]
Istiompax Black marlin Istiompax indica (Cuvier, 1832) 465 cm 380 cm 750 kg years 4.50 [5] Data deficient[14]
Makaira Indo-Pacific blue marlin Makaira mazara (Jordan and Snyder, 1901) cm cm kg years 4.46 [6] Not assessed
Atlantic blue marlin Makaira nigricans (Lacépède, 1802) 500 cm 290 cm 820 kg years 4.50 [15] Vulnerable[2]
Tetrapturus White marlin Tetrapturus albidus/Kajikia albida Poey, 1860 300 cm 210 cm 82.5 kg years 4.48 [16] Vulnerable[17]
Shortbill spearfish Tetrapturus angustirostris Tanaka, 1915 200 cm cm 52 kg years 4.50 [18] Data deficient[19]
Striped marlin Tetrapturus audax/Kajikia audax (Philippi, 1887) 350 cm cm 200 kg years 4.58 [20] [21] Near threatened[22]
Roundscale spearfish Tetrapturus georgii Lowe, 1841 184 cm cm 24 kg years 4.37 [23] Data deficient[22]
Mediterranean spearfish Tetrapturus belone Rafinesque, 1810 240 cm 200 cm 70 kg years 4.50 [24] Least concern[25]
Longbill spearfish Tetrapturus pfluegeri Robins and de Sylva, 1963 254 cm 165 cm 58 kg years 4.28 [26] Least concern[27]
True billfish
Swordfish, most commercially fished billfish
 
The Indo-Pacific sailfish, fastest of all fishes
 

Billfish-like fish

A number of other fishes have pronounced bills or beaks, and are sometimes referred to as billfish. However, they are not true billfish. Halfbeaks look somewhat like miniature billfish, and the sawfish and sawshark, which are cartilaginous fishes with spectacular rostrums shaped like a chain saw. Needlefish are sometimes confused with billfish, but they are "easily distinguished from the true billfish by having both jaws prolonged, the dorsal and anal fins both single and similar in size and shape, and the pelvic fins inserted far behind the pectorals."[4] Paddlefish have elongated rostrums containing electroreceptors that can detect weak electrical fields. Paddlefish are filter feeders and may use their rostrum to detect zooplankton.[28]

Structure and function of the bill

Billfish have a long, bony, spear-shaped bill, sometimes called a snout, beak or rostrum. The swordfish has the longest bill, about one-third its body length. Like a true sword, it is smooth, flat, pointed and sharp. The bills of other billfish are shorter and rounder, more like spears.[29]

Billfish normally use their bills to slash at schooling fish. They swim through the fish school at high speed, slashing left and right, and then circle back to eat the fish they stunned. Adult swordfish have no teeth, and other billfish have only small file-like teeth. They swallow their catch whole, head-first. Billfish don't normally spear with their bills, though occasionally a marlin will flip a fish into the air and bayonet it. Given the speed and power of these fish, when they do spear things the results can be dramatic. Predators of billfish, such as great white and mako sharks, have been found with billfish spears embedded in them.[30][31][32] Pelagic fish generally are fascinated by floating objects, and congregate about them.[33] Billfish can accidentally impale boats and other floating objects when they pursue the small fish that aggregate around them.[32] Care is needed when attempting to land a hooked billfish. Many fisherman have been injured, some seriously, by a billfish thrashing its bill about.[31]

Other characteristics

External video
Striped Marlin Bait Ball YouTube

Billfish are large swift predators which spend most of their time in the epipelagic zone of the open ocean. They feed voraciously on smaller pelagic fish, crustaceans and small squid. Some billfish species also hunt demersal fish on the seafloor, while others descend periodically to mesopelagic depths. They may come closer to the coast when they spawn in the summer. Their eggs and larvae are pelagic, that is they float freely in the water column.[30][32] Many grow over three metres (10 feet) long, and the blue marlin can grow to five metres (16 feet). Females are usually larger than males.[30][32]

Like scombroids (tuna, bonito and mackerel), billfish have both the ability to migrate over long distances, efficiently cruising at slow speeds, and the ability to generate rapid bursts of speed. These speed bursts can be quite astonishing, and the Indo-Pacific sailfish has been recorded making a burst of 68 miles per hour (110 km/h), nearly top speed for a cheetah and the highest speed ever recorded for a fish.[30]

