Acanthurus tractus Poey, 1860

Bernal, Moisés A. & Rocha, Luiz A., 2011, Acanthurus tractus Poey, 1860, a valid western Atlantic species of surgeonfish (Teleostei, Acanthuridae), distinct from Acanthurus bahianus Castelnau, 1855, Zootaxa 2905, pp. 63-68 : 64-67

Acanthurus tractus Poey, 1860

Ocean Surgeonfish

Cirujano Pardo

Figure 1 A–C

Acronurus nigriculus Poey, 1875 . Type locality: Cuba. Recognized as junior synonym of A. bahianus by Randall (1956). Should now be considered a synonym of A. tractus based on the type locality.

Acanthurus randalii Briggs and Caldwell, 1957 . Type locality: Panama City, Florida. Recognized as junior synonym of A. bahianus by Smith-Vaniz et al. (2002). Should now be considered a synonym of A. tractus based on the type locality.

Diagnosis. The Ocean Surgeonfish is distinguished from all of its Atlantic congeners by the following combination of characters: Dorsal fin IX, 23 to 26; anal fin III, 21-23; pectoral rays 15-17; gill rakers 18-24; very small ctenoid scales; spatulate teeth with denticulate edges, 14 in upper jaw and 16 in lower jaw. Body depth about two in standard length; concavity of caudal fin from five to 12 in standard length, more concave in adults; length of caudal spine between 3 and 3.5 in standard length; diameter of eye 3 to 3.5 in head length. Body yellowish to grayish brown, with no dark vertical bars; yellow lines radiating from posterior margin of eye; translucent pectoral fin with yellow tints; white or pale spot on base of caudal fin; caudal fin lunate and caudal concavity between five and 12 in standard length; posterior margin of caudal, anal and dorsal fins with light blue or white margin. Maximum reported length 38.1 cm SL, but most usually individuals not exceeding 25.0 cm TL.

Color in life. Overall coloration varies from tones of yellowish brown and grayish to olive-brown; short blue lines radiating from posterior margin of eye; dorsal and anal fins with a blue margin and alternating bands of dull white and bluish green (fewer and less conspicuous in anal fin); caudal fin olive-brown to bluish, with the base often abruptly white or at least paler than the rest of the body and the posterior margin bright blue to white ( Figure 1 A-1C).

Comparisons to western-Atlantic congeners. Among its congeners, the Ocean Surgeonfish is most similar to the Barber Surgeonfish, Acanthurus bahianus , having the same body proportions and counts. However these two species differ in the coloration of the margin of the caudal and dorsal fins. This coloration is white to bright blue in the Ocean surgeonfish ( Figures 1 A to 1C) and orange to bright yellow in the Barber surgeonfish ( Figure 1 D to 1F).

These color differences have previously been noticed by Rocha et al. (2002), but did not result in the separation of these two species until now. This color pattern is consistent through the entire range of A. bahianus , including Central Atlantic islands (e.g.: Edwards 1990). A. tractus also shares fin-ray counts with A. chirurgus , however the later is characterized by the presence of 10 narrow dark bars on side of body that are absent in A. tractus . In addition, the caudal fin of A. chirurgus lacks the distinct pale posterior margin that is present in A. tractus . The distinction between A. coeruleus and A. tractus is more evident as they do not share fin-ray counts. A. coeruleus has 24 to 26 anal-fin rays and 26 to 28 dorsal-fin rays, whereas A. tractus counts are 21 to 23 and 23 to 26 respectively. They also differ in color pattern, as adult A. coeruleus are blue to purplish- grey and juveniles are bright yellow. Finally, the eastern Atlantic A. monroviae has similar counts to A. tractus , but the former is characterized by having numerous horizontal lines on the side of the body, a very distinctive bright orange-yellow spot surrounding the peduncular spine and black to dark blue fins.

Distribution and habitat. From Massachusetts and Bermuda to Trinidad and Tobago, including the Gulf of Mexico and the islands of the Caribbean. This species inhabits mainly shallow rocky and coral reefs but can also be found around inshore rocky areas with patches of sand, feeding primarily on benthic algae and occasionally grazing on seagrass beds.

Genetics. The split between A. bahianus and A. tractus is evident from the results of the Bayesian tree. The sister species form a clade supported by 100% Bayesian posterior probability in the tree and are separated from each other by an average pairwise distance of 2.4% ( Figure 2 ). Rocha et al. (2002) had already observed that samples from the North and South Atlantic share no haplotypes and are separated by 11 mutations with Φ ST between 0.805–0.836 (P<0.001) in the same gene. Meanwhile, comparisons within the northwestern and southwestern Atlantic biogeographic regions showed no differences ( Rocha et al. 2002).

