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The Goldrim Surgeonfish (Acanthurus Nigricans; Acanthuridae) From Diego Garcia, Chagos Archipelago: First Record For The Central Indian Ocean

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Matthew Thomas Craig at NOAA National Marine Fisheries Service
Matthew Thomas Craig
  • NOAA National Marine Fisheries Service
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Abstract

The goldrim surgeonfish ( also known as the whitecheek surgeonfish in the aquarium trade), Acanthurus nigricans ( Linnaeus), is a common and widespread member of tropical reef fish communities throughout the Pacific Ocean. It has been reported at low densities at Cocos ( Keeling) and Christmas Islands in the eastern Indian Ocean. Named nigricans for its characteristically dark body color, the species usually inhabits shallow depths on the outer reef crest just below the surge zone, but has been observed as deep as 67 m ( Chave and Mundy, 1994). The species feeds on filamentous algae ( Randall, 2001). In this paper I report the first occurrence of Acanthurus nigricans in the central Indian Ocean ( Chagos Archipelago) and provide information on its biogeography and hybridization with A. leucosternon Bennett.
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Accepted by L. A. Rocha: 30 Jul. 2008; published: 15 Aug. 2008 65
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Copyright © 2008 · Magnolia Press
Zootaxa 1850: 65–68 (2008)
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The goldrim surgeonfish (Acanthurus nigricans; Acanthuridae) from
Diego Garcia, Chagos Archipelago: first record for the central Indian Ocean
MATTHEW T. CRAIG
Hawaii Institute of Marine Biolog, P.O. 1346, Kaneohe, HI 96744, USA. E-mail: mtcraig@hawaii.edu
The goldrim surgeonfish (also known as the whitecheek surgeonfish in the aquarium trade), Acanthurus nigricans (Lin-
naeus), is a common and widespread member of tropical reef fish communities throughout the Pacific Ocean. It has been
reported at low densities at Cocos (Keeling) and Christmas Islands in the eastern Indian Ocean. Named nigricans for its
characteristically dark body color, the species usually inhabits shallow depths on the outer reef crest just below the surge
zone, but has been observed as deep as 67 m (Chave and Mundy, 1994). The species feeds on filamentous algae (Randall,
2001). In this paper I report the first occurrence of Acanthurus nigricans in the central Indian Ocean (Chagos Archipel-
ago) and provide information on its biogeography and hybridization with A. leucosternon Bennett.
Acanthurus nigricans (Linnaeus, 1758)
Chaetodon nigricans Linnaeus, 1758
Acanthutus aliala Lesson, 1831
Acanthurus glaucopareius Cuvier, 1829
While collecting reef fishes at Diego Garcia Atoll on the southern end of the Chagos Archipelago (7º18’ S, 72º24’ E),
British Indian Ocean Territory (BIOT) in March 2008, a solitary A. nigricans was observed in a hole on the wall of a pre-
cipitous drop-off near Cannon Point in approximately 20m water depth. Recognizing the importance of this record, the
author collected the individual using a pole spear and photographed the specimen upon returning to the diving platform
(Figure 1). The individual was 93mm SL, and was presumed to be a stray as none had been observed on previous dives.
On several subsequent dives, however, the species was observed, photographed, and collected at two other locations
around the atoll (Barton Point and Horseburgh Point). The three sites lie on separate sides of the atoll indicating that the
species is not restricted to any one location at Diego Garcia (Figure 2) and is an uncommon but not rare member of the
ichthyofaunal community. In total, 10 specimens were collected and sampled for genetic analysis, while several others
were observed but not collected. Those not collected spanned a wide size range (~80mm – ~130mm) indicating that mul-
tiple year classes were present. Two specimens were deposited as vouchers at the Bishop Museum, Honolulu, Hawaii,
and Scripps Institution of Oceanography Marine Vertebrates Collection, La Jolla, California (BPBM 40895, SIO 08-95).
It is noteworthy that A. nigricans is a member of the Acanthurus achilles species complex known for their propen-
sity to hybridize (Randall and Frische, 2000). The four species in this complex (A. achilles Shaw, A. japonicus Schmidt,
A. leucosternon Bennett, and A. nigricans) are thought to hybridize where their distributional ranges overlap. Marie et al.
