Herein, we describe a new species of Plectranthias perchlet found at depths of 100–125 meters in mesophotic coral ecosystems of the Maldives in the Indian Ocean. Plectranthias raki sp. nov. is unique in both morphology and coloration. The following combination of characters distinguishes it from all known congeners: dorsal fin X, 15; anal-fin rays III, 7; pectoral-fin rays 13 | 13 (13 | 12), all unbranched; principal caudal-fin rays 9 + 8; lateral line complete with 30–32 tubed scales; gill rakers 5 + 12; circumpeduncular scales 11–12; and absence of antrorse or retrorse spines on ventral margin of preopercle. Coloration in life consists of a white to light pink body with two indistinct rows of irregularly shaped red-orange to yellow-orange patches along the dorsal two-thirds of the body, a golden-yellow opercle and maxilla, an indistinct yellow stripe on the dorsal fin, two yellow spots near the base of the anal fin, and two irregularly shaped yellow-orange spots located on either side of centermost caudal-fin rays. With this publication, the genus Plectranthias now comprises 67 valid species. This discovery adds to a strong body of research highlighting the novel biodiversity of mesophotic ecosystems, especially in locations like the Indian Ocean, where few prior ichthyological surveys have been conducted.

COI gene, deep reefs, ichthyology, Indian Ocean, rebreather diving, taxonomy

The anthiadine genus Plectranthias Bleeker, 1873, comprises 66 valid species found in tropical and temperate waters in the Atlantic, Pacific, and Indian oceans (Fricke et al. 2024). In general, they are small (20 cm maximum length, but most in the 5–10 cm range), benthic, feed on small mobile invertebrates, and hide in crevices and holes in relatively deep habitats (depths of 90–420 m) with complex rocky formations (Kuiter 2004; Allen and Walsh 2015). Due to their small size and cryptic habits, they are poorly represented in museum collections, and many species have been described based on a small number of specimens or single individuals (Randall 1980; Heemstra and Randall 2008; Wu et al. 2011; Bineesh et al. 2014; Allen and Walsh 2015; Gill et al. 2016; Shepherd et al. 2018; Gill and Roberts 2020; Shepherd et al. 2020; Wada et al. 2020; Fricke 2021; Koeda et al. 2022). The genus Plectranthias is distinguished from other genera in the subfamily Anthiadinae by the following characters: a dorsal fin with 10–12 spines and 13–20 rays, incised between the spinous and soft portions of the dorsal fin; anal fin III, 6–8; pelvic fin I, 5; pectoral fin with 12–18 rays; an absence of auxiliary scales on the head or body; teeth on the palatine and a V- or U-shaped vomer tooth patch, but no teeth on the tongue; 26 vertebrae (rarely 27); and 12–31 total gill rakers (Gill et al. 2021).

Mesophotic coral ecosystems (MCEs), coral reef habitats found at depths of 30–150 m commonly known as the coral reef “twilight zone,” are home to a diversity of organisms that are largely distinct from their shallow-water counterparts (Rocha et al. 2018). While conducting ichthyological and ecological surveys of MCEs at various locations around the globe, our team has encountered many undescribed species, especially from the families Labridae, Pomacentridae and Serranidae (Arango et al. 2019; Shepherd et al. 2019; Tea et al. 2019; Pinheiro et al. 2019; Shepherd et al. 2021). Recent research expeditions in the Maldives have revealed a similar pattern of discovery. In this paper, we describe a new species of Plectranthias perchlet seen in MCEs at two atolls in the Maldives.

All specimens were collected with hand nets while diving on a mixed-gas closed-circuit rebreather (Hollis Prism 2). Specimens were collected and immediately transported to a field laboratory, where they were photographed, tissues sampled, fixed in 10% formalin, and preserved in 75% ethanol. The preserved specimens were later measured and x-radiographed at the California Academy of Sciences. Measurements were taken with digital calipers to the nearest 0.01 mm and rounded to one decimal place, following the conventions described in Anderson and Heemstra (2012), Williams et al. (2013), and Gill and Roberts (2020). Diagrams in Gill et al. (2021) were especially useful to identify obscure characters. Principal caudal rays are those associated with hypurals. The lowermost principal caudal-fin ray is the ray articulating between the distal tips of the parahypural and the haemal spine of preural centrum 2 as described in Gill et al. (2021). Procurrent caudal-fin rays are those dorsal and ventral to the principal rays. Principal and procurrent caudal-fin ray counts are presented as upper + lower. Vertebral counts are presented as precaudal + caudal. The anterior-most vertebra with a haemal spine was counted as the first caudal vertebra, the urostylar complex the last. Gill raker counts are presented as upper (epibranchial) + lower (ceratobranchial) rakers on the anterior face of the first arch; the angle raker is included in the second count. The anterior supraneural-dorsal ray-pterygiophore-neural spine interdigitation pattern follows Ahlstrom et al. (1976), and is presented as a formula with “0” representing a supraneural, “/” a neural spine, and numerals indicating the number of spines borne by each pterygiophore. The elongated portions of dorsal-fin ray filaments were not included in the measurements, as they were damaged by preservation. Morphometric data for the holotype and paratype is presented in Table 1. Measurements in the text are proportions of standard length (SL) unless otherwise noted. Values in parentheses represent data from the paratype when different from the holotype. The holotype was deposited at the California Academy of Sciences ichthyological collection (CAS), and the paratype was deposited at the Natural History Museum of Los Angeles County (LACM).

