Pycnochromis, Fowler, 1941

Tang, Kevin L., Stiassny, Melanie L. J., Mayden, Richard L. & DeSalle, Robert, 2021, Systematics of Damselfishes, Ichthyology & Herpetology 109 (1), pp. 258-318 : 278-279

Pycnochromis .

— Support for the monophyly of this genus is high (100% bootstrap) and congruent with other studies. The species we are recognizing as Pycnochromis have been resolved as the sister group to Dascyllus in numerous phylogenetic analyses ( Tang, 2001; Jang-Liaw et al., 2002; Quenouille et al., 2004; Tang et al., 2004; Koh and Park, 2007; 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; Lobato et al., 2014; DiBattista et al., 2016; Mirande, 2016; Gaboriau et al., 2018; Delrieu-Trottin et al., 2019). Based on the distribution of the ceratomandibular ligament ( Frédérich et al., 2014), species of the Dascyllus þ Pycnochromis clade appear to be united by the reappearance of the ligament, which is otherwise lost in the subfamily (see above). All species of Dascyllus and almost all species of Pycnochromis have XII dorsal spines; P. acares , P. alleni , and P. ovatiformis can sometimes have XIII ( Randall and Swerdloff, 1973; Randall et al., 1981); P. howsoni occasionally has XI (Allen and Erdmann, 2014). However, this count is widespread in the subfamily (see above). A group matching the composition of Pycnochromis sensu novum has been recorded in other studies ( Hubert et al., 2011; 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). This taxon has been variously labeled ‘‘ Chromis II’’ ( Quenouille et al., 2004), ‘‘ Chromis b’’ ( Koh and Park, 2007), ‘‘ Chromis IP Clade 4’’ (Cowman and Bellwood, 2011), and ‘‘ Chromis III’’ (Cooper and Santini, 2016).

The genus we are recognizing as Pycnochromis is composed of two morphologically distinct clades: one that comprises the miniature species, P. vanderbilti and its allies, and another that represents an expanded margaritifer complex sensu Allen (1975a) [¼ Chromis amboinensis complex sensu Allen and Erdmann, 2020 ]. We were unable to sample Chromis lineata , but it is almost certainly a member of Pycnochromis given its similarity to P. acares , P. nigrurus , and P. vanderbilti (Allen, 1975a, 1997; Moyer and Ida, 1976; Randall et al., 1981; Myers, 1999; Allen and Erdmann, 2012). Hence, we refer it to Pycnochromis as P. lineatus . With that generic reassignment, Chromis lineata Cadenat, 1950 is no longer a secondary homonym of Chromis lineata Fowler and Bean, 1928 , but is considered permanently invalid because it was replaced by Whitley (1951) before 1961 ( ICZN, 1999: Art. 59.3) and its substitute name, Chromis cadenati , is in use (e.g., Wood, 1977; Edwards, 1986; Allen, 1991; Edwards et al., 2003; Lecchini and Williams, 2004; Domingues et al., 2005). The four miniature species are characterized by their small adult size (approximately 40–45 mm SL), elongate body, compact heads, and yellowish coloration with blue markings ( Randall and Swerdloff, 1973; Allen, 1975a; Randall et al., 1981, 1997; Myers, 1999; Allen and Erdmann, 2012). Three of the four have blue spots on the opercle and along the scale rows, forming stripes that run the length of the body. Pycnochromis acares possesses blue spots on the head but lacks the lateral bands on the body ( Randall and Swerdloff, 1973).