Some billfish also descend to considerable mesopelagic depths. They have sophisticated swim bladders which allow them to rapidly compensate for pressure changes as the depth changes. This means that when they are swimming deep, they can return swiftly to the surface without problems.[34] "Like the large tuna, some billfish maintain their body temperature several degrees above ambient water temperatures; this elevated body temperature increases the efficiency of the swimming muscles, especially during excursions into the cold water below the thermocline."[32]

In 1936 the British zoologist James Gray posed a conundrum which has come to be known as Gray's paradox. The problem he posed was how dolphins can swim and accelerate so fast when it seemed their muscles lacked the needed power.[35] If this is a problem with dolphins it is an even greater problem with billfish such as swordfish, which swim and accelerate faster than dolphins. In 2009, Taiwanese researchers from the National Chung Hsing University introduced new concepts of "kidnapped airfoils and circulating horsepower" to explain the swimming capabilities of swordfish. The researchers claim this analysis also "solves the perplexity of dolphin's Gray paradox". They also assert that swordfish "use sensitive rostrum/lateral-line sensors to detect upcoming/ambient water pressure and attain the best attack angle to capture the body lift power aided by the forward-biased dorsal fin to compensate for most of the water resistance power."[36]

Billfish have prominent dorsal fins. Like tuna, mackerel and other scombroids, billfish streamline themselves by retracting their dorsal fins into a groove in their body when they swim.[30] The shape, size, position and colour of the dorsal fin varies with the type of billfish, and can be a simple way to identify a billfish species. For example, the white marlin has a dorsal fin with a curved front edge and is covered with black spots. The huge dorsal fin, or sail of the sailfish is kept retracted most of the time. Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down.[30][37]

Distribution and migration

Billfish occur worldwide in temperate and tropical waters. They are highly migratory oceanic fish, spending much of their time in the epipelagic zone of international water following major ocean currents.[30][32] Migrations are linked to seasonal patterns of sea surface temperatures.[38] They are sometimes referred to as "rare event species" because the areas they roam over in the open seas are so large that researchers have difficulty locating them. Little is known about their movements and life histories, so assessing how they can be sustainably managed is not easy.[39][40]

Unlike coastal fish, billfish usually avoid inshore waters unless there is a deep dropoff close to the land.[31] Instead, they swim along the edge of the continental shelf where cold nutrient rich upwellings can fuel large schools of forage fish. Billfish can be found here, cruising and feeding "above the craggy bottom like hawks soaring along a ridge line".[41]

Commercial fishing

Global commercial capture of billfish reported by the FAO in tonnes 1950–2009[42]
Commercial catch of marlin at Jimbaran, Indonesia

In parts of the Pacific and Indian ocean such as the Maldives, billfishing, particularly for swordfish, is an important component of subsistence fishing.

Recreational fishing

Over the years, billfish tournaments have transformed into big business enterprises. Many prestigious tournaments now have enormous calcuttas and purses as well as large numbers of participating anglers. With huge purses and egos on the line, concern often arises whether all participants are adhering to the letter of the rules. 
 – IGFA Conservation Director, Jason Schratwieser[43]

Billfish are among the most coveted of big gamefish, and major recreational fisheries cater to the demand.[40] In North America, "the apex of the salt water pursuits is billfishing, the quest for elusive blue marlin and sailfish in the deep blue water about 60 miles out."[41] A lot of resources are committed to the activity, particularly in the construction of private and charter billfishing boats to participate in the billfishing tournament circuit. These are expensive purpose-built offshore vessels with powerfully driven deep sea hulls. They are often built to luxury standards and equipped with many technologies to ease the life of the deep sea recreational fisherman, including outriggers, flying bridges and fighting chairs, and state of the art fishfinders and navigation electronics.[41]

The boats cruise along the edge of the continental shelf where billfish can be found down to 200 metres (600 ft), sometimes near weed lines at the surface and submarine canyons and ridges deeper down. Commercial fishermen usually use drift nets or longlines to catch billfish, but recreational fishermen usually drift with bait fish or troll a bait or lure. Billfish are caught deeper down the water column by drifting with live bait fish such as ballyhoo, striped mullet or bonito. Alternatively, they can be caught by trolling at the surface with dead bait or trolling lures designed to imitate bait fish.[44]

