A new species of the perchlet genus Plectranthias is herein described from a single specimen found at Rapa Nui (Easter Island) in the South Pacific. Plectranthias ahiahiata sp. n. was collected at a depth of 83 m in a mesophotic coral ecosystem at Rapa Nui. The main difference between Plectranthias ahiahiata and other members of the genus is higher fin-ray counts (X, 18 dorsal; 18 pectoral) and its distinctive coloration. Compared to the three other known eastern South Pacific species, P. ahiahiata has more dorsal-fin rays, more pectoral-fin rays, fewer tubed lateral-line scales, fewer gill rakers, a longer head relative to SL, a very short first dorsal spine relative to SL, and a short third anal spine relative to SL. Plectranthias ahiahiata is distinguished from western Pacific species, by having more dorsal- and pectoral-fin rays. The closest relative based on genetic divergence (with 12.3% uncorrected divergence in the mitochondrial COI gene) is Plectranthias winniensis, a widely distributed species, suggesting important links between Rapa Nui and western Pacific islands. This new species adds to the high endemism of the Rapa Nui ichthyofauna, and is further evidence of the importance of mesophotic reefs as unique communities.

endemism, ichthyology, reef fish, South Pacific, taxonomy

Rapa Nui (Easter Island) is the most isolated inhabited island in the Indo-Pacific, located approximately 3,700 km west of Chile and 2,000 km from the nearest inhabited island, Pitcairn (Randall and Cea 2010; Easton et al. 2017). Due to the isolation, the fish fauna exhibits the highest degree of local and regional endemism in both shallow-water and deep-sea fishes in the Indo-Pacific, and very low overall diversity (Friedlander et al. 2013; Easton et al. 2017). Nearly 22% of the shore fishes of Easter Island are endemic (Randall and Cea 2010; Friedlander et al. 2013). Although ichthyological studies since the 1980’s have greatly expanded the knowledge of fish diversity in this ecoregion, there have been relatively few surveys of fishes at depths greater than 40 m (Friedlander et al. 2013; Wieters et al. 2014; Easton et al. 2017). Recent advances in technical mixed-gas rebreather diving are now allowing scientists to safely document small, active, cryptic, and demersal fishes in ways that have been previously impossible (Pyle 2000; Pyle et al. 2016; Rocha et al. 2017). Using these techniques, our team collected a distinctive new species of Plectranthias off Hanga Piko, Rapa Nui.

The anthiadine genus Plectranthias Bleeker 1873 currently comprises 56 valid species from tropical and temperate waters in the Atlantic, Pacific, and Indian oceans (Eschmeyer et al. 2017). Most of these fishes are found in relatively deep waters (90-420 m) on hard substrates such as mesophotic coral ecosystems and rocky formations (Allen and Walsh 2015; Gill et al. 2016). Due to their diminutive size and cryptic habits they are not often caught by hook and line or in trawls, so they are poorly represented in museum collections, and most species have been described based on a single or a small number of specimens (Randall 1980; Heemstra and Randall 2009; Bineesh et al. 2014; Allen and Walsh 2015; Gill et al. 2016). The three known eastern Pacific species within the genus are P. exsul Heemstra and Anderson 1983, P. nazcae Anderson 2008, and P. parini Anderson and Randall 1991. A fourth eastern Pacific species, P. lamillai Rojas and Pequeño 1998, was proposed, however it was later determined to be a junior synonym of P. exsul (Anderson and Baldwin 2002). Plectranthias parini is the only other congener known to occur at Rapa Nui, where it was recorded at depths of 150 m during drop-cam surveys (Friedlander et al. 2013). Here Plectranthias ahiahiata sp. n. is described and illustrated from a single specimen collected at Rapa Nui.

This species is described from a single specimen collected with hand nets while diving on mixed-gas, closed-circuit rebreather (Hollis Prism 2). Counts were performed with a microscope, and morphological characters were measured to the nearest 0.01 mm with digital calipers following the conventions described in Anderson and Heemstra (2012) and Williams et al. (2013). Comparative material included Plectranthias japonicus Steindachner 1883 (CAS33555), P. sagamiensis Katayama 1964 (CAS235596), and P. winniensis Tyler 1966 (CAS219169), along with recent publications, and keys and revisions to the genus made by Randall (1980, 1996) and Heemstra and Randall (2008). Morphometric and meristic data for the holotype and comparisons with related species are presented in Table 1 and the Supplementary Online Material. The holotype was deposited at the California Academy of Sciences ichthyological collection.

