Stegastes, Jenyns, 1840

Tang, Kevin L., Stiassny, Melanie L. J., Mayden, Richard L. & DeSalle, Robert, 2021, Systematics of Damselfishes, Ichthyology & Herpetology 109 (1), pp. 258-318 : 285-287

Stegastes .

— The 40 circumtropical species traditionally classified in this genus (farmerfishes or gregories) are characterized as aggressive, benthic damselfishes (Allen, 1975a; Allen and Emery, 1985; Allen and Smith, 1992; Smith-Vaniz et al., 1999) that engage in territorial behavior and algal farming ( Vine, 1974; Belk, 1975; Allen and Robertson, 1994; Ceccarelli et al., 2001; Hata and Kato, 2004, 2006; Frédérich et al., 2013; Hata and Ceccarelli, 2016; Pratchett et al., 2016). The genus has long posed a challenge to systematists (Bleeker, 1877; Longley and Hildebrand, 1941; Rivas, 1960; Emery and Burgess, 1974; Greenfield and Woods, 1974; Emery and Allen, 1980; Allen and Emery, 1985; Smith-Vaniz et al., 1999). Stegastes is not monophyletic as currently constituted ( Fig. 1 ), which corroborates other phylogenetic studies that have shown the same (e.g., Cooper et al., 2009; Cowman and Bellwood, 2011; Hofmann et al., 2012; Litsios et al., 2012a, 2012b; Frédérich et al., 2013; Rabosky et al., 2013, 2018; DiBattista et al., 2016; Mirande, 2016; Gaboriau et al., 2018; Delrieu-Trottin et al., 2019). Instead, the species fall into two disjunct clades, one of which is nested within Plectroglyphidodon and the other includes the type species, S. imbricatus . The latter group, Stegastes sensu stricto, would have been rendered non-monophyletic by the presence of ‘‘ Plectroglyphidodon ’’ lacrymatus , if not for its reclassification as Stegastes (see above). The two groups can be generally distinguished by the number of dorsal-fin spines: XII in Stegastes sensu stricto vs. XIII or XIV in ‘‘ Stegastes ’’ associated with Plectroglyphidodon . Allen (1975a: 42), who used the number of dorsal spines in his key for Stegastes (as Eupomacentrus ), suggested the genus could be divided into two subgenera based on this difference. However, although instructive, this character is not strictly diagnostic. Of the species sampled in this study, there are at least three exceptions: S. pelicieri (XIV, with Stegastes sensu stricto), S. altus , and S. aureus (XII, with ‘‘ Stegastes ,’’ S. altus occasionally has XIII; Allen, 1991). The presence of XII spines in Stegastes sensu stricto mostly fits the original conception of Stegastes and Eupomacentrus , a widely used junior synonym, which were both characterized by XII dorsal spines ( Jenyns, 1840; Bleeker, 1877; Emery and Allen, 1980). The results of this analysis imply that later additions to this genus, namely Indo-Pacific species with XIII spines (Allen, 1975a; Emery and Allen, 1980), are not part of Stegastes sensu stricto.

There are three junior synonyms of Stegastes : Brachypomacentrus , Eupomacentrus , and Omopomacentrus . Atlhough simultaneous, Eupomacentrus Bleeker, 1877 has priority over Brachypomacentrus Bleeker, 1877 ( ICZN, 1999: Art. 24.1) because Eupomacentrus was proposed at a higher rank (Bleeker, 1877). The type species of all three genera appear to be members of Stegastes sensu stricto. We recovered S. albifasciatus, the type of Brachypomacentrus , in the same clade as S. imbricatus . Stegastes lividus , the type of Eupomacentrus , presents an interesting situation. Randall (2004) discovered that the widespread species universally accepted as S. lividus (e.g., Bleeker, 1877; Fowler and Bean, 1928; Montalban, 1928; Whitley, 1929; de Beaufort, 1940; Aoyagi, 1941; Smith, 1960; Woods and Schultz, 1960; Allen, 1975a, 1991, 1997; Masuda et al., 1975, 1984; Allen and Randall, 1981; Allen and Emery, 1985; Randall et al., 1997; Myers, 1999; Jang-Liaw et al., 2002; Lieske and Myers, 2002) should be called S. punctatus because the S. lividus of Forster in Bloch and Schneider (1801) is endemic to the Marquesas Islands, where it had been recognized by Randall (2001) as Stegastes robertsoni . The taxonomic species that Bleeker (1877) fixed as the type of Eupomacentrus was actually S. punctatus . In cases where the type species was misidentified but correctly fixed, either species involved in the misidentification may be subsequently designated as the type ( ICZN, 1999: Arts. 67.9, 70.3). However, Randall (2004) did not take any taxonomic action, presumably because Eupomacentrus was not in use. In our phylogeny, S. punctatus is in the same clade as S. imbricatus . Even though we were unable to examine S. lividus , based on the number of the dorsal spines (XII) and the phylogenetic position of S. albifasciatus , which Randall (2001, 2005) proposed as its closest relative, it is likely that S. lividus is also part of Stegastes sensu stricto. Finally, S. acapulcoensis , the type species of Omopomacentrus , is an eastern Pacific species with XII dorsal spines found in the same clade as S. imbricatus . Based on the phylogenetic position of S. acapulcoensis , Omopomacentrus is a synonym of Stegastes , not Pomacentrus , as stated by Kottelat (2013). This is in agreement with numerous authors who have regarded acapulcoensis as a species of Stegastes (e.g., Allen and Woods, 1980; Allen, 1991; Allen and Robertson, 1994; Grove and Lavenberg, 1997; Thomson et al., 2000; Robertson and Allen, 2015).

