Molecular support for species status of the Nazca booby (Sula granti)

Auk, The, Jul 2002 by Friesen, V L, Anderson, D J, Steeves, T E, Jones, H, Schreiber, E A

ABSTRACT.-Pitman and Jehl (1998) recently argued that Masked Boobies (formerly Sula dactylatra grand) breeding on the Nazca Plate in the eastern Pacific Ocean are morphologically and ecologically distinct from other Masked Boobies and may represent a full species. The American Ornithologists' Union subsequently elevated that subspecies to a full species: the Nazca Booby (S. grand). To evaluate that change in classification, we compared sequence variation in the mitochondrial cytochrome-b gene among 75 Nazca Boobies and 37 Masked Boobies representing three subspecies from the central and eastern Pacific and Atlantic oceans. Results indicated strong differentiation of cytochrome-b variation among taxa. Sequences constituted three distinct groups: Nazca Boobies, Masked Boobies from the central and eastern Pacific (S. d. personata and S. d. californica), and Masked Boobies (S. d. dactylatra) from the Caribbean and Atlantic. Those three groups probably diverged within a very short period, 400,000-500,000 years ago. Our results support the proposal that S. grand represents a distinct species.

RESUMEN.-Pitman et Jehl (1998) ont recemment soutenu que les Fous masques (anciennement Sula dactylatra grand) nichant sur la plaque de Nazca dans Test de l'Ocean Pacifique sont morphologiquemenl et ecologiquement distincts des autres Fous masques et pourraient representer une espece a part entiere. Par consequent, l'Union des Ornithologistes Americains a eleve cette sous-espece au statut d'espece: lE Fou de Nazca (S. granti). Pour evaluer ce changement dans la classification, nous avons compare la variation de sequences dans le gene mitochondrial cytochrome-b parmi 75 Fous de Nazca et 37 Fous masque representant trois sous-especes du centre et de Fesi des Oceans Pacifique et Atlantique. Les resultats indiquaient une forte differenciation de variation du cytochrome-b parmi les taxa. Les sequences constituaient trois groupes distincts: les Fou de Nazca, leE Fous masques du centre et de l'est du Pacifique (S. d personata et S. d. californica), et les Fous masques (S d. dactylatra) des Caraibes et de 1 Atlantique. Ces troit groupes ont probablement diverge au cours dunk courte periode (i.e. il y a 400,000-500,000 ans). Nor resultats supportent la proposition que S. grand re presente une espece distincte.

Masked Boobies (Sula dactylatra; Pelecaniformes: Sulidae) are large, plunge-diving seabirds that breed on oceanic islands throughout the tropics. Historically they were divided into six subspecies on the basis of morphology and geographic distribution (Nelson 1978). Three of those traditional subspecies breed in the central and eastern Pacific Ocean: Sula d. grand nests in the Galapagos Islands and on a few other islands in the far eastern tropical Pacific; S. d. californica nests on islands off western Mexico and Clipperton Island in the eastern Pacific north of S. d. grand; and S. d. personata nests primarily north and west of S. d. grand. The other three subspecies breed in the central and western Indian Ocean (S. d. melanops), northwestern Australia (S. d. bedouti), and the Caribbean and Atlantic Ocean (S. d. dactylatra). A seventh subspecies (S. d. fullagari), distinguished primarily by eye color, also has been recognized on islands off eastern Australia (O'Brien and Davies 1990).

Recently, S. d. grand was the subject of a taxonomic reassessment by Pitman and Jehl (1998), who argued that individuals of that subspecies differ markedly in morphology and ecology from other Pacific subspecies: they have orange bills instead of yellow; they are generally smaller with longer wings; they have a distinct juvenal plumage; they have a different foraging distribution; and they tend to nest on rocky volcanic islands with rugged topography as opposed to flat atolls. In addition, S. d. grand individuals eat predominantly sardines (Anderson 1989, D. J. Anderson unpubl. data; but see Murphy 1936), whereas Masked Boobies elsewhere in the Pacific eat predominantly flying fish (Nelson 1978, Harrison 1983). Most importantly, orange- and yellow-billed "masked" boobies pair assortatively on islands where they coexist; for example, on Clipperton Island 150 orangebilled boobies nest sympatrically but assortatively with over 60,000 yellow-billed birds. On the basis of those data, Pitman and Jehl (1998) argued that S. d. grand satisfies the requirements of both a morphological species and a biological species, and recommended that it be recognized as a distinct species, Sula grand, with the common name "Nazca Booby." The American Ornithologists' Union (AOU 2000) has since recognized the Nazca Booby as a full species. However, Pitman and Jehl invited a molecular evaluation of genetic relationships among the "masked" boobies. In this article, we compare variation in a neutral molecular marker-the mitochondrial cytochrome-b gene-among Nazca Boobies and three subspecies of Masked Boobies to evaluate further whether S. grand merits recognition under either of the two most common species definitions: the biological (Mayr 1963) and phylogenetic (Cracraft 1989) species concepts.