(2007) confirmed the existence of the hybrid A. leucosternon x A. nigricans from Cocos (Keeling) and Christmas Islands
(eastern Indian Ocean) using genetic techniques. At Diego Garcia, one individual was observed and collected (SIO 08-
95) which displayed the color pattern described by Randall and Frische (2000) and Marie et al. (2007) as this hybrid.
Although yet to be confirmed genetically, we presume this to be the case and document the presence of this hybrid at
Diego Garcia. Following the collecting trip, the author was provided with a photographic record of a recently trans-
formed juvenile A. leucosternon x A. nigricans hybrid from Maldives by Mr. John Coppolino. This marks the first record
of the hybrid at Maldives and may indicate the presence of A. nigricans as well.
Diego Garcia is a small atoll situated on the southernmost end of the Great Chagos Bank (Figure 2, inset). This area,
which is the southerly portion of the Maldive/Laccadive ridge, is a scattered series of atolls and banks separated by water
depths of 500–2000m. The Chagos Archipelago is one of the most isolated in the world, separated by approximately
CRAIG
66 · Zootaxa 1850 © 2008 Magnolia Press
2700km from its nearest neighbor to the East (Cocos [Keeling] Islands), 1900km to the West (Seychelles), and 500km to
the North (southern Maldives).
FIGURE 1. Photographs of Acanthurus nigricans at Diego Garcia, Chagos Archipelago, British Indian Ocean Territory.
FIGURE 2. Map of Diego Garcia, Chagos Archipelago, British Indian Ocean Territory. Asterisks (*) indicate approxi-
mate collecting localities.
The fish fauna of the Chagos Archipelago was until recently relatively unknown. Early collections were made by
Regan (1908) who recorded 32 species of shore fishes. Part of this collection has been lost casting some uncertainty on
Zootaxa 1850 © 2008 Magnolia Press · 67
GOLDRIM SURGEONFISH FROM DIEGO GARCIA
those records (Winterbottom et al., 1989). A collection of subtidal fishes was made by Adair Fehlmann of the Smithso-
nian Oceanographic Sorting Center in 1967, and these specimens are currently housed at the U.S. National Museum of
Natural History. From 1971 to 1975 a Japanese fisheries feasibility study for the Indian Ocean performed exploratory
long lining and reported 78 species from Chagos (Kyushin et al., 1977). The 1984 FAO Species Identification Sheets for
the western Indian Ocean report 168 species from Chagos. The most comprehensive collection of fishes at Chagos was
that of Richard Winterbottom and Alan Emery who visited the area for two and half months in 1978/9. This survey cov-
ered depths from the intertidal to 48m and the specimens are deposited at the Royal Ontario Museum. Subsequently, R.
Charles Anderson participated in a survey of fishes in Chagos citing several new records. These two surveys yielded a
total of 773 species (Winterbottom et al., 1989; Winterbottom and Anderson, 1997). Despite the thorough surveys of the
fish fauna of the Chagos Archipelago, no records exist for A. nigricans.
As part of the Maldive/Laccadive ridge, the Chagos Archipelago has similar faunal characteristics to its northerly
neighbors, the Maldives (Randall and Anderson, 1993). Chagos is only known to have three endemic species and shows
greater affinities to the southern rather than the northern Maldives (Winterbottom and Anderson, 1997). The Maldives
ichthyofauna is recognizable as two distinct assemblages, with the northern portion of that archipelago sharing affinities
with eastern Indian Ocean fishes and the southern portion (including Chagos) having more representative species of the
western Indian Ocean (Winterbottom and Anderson, 1997). This difference has been attributed to the proximity of the
eastward flowing Equatorial Counter Current for at least a portion of the year (Winterbottom and Anderson, 1997). .
Our observations and collections of A. nigricans do not fit this mechanism of dispersal from a western “source” and
present a challenge to the traditional view of Indian Ocean reef fish biogeography. The distribution of A. nigricans indi-
cates a route of dispersal to Chagos from the eastern Indian Ocean (perhaps from Christmas and Cocos (Keeling) Islands
where they are relatively abundant). There are three possible mechanisms that readily explain this eastward dispersal: 1.