Most recreational fishermen now tag and release billfish.[41] A 2003 study surveyed 317,000 billfish known to have been tagged and released since 1954. Of these, 4122 were recovered. The study concluded that, while tag and release programs have limitations, they provided important information about billfish that cannot currently be obtained by other methods.[32][40]

Hooked striped marlin
 
Hooked billfish can leap spectacularly out of the water
 
 
 
 

As food

Billfish make good eating fish, and are high in omega-3 oils. Blue marlin has a particularly high oil content.[45] However, because billfish have high trophic levels, near the top of the food web, they also contain significant levels of mercury and other toxins. According to the United States Food and Drug Administration, swordfish is one of four fishes, along with tilefish, shark, and king mackerel, that children and pregnant women should avoid due to high levels of methylmercury found in these fish and the consequent risk of mercury poisoning.[46] [47]

Raw swordfish
Nutritional value per 100 g (3.5 oz)
Energy 144 kJ (34 kcal)
6.65 g
19.66 g
Vitamins
Vitamin A 120 IU
Vitamin D
(93%)

558 IU

Minerals
Calcium
(1%)

5 mg

Iron
(3%)

0.38 mg

Magnesium
(8%)

29 mg

Phosphorus
(36%)

255 mg

Potassium
(9%)

418 mg

Sodium
(5%)

81 mg

Zinc
(7%)

0.66 mg

Other constituents
Water 73.38 g
Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
Comparative mercury levels[48]
Species Mean ppm
Tilefish 1.450 Gulf of Mexico
Swordfish 0.995
Shark 0.979
King mackerel 0.730
Bigeye tuna 0.689 Fresh/frozen
Orange roughy 0.571
Marlin * 0.485
King mackerel 0.182 South Atlantic
Herring 0.084
Flatfish * 0.056 Flounder, plaice and sole
Catfish 0.025
Salmon * 0.022 Fresh/frozen
Sardine 0.013
Tilapia * 0.013
* indicates methylmercury only was analyzed (all other results are for total mercury)

Billfish are primarily marketed in Japan, where they are eaten raw as sashimi. They are marketed fresh, frozen, canned, cooked and smoked.[32] It is not usually a good idea to fry billfish. Swordfish and marlin are best grilled or broiled, or eaten raw as in sashimi. Sailfish and spearfish are somewhat tough and are better cooked over charcoal or smoked.[45]

Billfish as food
Swordfish auction in the fish market of Vigo 
Slice of Atlantic blue marlin 
Swordfish marinade 
Fried sailfish with noodle salad 

Conservation

Billfish are exploited both as food and as game fish. Marlin and sailfish are eaten in many parts of the world, and many sport fisheries target these species. Swordfish are subject to particularly intense fisheries pressures, and although their survival is not threatened worldwide, they are now comparatively rare in many places where once they were abundant. The istiophorid billfishes (marlin and spearfish) also suffer from intense fishing pressures. High mortality levels occur when they are caught incidentally by longline fisheries targeting other fish.[49] Overfishing continues to "push these declines further in some species".[50] Because of these concerns about declining populations, sport fishermen and conservationists now work together to gather information on billfish stocks and implement programs such as catch and release, where fish are returned to the sea after they have been caught. However, the process of catching them can leave them too traumatised to recover.[30] Studies have shown that circle fishing hooks do much less damage to billfish than the traditional J-hooks, yet they are at just as effective for catching billfish. This is good for conservation, since it improves survival rates after release.[51][52]

The stocks for individual species in billfish longline fisheries can "boom and bust" in linked and compensatory ways. For example, the Atlantic catch of blue marlin declined in the 1960s. This was accompanied by an increase in sailfish catch. The sailfish catch then declined from the end of the 1970s to the end of the 1980s, compensated by an increase in swordfish catch. As a result, overall billfish catches remained fairly stable.[53]

"Many of the world's fisheries operate in a data poor environment that precludes predictions about how different management actions will affect individual species and the ecosystem as a whole."[54] In recently years pop-up satellite archival tags have been used to monitor billfish. The capability of these tags to recover useful data is improving, and their use should result in more accurate stock assessments.[55] In 2011, a group of researchers claimed they have, for the first time, standardized all available data about scombrids and billfishes so it is in a form suitable for assessing threats to these species. The synthesis shows that those species which combine a long life with a high economic value, such as the Atlantic blue marlin and the white marlin, are generally threatened. The combination puts such species in "double jeopardy".[56]