Stegastes sensu stricto is resolved as a monophyletic group, with the inclusion of S. lacrymatus . Within it, there is 100% bootstrap support for a clade comprising the Atlantic and eastern Pacific species, which all have XII dorsal spines. The phylogeny suggests that Stegastes sensu stricto diversified following a colonization of Neotropical waters. In their review of eastern Pacific Stegastes, Allen and Woods (1980) recognized that those species were closely related to the Atlantic ones. We recovered a sister-group relationship between S. acapulcoensis and S. rectifraenum , which was posited by Thomson et al. (2000) based on similarities in juvenile color patterns. The phylogeny supports a S. partitus – S. pictus relationship (100% bootstrap) first identified by Emery and Allen (1980), on the basis of both species possessing one fewer row of cheek scales (3) than other Stegastes (4). In their analysis of Atlantic Stegastes, Souza et al. (2016) also recovered a S. partitus – S. pictus clade. Our topology shares other similarities with theirs: a clade composed of S. adustus , S. beebei , S. diencaeus , S. flavilatus , S. variabilis , and S. xanthurus (treated as a population of S. variabilis therein); another clade of S. arcifrons , S. fuscus , S. planifrons , S. rectifraenum , and S. rocasensis ; a third clade of S. albifasciatus , S. limbatus , S. nigricans , and S. punctatus . They demonstrated a divergence between the Brazilian and Caribbean populations of S. variabilis , which corroborates the restriction of S. variabilis to Brazil and recognition of S. xanthurus as the widespread species that occurs in the Caribbean ( Robertson and Van Tassell, 2012; Smith-Vaniz and Jelks, 2014; Robertson et al., 2016a).

Stegastes sensu stricto includes most of the Indo-West Pacific species that possess XII dorsal-fin spines plus S. pelicieri , which has XIV spines. In their revision of the Indo-Pacific Stegastes sensu lato, Allen and Emery (1985) noted that, unlike many other widespread genera, much of its diversity falls outside of the region. They differentiated four species ( S. albifasciatus , S. limbatus , S. lividus [¼ S. punctatus ], and S. nigricans ) from the remainder of Stegastes . They linked S. albifasciatus and S. nigricans together as part of a species complex, but separated S. limbatus and S. lividus [¼ S. punctatus ] each into their own lineages. In our phylogeny, all four species form a monophyletic group (100% bootstrap) that is the sister taxon of the eastern Pacific/Atlantic Stegastes . Hubert et al. (2017: fig. A1, tables S3, S4) reported spatial structure within S. nigricans , with discrete lineages representing the Indian and Pacific Oceans. Even though they described it as geographic paraphyly (their ‘‘pattern II.2’’), their figure ( Hubert et al., 2017: fig. A1) shows geographic monophyly (their ‘‘pattern II.1’’). If those populations are recognized as cryptic diversity, there is an available name in the synonymy of S. nigricans that could apply to the western Indian Ocean population, Pomacentrus scolopseus .

The Indo-Pacific ‘‘ Stegastes ’’ also form a monophyletic group (100% bootstrap support; Fig. 1 ), but their placement renders Plectroglyphidodon paraphyletic. Species in this ‘‘ Stegastes ’’ are confined to the Indo-Pacific and typically have XIII or more dorsal spines, except for S. altus and S. aureus , which usually have XII dorsal spines (rarely XIII in altus ; Allen and Emery, 1985). The group includes the species circumscribed as the ‘‘ fasciolatus complex’’ by Allen and Emery (1985): S. aureus , S. emeryi , S. fasciolatus , S. gascoynei , and S. insularis . Our phylogeny found all of them together in a clade, along with S. luteobrunneus and S. marginatus , which were considered synonyms of S. fasciolatus by Allen and Emery (1985), as well as S. altus . This fasciolatus complex plus S. altus is well supported (100% bootstrap). Allen and Emery (1985) considered S. altus part of a different species complex that also comprised S. apicalis and S. obreptus , with the latter two species hypothesized to be most closely related. Although the topology did resolve S. apicalis and S. obreptus as sister species (100% bootstrap), our results placed S. altus inside the fasciolatus clade. The S. apicalis þ S. obreptus pair is sister to the fasciolatus complex. All of these species form a monophyletic group (100% bootstrap) within Plectroglyphidodon , which is also composed entirely of Indo-Pacific taxa. In addition to rendering Plectroglyphidodon paraphyletic, this clade of ‘‘ Stegastes ’’ lacks an available name. Cowman and Bellwood (2011: figs. 2b, S6) called a group with similar composition ‘‘ Stegastes et al. IP.’’ Cooper and Santini (2016: fig. 1) called it ‘‘ Stegastes II / Plectroglyphidodon II / Parma II’’ and suggested that several of these species (e.g., S. altus , S. apicalis , S. fasciolatus , S. obreptus ) would eventually need to be classified as Plectroglyphidodon , but they did not formally make the taxonomic change. The diagnoses for both genera largely overlap (Allen, 1975a; Allen and Emery, 1985), with two conspicuous differences: margin of preopercle smooth in Plectroglyphidodon vs. serrated in adult Stegastes ; margin of suborbital smooth in Plectroglyphidodon vs. variable (serrated, crenulated, or smooth) in Stegastes . We hereby assign the following species to Plectroglyphidodon : altus , apicalis , aureus , emeryi , fasciolatus , gascoynei , insularis , luteobrunneus , marginatus , and obreptus . Plectroglyphidodon and Stegastes are both masculine, so no changes for gender agreement are necessary.

