Acanthurus nigricans (Linnaeus, 1758)

Craig, Matthew T., 2008, The goldrim surgeonfish (Acanthurus nigricans; Acanthuridae) from Diego Garcia, Chagos Archipelago: first record for the central Indian Ocean, Zootaxa 1850, pp. 65-68 : 65-67

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 precipitous 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 multiple 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 propensity 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 transformed 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 2700km from its nearest neighbor to the East (Cocos [Keeling] Islands), 1900km to the West ( Seychelles), and 500km to the North (southern Maldives).

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 those records ( Winterbottom et al., 1989). A collection of subtidal fishes was made by Adair Fehlmann of the Smithsonian 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 covered 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 indicates 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 specimens. 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 evidence of the presence of A. nigricans there. Thus dispersal via a continental route is also unlikely. The remaining hypothesis 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.

Figures

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 approximate collecting localities.