See also

References

  1. ↑ Makaira nigricans bioSearch. Updated: 20 January 2011.
  2. 1 2 Collette B and 30 others (2011). "Makaira nigricans". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  3. ↑ Ken Schultz: Ken Schultz's Fishing Encyclopedia. 1999. "http://www.gofishn.com/content/billfish#"
  4. 1 2 3 4 Nakamura, Izumi (1985) Billfishes of the world. An annotated and illustrated catalogue of marlins, sailfishes, spearfishes and swordfishes known to date FAO Fisheries Synopsis, 125 (5). Rome.
  5. 1 2 Froese, Rainer and Pauly, Daniel, eds. (2013). "Istiompax indica" in FishBase. April 2013 version.
  6. 1 2 Froese, Rainer and Pauly, Daniel, eds. (2012). "Makaira mazara" in FishBase. March 2012 version.
  7. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Xiphias gladius" in FishBase. March 2012 version.
  8. ↑ Xiphias gladius (Linnaeus, 1758) FAO, Species Fact Sheet. Retrieved 2 March 2012.
  9. 1 2 Collette B and 34 others (2011). "Xiphias gladius". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  10. ↑ Arocha F and Ortiz M (2006) Description of Sailfish (SAI) ICCAT Manual.
  11. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Istiophorus albicans" in FishBase. March 2012 version.
  12. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Istiophorus platypterus" in FishBase. March 2012 version.
  13. ↑ Istiophorus platypterus (Shaw, 1792) FAO, Species Fact Sheet. Retrieved 2 March 2012.
  14. ↑ Collette B and 13 others (2011). "Istiompax indica". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  15. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Makaira nigricans" in FishBase. March 2012 version.
  16. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus albidus" in FishBase. March 2012 version.
  17. ↑ Collette B and 21 others (2011). "Kajikia albida". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  18. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus angustirostris" in FishBase. March 2012 version.
  19. ↑ Collette B and 13 others (2011). "Tetrapturus angustirostris". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  20. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus audax" in FishBase. March 2012 version.
  21. ↑ Tetrapturus audax (Philippi, 1887) FAO, Species Fact Sheet. Retrieved 2 March 2012.
  22. 1 2 Collette B and 13 others (2011). "Kajikia audax". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  23. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus georgii" in FishBase. March 2012 version.
  24. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus belone" in FishBase. March 2012 version.
  25. ↑ Collette B and 10 others (2011). "Tetrapturus belone". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  26. ↑ Froese, Rainer and Pauly, Daniel, eds. (2012). "Tetrapturus pfluegeri" in FishBase. March 2012 version.
  27. ↑ Collette B and 15 others (2011). "Tetrapturus pfluegeri". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved 14 March 2012.
  28. ↑ Wiley, Edward G. (1998). Paxton, J.R.; Eschmeyer, W.N., eds. Encyclopedia of Fishes. San Diego: Academic Press. pp. 77–78. ISBN 0-12-547665-5.
  29. ↑ Schultz 2011
  30. 1 2 3 4 5 6 7 8 Aquatic Life of the World pp. 332–333, Marshall Cavendish Corporation, 2000. ISBN 9780761471707.
  31. 1 2 3 Iversen ES and Skinner RH (2006) Dangerous sea life of the west Atlantic, Caribbean, and Gulf of Mexico: a guide for accident prevention and first aid Page 77–78, Pineapple Press. ISBN 9781561643707.
  32. 1 2 3 4 5 6 7 8 Heemstra PC and Heemstra E (2004) Coastal fishes of Southern Africa Page 424, NISC. ISBN 9781920033019.
  33. ↑ Hunter, JR and Mitchell CT (1966). "Association of fishes with flotsam in the offshore waters of Central America". Fishery Bulletin. 66: 13–29.
