Azurina Jordan & McGregor 1898

Azurina.

— The genus was first described as being ‘‘closely allied to Chromis ’’ (Jordan and McGregor, 1898 [in Jordan and Evermann, 1898]). It originally encompassed long, slender damselfishes with a continuous lateral line and deeply forked caudal fin (Jordan and McGregor, 1898 [in Jordan and Evermann, 1898]; Heller and Snodgrass, 1903). It has been consistently recovered within Chromis sensu lato (Tang et al., 2004; Cooper et al., 2009; Cowman and Bellwood, 2011; Litsios et al., 2012a, 2012b; Frédérich et al., 2013; Rabosky et al., 2013, 2018; Lobato et al., 2014; DiBattista et al., 2016; Mirande, 2016; Gaboriau et al., 2018; Delrieu-Trottin et al., 2019). Cowman and Bellwood (2011: figs. 2b, S6) included Azurina in a group they called ‘‘ Chromis EP /WA.’’ Cooper et al. (2009) placed Azurina in the synonymy of Chromis, a change that was supported by Aguilar-Medrano (2013) but largely ignored by others (e.g., Cowman and Bellwood, 2011; Litsios et al., 2012a, 2012b; Betancur-R. et al., 2013a, 2015, 2017; Frédérich et al., 2013, 2014; Cooper et al., 2014; Lobato et al., 2014; Aguilar-Medrano et al., 2015; DiBattista et al., 2016). Cooper and Santini (2016) formally reversed this decision and resumed treating Azurina as a distinct genus. In studies with sufficient sampling, Azurina hirundo is always found in a clade with some combination of A. atrilobata, A. cyanea, and/or A. multilineata (Cooper et al., 2009; Cowman and Bellwood, 2011; Litsios et al., 2012a, 2012b; Frédérich et al., 2013; Rabosky et al., 2013, 2018; Lobato et al., 2014; DiBattista et al., 2016; Mirande, 2016; Gaboriau et al., 2018; Delrieu-Trottin et al., 2019), which is usually apart from Chromis sensu stricto. The sister-group relationship of A. atrilobata and A. multilineata shown herein corroborates some previous studies (Quenouille et al., 2004; Rocha et al., 2008) but those did not include A. hirundo. Others that did examine A. hirundo found it more closely related to either A. atrilobata (Frédérich et al., 2013; Rabosky et al., 2013, 2018; Gaboriau et al., 2018; Delrieu-Trottin et al., 2019) or A. multilineata (Cowman and Bellwood, 2011; DiBattista et al., 2016). The two species of Azurina sensu stricto plus A. atrilobata are the only three species in Chrominae with no exposed spiniform procurrent rays (Greenfield and Woods, 1980; Aguilar-Medrano, 2013). Those three species also lack gill rakers on the upper branchial limb, though that character is also observed in a few other species (C. punctipinnis and C. scotti in Chrominae; Microspathodon frontatus and Stegastes diencaeus in Microspathodontinae; Allen, 1991). Azurina sensu lato also includes A. elerae and A. lepidolepis, two species from the Indo-Pacific. This relationship was also recorded in other phylogenies (Frédérich et al., 2013; Delrieu-Trottin et al., 2019). However, some alternate hypotheses more closely affiliate A. lepidolepis with C. ternatensis (Mirande, 2016; Gaboriau et al., 2018; Rabosky et al., 2018). Our expanded Azurina includes all species of ‘‘ Chromis ’’ from the Atlantic and eastern Pacific with XII dorsal-fin spines (viz., A. atrilobata, A. cyanea, A. multilineata) plus two Indo-West Pacific species (A. elerae and A. lepidolepis) that also have XII dorsal-fin spines.