Based on geographic variations in color, Allen and Emery (1985: 16, pl. I–II) recognized three distinct populations within the widespread Stegastes fasciolatus [¼ Plectroglyphidodon fasciolatus ]: Hawaiian Islands; Indo-West Pacific Ocean (type locality: Lord Howe Island); western Indian Ocean and Cocos-Keeling Islands. Citing unpublished sequence data from D. R. Roberston, Randall (2005, 2007) stated that each population should be recognized as a separate species. Randall (2005) indicated that S. fasciolatus sensu stricto should be restricted to the population in the eastern Indian Ocean ( Christmas Island and western Australia), Oceania (excluding Hawaii), and the western Pacific Ocean. Randall (2007) resurrected Stegastes marginatus for the Hawaiian population, removing it from the synonymy of S. fasciolatus . Following Randall (2005), Fricke et al. (2009) resurrected Stegastes luteobrunneus for the western Indian Ocean population (including Cocos-Keeling). The sequences for Plectroglyphidodon luteobrunneus used in this analysis ( JF435094 , JF457578 , JF458207 ) come from a sample currently identified as Pomacentrus agassizii (BOLD record: IPCOM438-10; BOLD sample ID: REU0983). Moreover, all currently published sequences labeled as ‘‘ Pomacentrus agassizii ’’ ( JF435094 – JF435098 ; JF457578 – JF457582 ; JF458207 – JF458211 ; JQ350239 – JQ350240 ; MF409473 ) appear to be Plectroglyphidodon luteobrunneus . Those specimens were all collected from Réunion, and their sequences are most similar to fish identified as Stegastes fasciolatus [¼ Plectroglyphidodon fasciolatus ] (Collet et al., 2018: table 2). Plectroglyphidodon luteobrunneus is the only member of the fasciolatus clade known from the western Indian Ocean ( Fricke, 1999; Heemstra et al., 2004; Letourneur et al., 2004; Fricke et al., 2009, 2018). This misidentification would explain the anomalous position of ‘‘ Pomacentrus agassizii ’’ seen in several studies (e.g., Hubert et al., 2011, 2012, 2017; DiBattista et al., 2016; Mirande, 2016; Rabosky et al., 2018).

Of the species that we could not examine, each can be tentatively assigned to Stegastes sensu stricto based on the number of dorsal-fin spines, unpublished barcode sequences, previous statements in the literature, and/or geographic distribution. All remaining Atlantic and eastern Pacific species ( S. leucorus , S. lubbocki , S. redemptus , S. sanctaehelenae , and S. uenfi ) are retained in Stegastes sensu stricto. We follow Souza et al. (2016) in treating Stegastes sanctipauli as a synonym of S. rocasensis . Tree-based identifications (not shown) that assayed unreleased sequences in the BOLD database support this placement of S. leucorus , S. lubbocki , S. redemptus , and S. sanctaehelenae . Stegastes leucorus was most similar to S. beebei . This would corroborate the hypothesis of Allen and Woods (1980) who suggested that the two lineages ( S. beebei as a subspecies of S. leucorus ) plus S. baldwini are most closely related. Stegastes lubbocki and S. sanctaehelenae were found to be most similar to S. pictus and each other. Stegastes redemptus was linked to S. arcifrons in the NJ tree generated by BOLD. The Marquesan endemic Stegastes lividus is retained in Stegastes sensu stricto based on its possession of XII dorsal spines and overall resemblance to S. albifasciatus and S. nigricans ( Randall, 2001, 2005).

Figures

FIG. 1. The phylogenetic relationships of damselfishes (Teleostei: Pomacentridae): (A) outgroup taxa; (B) subfamilies Microspathodontinae and Glyphisodontinae; (C) subfamily Chrominae; (D) subfamily Pomacentrinae (part I); (E) subfamily Pomacentrinae (part II). The phylogeny is based on the maximum-likelihood topology with the best log likelihood score (ln L ¼ –45,4518.728) recovered from ten independent maximum-likelihood analyses of a data matrix composed of 462 taxa and 6,865 nucleotides. Bootstrap values from 1,000 ultrafast replicates are displayed for each node