Mitochondrial cytochrome c oxidase subunit I (COI) DNA was sequenced and analyzed for the new species. DNA extraction and PCR amplification of the COI gene were performed following protocols detailed in Weigt et al. (2012). DNA sequences were compared to the 19 Plectranthias species available in GenBank (P. ahiahiata Shepherd, Phelps, Pinheiro, Perez-Matus & Rocha, 2018: MH025944; P. alleni Randall, 1980: FOAO1479; P. bennetti Allen & Walsh, 2015: KT601636; P. bilaticlavia Paulin & Roberts, 1987: MN915262; P. flammeus Williams, Delrieu-Trottin & Planes, 2013: KC565477–KC565480; P. fourmanoiri Randall, 1980: KC567662, KC567663; P. inermis Randall, 1980: OQ386015; P. japonicus Steindachner, 1883: JQ681323, JQ681324; P. kamii Randall, 1980: KU943548; P. kelloggi Jordan & Evermann, 1903: KP267643; P. longimanus Weber, 1913: JF494178; P. maculicauda Regan, 1914: FNZ095; P. nanus Randall, 1980: JQ432001–JQ432004, KC565481, KC567661; P. polygonius Shepherd, Phelps, Pinheiro, Rocha & Rocha, 2020: MN922331; P. wheeleri Randall, 1980: LC730852; P. randalli Fourmanoir & Rivaton, 1980: KP267613; P. retrofasciatus Fourmanoir & Randall, 1979: JN313133; P. winniensis Tyler, 1966: KC565482, KC565483; P. yamakawai Yoshino, 1972: OP614925) and two in the Barcode of Life Database (P. ferrugineus Gill, Pogonoski, Moore & Johnson, 2021: FOAO1382-18; P. mcgroutheri Gill, Pogonoski, Moore & Johnson, 2021: FOAO2412-20). Alignments of DNA sequences were done using a standard Geneious global alignment with free end gaps and 65% similarity in the program Geneious Prime 2020.0.3 (Biomatters, Auckland; Kearse et al. 2012).

The Maldives Archipelago shelters a rich biodiversity of reef fishes. However, only Plectranthias winniensis Tyler, 1966 was previously known for the region (Randall and Anderson 1993). Plectranthias raki is distinguished from P. winniensis by the number of pectoral-fin rays (12–13, versus 16–18), tubed lateral line scales (32, versus 8–27), and in coloration, especially in having a yellow stripe on the dorsal fin and lacking a white spot at the caudal peduncle, as in P. winniensis. Four species, Plectranthias garrupellus Robins & Starck, 1961, Plectranthias hinano Shepherd, Phelps, Pinheiro, Rocha & Rocha, 2020, Plectranthias kojiorum Koeda, Muto & Wada, 2021, and Plectranthias longimanus Weber, 1913, share the following diagnostic characters with Plectranthias raki: dorsal fin X, 15; anal fin III, 7; and pectoral-fin rays 12–13. In addition to the morphological differences that follow, P. raki is easily distinguished from these four species based on its living coloration. Plectranthias raki can be distinguished from Plectranthias garrupellus by having 30–32 tubed lateral-line scales, compared to 28–29 in P. garrupellus, and by lacking antrorse spines on the preopercle (versus 2 spines in P. garrupellus). Plectranthias raki has 5 gill rakers on the upper arch compared to 7–8 in P. hinano, a shorter dorsal-fin base length, 31% SL versus 48.5% in P. hinano, and lacks antrorse spines on the preopercle, whereas P. hinano has 3 antrorse spines. Plectranthias raki has 16 branched caudal-fin rays, while Plectranthias kojiorum has only 13. Plectranthias raki has 3 supraneural bones and lacks antrorse spines on the preopercle, while P. kojiorum has 2 predorsal bones and 2 antrorse spines. Plectranthias raki differs from P. longimanus by having a complete lateral line with 30–32 tubed scales, compared to the incomplete line of 12–15 tubed scales in P. longimanus.