The inclusion of A. brevirostris in this clade is unexpected, although Gaboriau et al. (2018: fig. A5) did report a similar relationship. Some key meristic counts differ from the rest of Azurina sensu lato: 14 (rarely 13) dorsal-fin rays versus 10–13; 15–16 anal-fin rays versus 10–13; XIII dorsal-fin spines versus XII (Allen, 1991; Pyle et al., 2008), except for A. eupalama (XIII, occasionally XIV; Heller and Snodgrass, 1903). Furthermore, its placement is highly unstable. There is no consensus for its phylogenetic position in past studies, with differing hypotheses placing it sister to: C. acares þ C. vanderbilti (Litsios et al., 2012a); C. cyanea (Gaboriau et al., 2018); C. notata (Frédérich et al., 2013); Chromis (Hoplochromis; DiBattista et al., 2016); Azurina sensu novum þ (Dascyllus þ Pycnochromis; Delrieu-Trottin et al., 2019). Rabosky et al. (2018: Dryad file ‘‘dropped_rogues.csv’’) identified this species as a rogue taxon (Wilkinson, 1996; Aberer et al., 2013) and pruned it from their analyses. All studies that have examined it, including this one, are based on the same COI sequences from Pyle et al. (2008); additional data are needed to improve its resolution. The placement of brevirostris in Azurina is provisional.

Of the species not examined herein, Chromis intercrusma is likely a member of Azurina sensu lato considering it has XII dorsal-fin spines and an eastern Pacific distribution, a combination of traits it shares only with A. atrilobata and A. hirundo among chromines. If we expand the criteria to all New World chromines with XII dorsal-fin spines, that would include A. cyanea and A. multilineata, both of which have been reclassified herein as members of Azurina sensu lato. Moreover, its combination of a New World distribution and two exposed spiniform procurrent caudal-fin rays on each caudal lobe is shared with only A. cyanea (variably 2–3 rays), A. multilineata, and C. meridiana among chromines (Greenfield and Woods, 1980; McEachran and Fechhelm, 2005). Except for C. meridiana, which we were unable to examine (see below), the only chromine species with this combination of features were all recovered in Azurina. A neighbor-joining (NJ) tree (Saitou and Nei, 1987) generated by a tree-based search using the Identification System in BOLD v.4 (Ratnasingham and Hebert, 2007) appears to confirm this by placing unreleased sequences of C. intercrusma with A. atrilobata, A. hirundo, and A. multilineata (not shown). We hereby assign intercrusma to Azurina sensu lato (Table 2).

Chromis meridiana is likely a member of Azurina. Greenfield and Woods (1980) originally suggested that C. meridiana might have closer affinities to Indo-Pacific species based on the number of exposed spiniform procurrent rays on the upper and lower caudal peduncle (2 vs. 3 in other deep-bodied Chromis from the eastern Pacific except C. intercrusma). They proposed C. randalli as a possible close relative because it also has a high dorsal-fin spine count (XV in C. randalli; XIV, occasionally XIII, in C. meridiana). Lecchini and Williams (2004) compared it to C. planesi and C. struhsakeri based on similarities in coloration. Shepherd et al. (2020) stated that it shares a white spot at the junction of the dorsal-fin base and the caudal peduncle with C. mamatapara, C. notata, and C. planesi; they noted that it also shares an array of meristic characters, including XIV dorsal-fin spines, with C. mamatapara, C. mirationis, and C. verater. Aguilar-Medrano (2013: fig. 6) found it sister to A. atrilobata whereas Aguilar-Medrano et al. (2013: fig. 9) found it sister to C. crusma. Its eastern Pacific distribution is consistent with either Azurina sensu lato or the C. insolata clade of Chromis sensu stricto. The number of dorsal spines (XIII or XIV) observed in C. meridiana is seen in species from both of those groups. However, among chromines, only three other species have a New World distribution and two spiniform rays: A. cyanea, A. intercrusma, and A. multilineata (Greenfield and Woods, 1980; McEachran and Fechhelm, 2005). Both A. cyanea and A. multilineata are part of Azurina. We were unable to examine A. intercrusma, but provisionally classified that species in Azurina partially on the basis of the same combination of traits (see above). Therefore, we tentatively assign meridiana to Azurina sensu lato (Table 2).