Species of the margaritifer complex differ markedly in their coloration and overall appearance from Pycnochromis vanderbilti and its immediate relatives. Allen (1975a: 38) characterized them as possessing XII dorsal spines, an exposed suborbital (i.e., infraorbital) margin, and prolonged bifurcate filaments at the tips of the caudal lobes. Moyer and Ida (1976) identified the group by their filamentous caudal-fin rays. The presence of filamentous caudal-fin margins in these species was also noted by Randall et al. (1981). As stated above, due to the widespread presence of XII dorsal spines across chromines, that meristic count has limited value for distinguishing among genera. Similarly, the condition of the suborbital margin is not unique to the group; there are other species of Chrominae that exhibit the same character state (e.g., Azurina elerae , A. lepidolepis , Chromis abyssus , C. albomaculata , C. athena , C. circumaurea , C. degruyi , C. earina , C. flavapicis , C. mirationis , C. monochroma , C. okamurai , C. planesi , C. pura , C. unipa , and C. xouthos ; Randall et al., 1981; Yamakawa and Randall, 1989; Randall, 2001; Allen and Randall, 2004a; Lecchini and Williams, 2004; Allen and Erdmann, 2005, 2008a, 2009a; Pyle et al., 2008). However, species of the sister clade ( P. vanderbilti and relatives) lack an exposed suborbital ( Fowler and Bean, 1928; Fowler, 1941; Randall and Swerdloff, 1973; Moyer and Ida, 1976; Randall et al., 1981), so the condition of the suborbital in the species of the margartifer complex may distinguish the group from the rest of Pycnochromis . A more exhaustive survey of this trait is necessary. Of the three diagnostic characters detailed by Allen (1975a), the presence of extended, branching filaments on each caudal-fin lobe seems to be most reliable for identifying members of this complex. They are present in all species of the group ( Smith, 1960; Randall and Swerdloff, 1973; Randall et al., 1981; Randall, 1988a, 1988b, 2001; Randall and McCosker, 1992; Randall and DiBattista, 2013; Allen and Erdmann, 2014; Froese and Pauly, 2019) and absent in other chromine species. A few have extensions of the caudal fins but not like what is seen in the margaritifer complex: Chromis albomaculata (as juveniles), C. atripectoralis , and C. xanthura have filamentous caudal-fin rays but none are described as being branched ( Randall et al., 1981). Pycnochromis includes many of the black-and-white species (e.g., P. fieldi , P. iomelas , P. leucurus , and P. margaritifer ), but some white-tailed species were recovered elsewhere (e.g., C. chrysura and C. xanthura ). The placement of C. chrysura outside of Pycnochromis is expected; in comparing other species to P. fatuhivae, Randall (2001) noted in passing that C. chrysura ‘‘is in a different lineage.’’ He specifically commented on its XIII dorsal spines and lack of filamentous caudal rays. Even though the bicolor species with abrupt dark-tolight color patterns do not form a monophyletic group within Pycnochromis , a subset of them ( P. abruptus, P. dimidiatus , P. fieldi , and P. margaritifer ) are closely related (100% bootstrap), which has been proposed before ( Randall, 2001; Allen and Erdmann, 2012; Randall and DiBattista, 2013; He et al., 2019b). Our results corroborate the sister relationship hypothesized for P. abruptus (¼ Chromis sp. ‘‘F’’ in Allen, 1975a) and P. margaritifer (Allen, 1975a; Randall, 2001). Pycnochromis iomelas , which shares a similar halfand-half color pattern with these species and has been previously associated with them (e.g., Allen, 1975a; Randall et al., 1997; Randall, 2005; Randall and DiBattista, 2013), was recovered as a more distant relative within Pycnochromis . Our phylogeny resolves P. amboinensis and P. howsoni as sister species, as hypothesized by Allen and Erdmann (2014).

Provisional classifications.— Chromis agilis is probably a member of this genus because of its similarity to P. atripes ( Randall, 1988a; Allen, 1991; Myers, 1999; Randall, 2005; C. agilis treated as C. xutha ) and P. pacifica (Allen and Erdmann, 2020) . Chromis flavipectoralis is likely another species of Pycnochromis , based on its similarity to P. amboinensis (Allen, 1991; Allen and Erdmann, 2012) and P. ovatiformis ( Randall, 1988a) . Both C. agilis and C. flavipectoralis possess the diagnostic features of the margaritifer complex: XII dorsal spines, exposed suborbital margin, and caudal-fin lobes with filamentous branched tips ( Randall, 1988a). Therefore, we provisionally classify these two species as Pycnochromis ( Table 2 ). This would bring the total number of species in this complex to 20, matching the diversity of the Chromis amboinensis complex mentioned by Allen and Erdmann (2020: 110), who placed C. xutha in the synonymy of C. agilis [¼ P. agilis ]. We treat Chromis lineata as Pycnochromis lineatus because of its similarity to P. acares , P. nigrurus , and P. vanderbilti (see above).