Mitochondrial Cytochrome c oxidase subunit I (COI) DNA was sequenced and analyzed for the new species. DNA extraction and PCR amplification of the COI were performed following Weigt et al. (2012) protocols. DNA sequences were compared to the ten Plectranthias species available in GenBank (P. bennetti Allen and Walsh 2015: KT601636; P. flammeus Williams, Delrieu-Trottin and Planes 2013: KC565477–KC565480; P. fourmanoiri Randall 1980: KC567662, KC567663; P. japonicus: JQ681323, JQ681324; P. kamii Randall 1980: KU943548; P. kelloggi Jordan and Evermann 1903: KP267643; P. longimanus Weber 1913: JF494178; P. nanus Randall 1980: JQ432001–JQ432004, KC565481, KC567661; P. randalli Fourmanoir and Rivaton 1980: KP267613; P. winniensis: KC565482, KC565483). GenBank accession number for the new species is MH025944.

The generic classification of the Anthiadiniae is problematic, and the genus Plectranthias is especially in need of revision, as it is not currently defined on the basis of synapomorphies, and there is high variation within many of the defining characters (Anderson and Heemstra 2012; Gill et al. 2016). Plectranthias ahiahiata is placed within the genus Plectranthias due to the combination of the following characters: dorsal fin X, 18; dorsal fin deeply notched before soft rayed part; anal fin III, 7; second anal spine the longest; pectoral fin rays 18; pelvic fin I, 5; body elongate, oval to oblong; body depth approximately 30-45% of SL; benthic, cryptic habits.

However, placement is provisional, as ongoing genetic and morphological studies indicate Plectranthias is not monophyletic, and it is likely that future work will split the genus (Anderson and Heemstra 2012; Gill, et al. 2016). The results of our molecular analysis indicate 12.3% uncorrected divergence in the mitochondrial COI gene from P. winniensis, its closest relative among the sequenced members of the genus (Table 2). Plectranthias species are minimally represented in GenBank, with COI sequences only available for ten species. Since there are 56 valid species in the genus, presenting a phylogenetic tree with just ten species (less than 20% taxon coverage) could be misleading (Zwickl and Hillis 2002); therefore, a phylogenetic analysis was not performed with the limited available data. As more discoveries are made and more species are sampled for phylogenetics, we can begin to unravel the taxonomic confusion within this genus.

Due to the high degree of endemism among shore fishes of Rapa Nui, as well as the large number of known Plectranthias species from the western Pacific, it is likely that P. ahiahiata is also a locally or regionally endemic species. The fish was found at mesophotic depths on a rocky outcrop with a stand of the stony coral Leptoseris scabra (Figure 4). The ambient seawater temperature was approximately 19°C at the time of the dive (7 March 2017). Other fishes at this site included two unknown species of the family Serranidae and one from Pomacentridae, which our group will be describing. Although it is possible that the other species described from the Nazca and Sala y Gómez ridge region, P. exsul and P. nazcae, are geographically sympatric, the only member of the genus previously documented from Rapa Nui is P. parini. The affinity with P. winniensis, a widespread species ranging across the Indo-Pacific from the Red Sea to Pitcairn, suggests important links between the ichthyofauna of Rapa Nui and the western Pacific islands.

Figure 4. 

Collection site of the holotype 83 m depth, Hanga Piko, Rapa Nui (photograph: LA Rocha).

Mesophotic ecosystems are known to shelter numerous specialist and exclusive species, which contribute to making this environment biologically unique in tropical and semitropical environments (Pinheiro et al. 2016). The few surveys of fishes at depths greater than 40 m at Rapa Nui (Easton et al. 2017) and the high degree of endemism at the island suggest that many other unknown species remain to be discovered. Scientists’ use of closed-circuit, mixed-gas rebreathers is allowing exploration and comparison between mesophotic and shallow coral ecosystems. The many new species descriptions resulting from the exploration of mesophotic reefs reveal the importance and uniqueness of these ecosystems (Pyle et al. 2016; Baldwin et al. 2016, 2018; Rocha et al. 2017).

This work was funded by the generous support of donors to the California Academy of Sciences’ Hope for Reefs Initiative, and Fondecyt # 1151094 granted to APM. This research was conducted under permit 2231 from the Ministerio de Economia Fomento Y Turismo de Chile and exported under certificate 669081 from the National Fisheries Service of Chile (SERNAPESCA). We are grateful to many colleagues who helped in the field, lab, and with discussions: Cristina Castillo, Mauritius V. Bell, Mark Lane, David Catania, Michel Garcia, Wilo Teao, Vladimir (Tuto) Garmendia, and Orca Diving Center. We would like to thank Jon Fong for taking the photographs and radiograph of the preserved holotype. Charlotte Shepherd suggested the common name, sunset, which we have done our best to translate into the Rapa Nui language. We thank Carol Baldwin for her many helpful suggestions, which greatly improved the final manuscript.