  34. ↑ Schultz, 2007, pp. 22–24.
  35. ↑ Gray, J (1936). "Studies in animal locomotion VI. The propulsive powers of the dolphin" (PDF). Journal of Experimental Biology. 13: 192–199.
  36. ↑ Lee, Hsing-Juin; Jong, Yow-Jeng; Change, Li-Min; and Wu, Wen-Lin (2009). "Propulsion Strategy Analysis of High-Speed Swordfish". Transactions of the Japan Society for Aeronautical and Space Sciences. 52 (175): 11–20. doi:10.2322/tjsass.52.11.
  37. ↑ Dement J Species Spotlight: Atlantic Sailfish (Istiophorus albicans) littoralsociety.org. Retrieved 1 April 2012.
  38. ↑ Breman, Joe (2002) Marine geography: GIS for the oceans and seas Page 46, ESRI. ISBN 9781589480452.
  39. ↑ Lutcavage, M (2001). "Bluefin Spawning in Central North Atlantic" (PDF). Pelagic Fisheries Research Program, Newsletter. 6 (2): 1–6.
  40. 1 2 3 Ortiz M, Prince ED, Serafy JE, Holts DB, Davy KB, Pepperell JG, Lowry MB and Holdsworth JC (2003). "Global overview of the major constituent-based billfish tagging programs and their results since 1954" (PDF). Marine and Freshwater Research. 54: 489–507. doi:10.1071/MF02028.
  41. 1 2 3 4 Janiskee RL (2008) Tourism and recreation in the Carolinas In: DG Bennett and JC Patton, A geography of the Carolinas, pp. 201–202, Parkway Publishers. ISBN 9781933251431.
  42. ↑ Based on data sourced from FAO Species Fact Sheets
  43. ↑ IGFA’s Observer Training Class in Virginia Beach IGFA News, March 2010.
  44. ↑ Williams RG and Nichols CR (2009) Encyclopedia of Marine Science Page 505, Infobase Publishing. ISBN 9781438118819.
  45. 1 2 Livingston AD (1996) Complete Fish & Game Cookbook Page 158, Stackpole Books. ISBN 9780811704281.
  46. ↑ FDA (1990–2010). "Mercury Levels in Commercial Fish and Shellfish". Retrieved 14 September 2011.
  47. ↑ Natural Resources Defense Council. "Protect Yourself and Your Family". Retrieved 14 September 2011.
  48. ↑ The mercury levels in the table, unless otherwise indicated, are taken from: Mercury Levels in Commercial Fish and Shellfish (1990–2010) U.S. Food and Drug Administration. Accessed 8 January 2012.
  49. ↑ Myers, Ransom A. and Boris Worm (2003). "Rapid worldwide depletion of predatory fish communities". Nature. 423 (6937): 280–283. doi:10.1038/nature01610. PMID 12748640.
  50. ↑ ICCAT (2002) Report of the 2001 billfish species group session. ICCAT Collected Volume of Scientific Papers 54: 649–764
  51. ↑ Prince ED, Ortiz M, and Venizelos A (2002) "A Comparison of Circle Hook and "J" Hook Performance in Recreational Catch-and-Release Fisheries for Billfish" American Fisheries Society Symposium 30: pp. xxx–xxx.
  52. ↑ Prince ED, Snodgrass D, Orbesen ES (2007). "Circle hooks,'J' hooks and drop‐back time: a hook performance study of the south Florida recreational live‐bait fishery for sailfish, Istiophorus platypterus" (PDF). Fisheries Management and Ecology. 14: 173–182. doi:10.1111/j.1365-2400.2007.00539.x.
  53. ↑ Duffy, J. Emmett (2008) Marine biodiversity and food security Encyclopedia of Earth. Updated 25 July 2008.
  54. ↑ Richardson DE (2008) "Physical and biological characteristics of billfish spawning habitat in the Straits of Florida" Open Access Dissertations. Paper 26.
  55. ↑ Kerstetter DW, Luckhurst BE, Prince ED and Graves JE (2003). "Use of pop-up satellite archival tags to demonstrate survival of blue marlin (Makaira nigricans) released from pelagic longline gear" (PDF). Fishery Bulletin. 101: 939–948.
  56. ↑ Collette, BB; Carpenter, KE; et al. (2011). "High Value and Long Life—Double Jeopardy for Tunas and Billfishes" (PDF). Science. 333 (6040): 291–292. doi:10.1126/science.1208730.

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