Polybranchia Pease, 1860

Polybranchia Pease, 1860 View in CoL is a genus of sacoglossan sea slugs ( Gastropoda: Heterobranchia: Caliphyllidae ) that are thought to be specialized consumers of the green algae Caulerpa spp. ( Burn, 1989; Clark, 1994; Jensen, 1997). However, comparatively little work has been done on the feeding ecology or other aspects of the biology of Polybranchia . Unlike most sacoglossans, which

*Corresponding author: E-mail: aavaldes@cpp.edu

[Version of Record, published online 21 August 2018; http://zoobank.org/ urn:lsid:zoobank.org:pub:0B97B947-E1BA-41DD-AD01-C4C464447E48 ]

are typically found on or beneath the host algae on which they feed, large-bodied Polybranchia spp. hide under rocks by day and emerge to forage nocturnally ( Clark, 1994; Coleman, 2001; Apte, 2009). Their nocturnal habits and tendency to hide away from food algae make specimens of Polybranchia difficult to find and study, contributing to the paucity of biological data on this enigmatic group; in turn, Polybranchia has become a taxonomically neglected genus in need of revision.

Polybranchia View in CoL contains seven or eight nominal species generally recognized as valid, depending on the author (see: Jensen, 2007; Bouchet & Gofas, 2011). These include, P. viridis ( Deshayes, 1857) View in CoL , P. orientalis ( Kelaart, 1858) View in CoL , P. pellucida Pease, 1860 , P. papillosa ( Pease, 1866) , P. prasina ( Bergh, 1871) , P. borgninii ( Trinchese, 1896) View in CoL , P. pallens ( Burn, 1957) View in CoL and P. westralis Jensen, 1993 View in CoL . The geographic ranges of some nominal species are widespread, e.g. P. viridis View in CoL has been reported from the Eastern Pacific, Caribbean and Eastern Atlantic ( Bertsch & Smith 1973; Valdés et al. 2006; Ortea 1981), and P. orientalis View in CoL is found across the Indo-West Pacific (IWP) region, from warm-temperate southern Japan to southern Australia, and from the Red Sea to the Hawaiian Islands ( Jensen 2007). In contrast, several other species ( P. pellucida , P. papillosa , P. pallens View in CoL and P. westralis View in CoL ) are only known from their type localities (respectively: the Hawaiian Islands, an undermined locality in the Pacific Islands, south-eastern Australia and south-western Australia).

The phylogenetic position of Polybranchia View in CoL is also controversial. Cladistic analysis of morphological data recovered Polybranchia View in CoL sister to Cyerce Bergh, 1870 View in CoL , which lacks digestive diverticula in the large, flattened cerata that characterize members of the family Caliphylidae ( Jensen, 1996) . However, a four-gene molecular phylogeny recently recovered Polybranchia View in CoL as nested within a non-monophyletic Mourgona Er. Marcus & Ev. Marcus, 1970 View in CoL , although without significant support for the paraphyly of Mourgona ( Krug et al., 2015) View in CoL . Both Mourgona View in CoL and Polybranchia View in CoL have diverticula in their cerata, and neither was closely related to Cyerce View in CoL in molecular phylogenetic analyses. The species level taxonomy of Polybranchia View in CoL is equally problematic, with a complex history ( Jensen, 1993). In modern literature, only two species, P. viridis View in CoL and P. orientalis View in CoL , are consistently reported as valid (e.g. Coleman, 2001, 2008; Nakano, 2004; Camacho-García et al., 2005; Valdés et al., 2006; Yonow, 2008; Gosliner et al., 2015) and cited in largescale phylogenetic studies (e.g. Jensen, 1996; Christa et al., 2015; Krug et al., 2015). However, the status of these two species is problematic. The disjunct range of P. viridis View in CoL includes the tropical Atlantic and Eastern Pacific ( Camacho-García et al., 2005; Valdés et al., 2006), suggesting it might constitute a species complex. As currently understood, ‘ P. orientalis View in CoL ’ is highly polymorphic, possibly encompassing multiple cryptic species ( Gosliner et al., 2015). Indeed, at least three morphologically similar but genetically distinct species from the tropical Pacific were included in a recent phylogenetic study, indicating multiple species occur in regions where only P. orientalis View in CoL is typically reported ( Krug et al., 2015). The taxonomic status of most named Polybranchia spp. is unclear, however, as they lack detailed original descriptions, and few specimens have been reported to date.

Here, we use an integrative approach to address the systematics of Polybranchia , focusing on outstanding taxonomic issues in nominal species with disjunct or widespread ranges that might comprise cryptic species complexes. We describe cryptic diversity revealed by molecular genetic analyses using an integrative total-evidence approach, including new data on algal feeding and host association, and morphological data, including scanning electron microscope (SEM) images of the penis and radulae. We also conducted a review of the literature to determine the validity of previously introduced names, and carried out a comprehensive examination of online photographic records in an attempt to establish the range of all the species recognized herein. Our findings attempt to stabilize the taxonomy of this group, increase our understanding of heterobranch diversity and challenge long-standing vicariant hypotheses for species origins in the tropical Eastern Pacific.