The species dispersed via a continental route, through the Bay of Bengal, across India, and downward through the
Maldives to Chagos, 2. The species dispersed via direct larval transport to Chagos, and 3. Release of aquarium speci-
mens. Accidental or intentional release of aquarium specimens is highly unlikely; Diego Garcia is currently the site of a
United States Naval Support Facility and no aquarium trade is present. Acanthurs nigricans has not been recorded from
continental localities in the northern Indian Ocean, nor has it been recorded in Maldives despite a booming recreational
diving industry and the numerous surveys of R. C. Anderson who has resided in Maldives for over a decade (R. C.
Anderson, pers. comm.). In addition, the photographic record of the hybrid from Maldives only provides equivocal evi-
dence of the presence of A. nigricans there. Thus dispersal via a continental route is also unlikely. The remaining hypoth-
esis invokes direct larval transport to Chagos.
Although eastward flowing surface currents may be responsible for transporting larvae from the western Indian
Ocean to Chagos, A. nigricans has clearly dispersed from the eastern Indian Ocean, likely via direct larval transport.
Unlike the largely predictable Pacific Ocean gyre, surface currents in the Indian Ocean are variable. For example, during
northeast monsoonal periods (November–March), the North Equatorial Current flows westward between 5ºN and 2ºS,
yet during the southwest monsoon season (April–October) this same current reverses and flows eastward. In addition,
this current is known to oscillate between the northern and southern hemispheres during inter-monsoonal periods with
the formation of an eastward flowing “jet” (Wyrtki, 1973). Coupled with the complex atmospheric and oceanographic
changes associated with the Indian Ocean Dipole (IOD), a phenomenon not unlike the well known El Niño Southern
Oscillation (ENSO), the surface currents that likely facilitate long-distance larval dispersal in the Indian Ocean are a
complex system. The presence of A. nigricans at Diego Garcia indicates that stochastic instances of larval transport from
the eastern Indian Ocean to Chagos have likely occurred.
Acknowledgements
I thank C. Sheppard, University of Warwick, Cmdr. G. Brooks, British Indian Ocean Territory, and Capt. G. Looney, U.S.
Naval Support Facility Diego Garcia, for facilitating collecting activities. B. Bowen, Hawaii Institute of Marine Biology,
D. Marx, NAVFAC ESC, J. Puritz, Hawaii Institute of Marine Biology, J. Turner, Bangor University, and S. Smith, NAV-
FAC Pacific EV, provided assistance in the field. I would also like to thank Master Diver D. Horvath, T. Casey, C. Echev-
erry, and J. Peters, U.S. Navy Divers, Guam, for their support of diving activities, assistance with aluminum engineering/
architecture, providing astute lunar observations, and facilitating GNR rendition. A. Suzumoto, Bishop Museum, and
H.J. Walker, SIO Marine Vertebrates Collection, provided curatorial assistance. Fieldwork at Diego Garcia was sup-
ported by U.S. National Science Foundation grant OCE-0454873.
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68 · Zootaxa 1850 © 2008 Magnolia Press
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... This effect is likely facilitated by larval dispersal of hybrid and backcrossed individuals out of the hybrid zone. Indeed, individuals of hybrid coloration have been observed outside the hybrid zone (Craig, 2008; J.P.A. Hobbs pers. obs.). ...
... In these circumstances, perhaps it is inevitable that the hybrid larvae can also disperse widely and backcross far from the point of origin. A 2008 expedition to the Chagos Archipelago revealed a previously unknown population of A. nigricans in the central Indian Ocean, and new A. leucosternon  A. nigricans hybrid records at Chagos and other archipelagos in the Indian Ocean support a broad geographic distribution of introgressed individuals in these two species (Craig, 2008). We conclude that hybridization coupled with high dispersal is the key to understanding the pattern of hybrid ancestry across the A. nigricans complex. ...