The divergence found between these two species is comparable to that found among other recognized closely related species. For example, in a survey of 81 fish genera, Johns and Avise (1998) found 39 pairs of sister species in which the pair-wise divergence was below 2.5% for the mitochondrial marker Cyt B. More recent examples with comparable divergence in reef fishes include grunts ( Rocha et al. 2008), angelfishes ( Bellwood et al. 2004) and wrasses ( Rocha 2004).

The separation between the two clades can be explained by the freshwater outflows of the Orinoco and Amazon basins. These rivers change the conditions from clear waters with shallow coral reefs, to murky low salinity waters along the northeast coast of South America ( Rocha 2003). In this particular case, A. bahianus and A. tractus are restricted to shallow reefs, so individuals are unable to cross this barrier. Similar cases of sister species on both sides of the freshwater plume have been described for cardinalfishes ( Gilbert 1977), damselfishes ( Greenfield & Woods 1974), parrotfishes ( Moura et al. 2001) and wrasses ( Rocha 2004).

With the revalidation of Acanthurus tractus the number of surgeonfish species present in the western Atlantic increases from three to four. The history of the genus Acanthurus is filled with changes in nomenclature and misidentifications ( Randall 1956 & 2002), probably caused by the wide distribution, ability of many species to change color and similarity between closely related species. However, to the knowledge of the authors, there are no other lineages in this group that could potentially warrant species level recognition in the Atlantic.

Notes on Acanthurus bahianus ( Castelnau 1855) . The common name “Ocean Surgeonfish” has been historically used for the Caribbean species, and we propose that this common name be kept for A. tractus . Because the bulk of the southern species distribution is the Brazilian coast, we suggest the adoption of the Brazilian common name of Barber Surgeonfish (“Cirurgião Barbeiro” in Portuguese) for A. bahianus . The Barber Surgeonfish is thus found in tropical and subtropical waters of the coast of Brazil, from Parcel Manuel Luiz to Santa Catarina, including the Oceanic Islands of Fernando de Noronha, Atol das Rocas, Trindade, Ascension and St. Helena, in the South and Central Atlantic.

FIGURE 3. Geographical distribution of Acanturus tractus () and A. bahianus (Δ) in the tropical western and central Atlantic. Neither species is present between the Amazon and Orinoco rivers in the northern coast of Brazil as an effect of the fresh water influx of these rivers.

Material examined. Institutional abbreviations are as listed at http://www.asih.org/codons.pdf, except for CIUFES (Colecao Ictiologica da Universidade Federal do Espirito Santo, Brazil) and UFPB (Universidade Federal da Paraiba, Brazil). Materials are listed alphabetically by locality, and number of specimens examined in each lot is given in parenthesis after the collection number.

Acanthurus tractus

Bahamas: UF 171552 (10), CAS 23800 (2). Belize: UF 209809 (3). Cayman Islands: UF 28781 (7), UF 32490 (11). Colombia: UF 224106 (5). Panama: CAS 31730 (7). Navassa Island: USNM 360416 (2). Florida: UF 31235 (2), UF 67552 (28). Trinidad and Tobago: USNM 319192 (7). US Virgin Islands: UF 159194 (4), UF 159195 (4), UF 159196 (3), UF159197 (6).

Acanthurus bahianus

Ascension Island: USNM 381170 (9). Bahia, Brazil: USNM 43281 (2), CIUFES 130751 (3). Espírito Santo, Brazil: ZUEC 2826 (1), CIUFES 130007 (5), CAS-SU 67767. Paraíba, Brazil: UFPB 3505 (11), UFPB 5066 (3), UFPB 5342 (12). Pernambuco, Brazil: CAS-SU 67765 (2). Trindade Island, Brazil: ZUEC 1530 (1), CIUFES 1538 (2), CIUFES 1714 (1).

Figures

FIGURE 1. Underwater photographs of Acanthurus tractus (A to C) and Acanthurus bahianus (D to F). Note the clear and consistent difference in the coloration of the margin of the caudal and dorsal fins. Photographs by J. Randall (1 A), D. Snyder (1 B, 1 C), O. Luiz-Junior (1 D, 1 E) and G. Allen (1 F).

FIGURE 2. Relationships among Atlantic Acanthurus based on Bayesian analysis of mitochondrial CytB sequences. Numbers above branches correspond to posterior probabilities and those below branches are bootstrap support. Since our analysis did not include Pacific or Indian Ocean surgeonfishes, relationships in this tree (especially the position of long branches) may change with the addition of other taxa.