Plectranthias raki is similar in many counts and measurements to Plectranthias klausewitzi Zajonz, 2006 from the Red Sea, but differs in having a longer snout length (13.1% SL versus 10.7% SL in P. klausewitzi, a smaller orbit diameter (9.8% SL in P. raki versus 13.9% in P. klausewitzi), and by having fewer pectoral-fin rays (12–13 in P. raki versus 14–15 in P. klausewitzi). The living coloration of Plectranthias klausewitzi is unknown at this time.

Plectranthias raki is distinct from the other Indian Ocean species, Plectranthias alcocki Bineesh, Gopalakrishnan & Jena, 2014, Plectranthias alleni Randall, 1980, and Plectranthias morgansi Smith, 1961 based on the following characteristics. Plectranthias raki has 12–13 pectoral-fin rays and 30–32 tubed lateral-line scales, and three scales above the lateral line to the origin of the dorsal fin, while P. alcocki has 14 pectoral-fin rays, 28 tubed lateral-line scales, and only one scale above the lateral line to the origin of the dorsal fin. Plectranthias raki differs from P. alleni in dorsal-fin counts (X, 15 versus X, 14), pectoral-fin rays (12–13, versus 15–17), the number of circumpeduncular scales (11–12 versus 14–15), and in coloration, by lacking the short narrow dark stripe in front of the eye and the faint dusky stripe from behind the eye across the upper side of the body that distinguishes P. alleni. Plectranthias raki differs from Plectranthias morgansi by having the 4^th dorsal spine the longest (versus the 3^rd in P. morgansi, which also has a pennant), shorter pelvic fins (not reaching the anus, as in P. morgansi), pointed anal fin (versus rounded in P. morgansi) and an emarginate caudal fin (versus rounded in P. morgansi) (Smith 1961).

Our specimens also resemble an undescribed species of Plectranthias collected in the Andaman Sea on the Tanintharyi coast of Myanmar (Gill and Psomadakis 2018). However, there are key differences that distinguish them. Plectranthias raki has 12–13 pectoral-fin rays versus 14 on the Myanmar specimen, and lacks the two enlarged antrorse spines on the lower part of the preopercle seen on the specimen from Myanmar. In addition, there are several morphometric differences, including a smaller orbit diameter (9.8% SL in Plectranthias raki versus 14.9% SL in the specimen from Myanmar), a shorter caudal peduncle (11.7% SL versus 20.4% SL in the Myanmar specimen), shorter pectoral fins (34.8% SL in Plectranthias raki versus 42.9% SL in the Myanmar specimen), and the longest segmented dorsal ray being the 3^rd in Plectranthias raki and the 7^th in the specimen from Myanmar. A second undescribed species, also collected in Myanmar, is documented in the same publication (Gill and Psomadakis 2018). The specimen was subsequently lost, so there are limited details for comparison, but it is also distinguishable from Plectranthias raki morphologically and in coloration. Plectranthias raki has 12–13 pectoral fin rays, while the second specimen from Myanmar has 14. Plectranthias raki also has a predominately white caudal fin with two indistinct, but pronounced, yellow-orange spots, while the caudal fin on the second specimen from Myanmar is solid yellow, and Plectranthias raki lacks the red marking on the middle spines of the dorsal fin as seen in the photograph of the specimen from Myanmar.

The barcode fragment of the COI gene of Plectranthias raki is not within 10% distance to any COI sequences of Plectranthias available on GenBank and the Barcode of Life Database. Therefore, we couldn’t identify a close genetic relative. However, the new species seems to form a group with other distantly related species (12–15% genetic divergence) that include P. bennetti, P. hinano, P. ferrugineus, and P. kojiorum. Since DNA sequences are available for just about 30% of the species in the genus, we prefer not to discuss their relationships here in detail because they will likely change with the addition of more species.

We are grateful to JD Fong for pictures and x-rays of the type specimens and T Clardy for providing the LACM collection number, and to the CalAcademy dive operations team for dive training and support in the field. We are also thankful to many colleagues who helped in the field and with discussions, including A Gill, K Lev, T Phelps, R Pyle, Y-K Tea, and M Zahuwaan. Hollis and Carpe Diem Maldives provided gear and logistical support respectively. The Maldivian Coast Guard authorized and supervised the technical diving and provided nautical support for all expeditions. Research and collecting permits were provided by the Maldives Ministry of Fisheries and Ocean Resources. This is a collaborative research initiative with the Maldives Marine Research Institute, and we especially thank MMRI Director S Naeem for her support.