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... This effect is likely facilitated by larval dispersal of hybrid and backcrossed individuals out of the hybrid zone. Indeed, individuals of hybrid coloration have been observed outside the hybrid zone (Craig, 2008; J.P.A. Hobbs pers. obs.). ...
... In these circumstances, perhaps it is inevitable that the hybrid larvae can also disperse widely and backcross far from the point of origin. A 2008 expedition to the Chagos Archipelago revealed a previously unknown population of A. nigricans in the central Indian Ocean, and new A. leucosternon  A. nigricans hybrid records at Chagos and other archipelagos in the Indian Ocean support a broad geographic distribution of introgressed individuals in these two species (Craig, 2008). We conclude that hybridization coupled with high dispersal is the key to understanding the pattern of hybrid ancestry across the A. nigricans complex. ...
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Article
  • Jan 1994
More than 250 benthic fish taxa were photographed and videotaped by Hawaii Undersea Research Laboratory submersibles at depths between 40 and 2000 m in the Hawaiian Archipelago, Johnston Atoll, and Cross Seamount. Most of the 213 identified fish species occurred close to hard substrates with holes, ledges, or caves. Twenty-two species (notably the larger sharks, lutjanids, and carangids) are cosmopolitan. Seventy-six species are restricted to various Indo-Pacific areas, 64 in the Pacific, and 51 in the Hawaiian Archipelago including Cross Seamount and Johnston Atoll. There is a rapid decrease in the number of species from 200 to 400 m depth. One hundred eight species were seen 20 m deeper than previously reported. Eleven of the deeper-dwelling animals were found 20 m shallower than previously recorded. Faunal zones were not recognized at any depth. Species newly recorded in Hawai'i include Bathypterois grallator (Goode & Bean), Bodianus cylindriatus (Tanaka), Centrophorus cf. granulosus (Bloch & Schneider), Chaunax fimbriatus Hilgendorf, Caelorinchus spilonotus Sazonov & Iwamoto, Notocanthus sp., Paratrachichthys prosthemius Jordan & Fowler, Prognathodes guezei (Mauge & Bauchot), and Sladenia remiger Smith & Radcliffe. New species collected and reported elsewhere are Centrodraco rubellus Fricke et al., Epigonus glossodontus Gon, Owstonia sp., and Pseudanthiasfucinus (Randall & Ralston). Caelorinchus sp. 2 and Callanthias sp. are probably undescribed. It appears that the Hawaiian deep-sea fish fauna has multiple origins and affinities with many regions.
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An Equatorial Jet in the Indian Ocean
Article
  • Aug 1973
At the surface of the Indian Ocean along the equator a narrow, jet-like current flows eastward at high speed during both transition periods between the two monsoons. The formation of the jet is accompanied by thermocline uplifting at the western origin of the jet and by sinking at its eastern terminus. This demonstrates that a time-variable current can have profound effects in changing the mass structure in the ocean.
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  • Jan 2001
  • J E Randall
Randall, J.E. (2001) Surgeonfishes of the World. Bishop Museum Bulletin in Zoology, No. 4. Mutual Publishing, Honolulu. 123pp.
Report on the marine fishes collected by
  • Jan 1908
  • 217-255
  • C T Regan
Regan, C.T. (1908) Report on the marine fishes collected by Mr. J. Stanley Gardiner in the Indian Ocean. Transactions of the Linnaean Society of London, Ser. 2, Zoology 12, 217-255.
Hybrid surgeonfishes of the Acanthurus achilles complex
  • Jan 2000
  • J WATER SUPPLY RES T
  • 51-56
  • J E Randall
  • J Frische
Randall, J.E. & Frische, J. (2000) Hybrid surgeonfishes of the Acanthurus achilles complex. Aqua 4, 51-56.
TERM OF USE This pdf is provided by Magnolia Press for private/research use. Commercial sale or deposition in a public library or website site is prohibited
  • Jan 1973
  • 262-264
  • K Wyrtki
Wyrtki, K. (1973) An equatorial jet in the Indian Ocean. Science 181, 262-264. TERM OF USE This pdf is provided by Magnolia Press for private/research use. Commercial sale or deposition in a public library or website site is prohibited.