Nexilosus, Heller & Snodgrass, 1903

Tang, Kevin L., Stiassny, Melanie L. J., Mayden, Richard L. & DeSalle, Robert, 2021, Systematics of Damselfishes, Ichthyology & Herpetology 109 (1), pp. 258-318 : 283

Nexilosus .

— The single species of this genus is a benthic omnivore found in shallow, rocky areas of the temperate eastern Pacific ( Hildebrand, 1946; Allen, 1991; Grove and Lavenberg, 1997; Angel and Ojeda, 2001; Aguilar-Medrano et al., 2011). In their description of the genus, Heller and Snodgrass (1903) remarked that most of the diagnostic characters for Nexilosus are shared with Hypsypops . Despite its classification in the tribe Microspathodontini (Cooper and Santini, 2016), the relationships of Nexilosus latifrons were uncertain in Cooper et al. (2009), who only had access to DNA acquired from formalin-fixed samples. The few target loci that were successfully sequenced had fragmentary data. Although it appeared in a polytomy within Microspathodontinae , Cooper et al. (2009: 12) noted that ‘‘molecular evidence for placing Nexilosus in the damselfish tree is weak’’ and its position was tentative. Cooper and Santini (2016) speculated that Nexilosus is a member of their tribe Microspathodontini , based on its large adult size, cranial morphology, ecology, and distribution. All subsequent works that have examined Nexilosus used Cooper et al.’s (2009) data. The position of this genus has been unstable in those phylogenetic studies. Cowman and Bellwood (2011: fig. S6) resolved it in the Indo-West Pacific ‘‘ Stegastes ’’ and not closely related to the putative genera of Microspathodontini ; Lobato et al. (2014) and Gaboriau et al. (2018) reported similar findings. Litsios et al. (2012a: fig. 2) found it in a clade with Chrysiptera starcki , sister to the remaining pomacentrines. Litsios et al. (2012b: figs. A1, A2) recovered Nexilosus either as the sister group of the Pomacentrinae or sister to Altrichthys þ Chrysiptera galba . DiBattista et al. (2016) recovered Nexilosus inside Chromis sensu stricto, sister to a group equivalent to Chromis (Thrissochromis) . In Delrieu-Trottin et al. (2019), Nexilosus is sister to Altrichthys and together they are the sister taxon of the Pomacentrinae minus Cheiloprionini . The inconsistency in its phylogenetic position is probably due to the incomplete nature of the sequences, a basic problem with DNA extracted from formalin-preserved tissue ( Shedlock et al., 1997; Schander and Halanych, 2003; Chakraborty et al., 2006). Because of their ambiguous nature, we did not include those data. Instead, we analyzed a COI sequence of Nexilosus latifrons retrieved from the BOLD database (Sequence ID: LIDMA1248-12; Supplemental Table 1; see Data Accessibility). Our results show that Nexilosus is a member of Microspathodontinae , as the sister group of a Hypsypops – Similiparma clade, which supports recent classifications (Cooper et al., 2009; Cooper and Santini, 2016).