A new species of Chromis (Teleostei, Pomacentridae) is described from four specimens collected between 95 and 110 m depth in mesophotic coral ecosystems in the Maldives, Indian Ocean. Chromis abadhah sp. nov. can be distinguished from all of its congeners by the following combination of characters: dorsal-fin rays XIII, 12–13; anal-fin rays II,11–12; pectoral-fin rays 17–18; tubed lateral-line scales 17; gill rakers 7+17–18 = 24–25; pearly white body with a large black marking covering the anterior two-thirds of the anal fin. The closest DNA barcode sequence (5.1% average uncorrected genetic distance on the mitochondrial COI gene), among those available, is Chromis woodsi, a similar mesophotic species known from the coastal western Indian Ocean (Somalia to South Africa). The new species is easily distinguished from C. woodsi by having 13 dorsal spines (versus 14 in C. woodsi), the absence of a black band on the base of the tail (present in C. woodsi), and by the genetic difference.

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

The family Pomacentridae (damselfishes and anemonefishes) is one of the largest and most conspicuous families of fish inhabiting tropical shallow coral reefs (Allen 1991). One of the key elements of their evolutionary success in coral reefs is resource partitioning, both at the spatial and trophic axes, with species falling into three main trophic levels: herbivores, zooplanktivores, and omnivores (Frédérich and Parmentier 2016). Despite its recent splitting into three genera (Azurina Jordan & McGregor, 1898, Pycnochromis Fowler, 1941, and Chromis Cuvier, 1814 sensu stricto; Tang et al. 2021), Chromis still is the most species-rich genus within Pomacentridae, comprising 78 valid species, all zooplanktivorous, at least 10 of which, including several recently described, are found only below 60 m (Pyle et al. 2008; Arango et al. 2019; Tea et al. 2019; McFarland et al. 2020). They are conspicuous residents of mesophotic coral ecosystems (coral reef habitats occurring at depths from 30–150 m) across all tropical seas, and are often seen in high numbers where they occur. Chromis is also the only genus of Pomacentridae that occurs below 80 m and down to at least 180 m depth (McFarland et al. 2020; Shepherd et al. 2020).

Because they are much harder to explore than their shallow-water counterparts, deep reefs are home to many undescribed species (Pyle et al. 2019). Their relative inaccessibility initially led scientists to consider deep reefs as potential refuges for threatened shallow-water species; however, the general consensus now is that species assemblages in mesophotic ecosystems are very different from shallow-water ones, and that deep reefs are also heavily impacted (Rocha et al. 2018; Pinheiro et al. 2023).

Exploration of the planet’s mesophotic ecosystems has been uneven. Hawaii and some locations in the Caribbean, the South Pacific and the Red Sea are relatively well-known, whereas the eastern Pacific, eastern Atlantic and Indian Ocean are largely unexplored (Bongaerts et al. 2019). Aiming to fill this gap through a partnership between the California Academy of Sciences, the Maldives Marine Research Institute, and Rolex, we conducted ichthyological surveys to depths of 150 meters at 21 locations across five atolls in the Maldives, Indian Ocean between January 2022 and October 2023. Several new species of fishes were discovered during these surveys, of which this is the first to be described.

Specimens were collected with hand nets and immediately transported to a field laboratory where they were photographed, tissue sampled, and fixed in 10% formalin. Measurements and x-radiographs were made at the California Academy of Sciences after fixation and preservation in 75% ethanol. Counts were made with the aid of a microscope; measurements were made with digital calipers to the nearest 0.01 mm and rounded to one decimal place, following the conventions described in Allen and Randall (2004) and Pyle et al. (2008). Spiniform procurrent caudal-fin rays are those situated anteriorly to segmented procurrent caudal-fin rays, especially visible in radiographs. Vertebral counts include the first vertebra fused to the skull, and the last vertebra fused to the hypural plate; vertebral counts are presented as precaudal + caudal, where the first caudal vertebrae is the anteriormost vertebrae having its haemal spine associated with the anal fin pterygiophore, and the last caudal vertebrae is fused to the hypural plate. 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. Counts and measurements for the holotype and paratypes are presented in Table 1. Measurements in the text are given as percent standard length (SL), unless otherwise noted. Data are presented first for the holotype, followed by a range of values for the paratypes, in parenthesis, where variation was noted. The holotype and paratypes were deposited at the California Academy of Sciences (CAS) and at the Smithsonian Institution’s National Museum of Natural History (USNM) ichthyological collections.

In December 2022, four individuals were collected with hand nets by members of our team diving on mixed-gas, closed-circuit rebreathers (Hollis Prism 2). Molecular analysis and PCR amplification of the standard barcode fragment of the mitochondrial cytochrome c oxidase subunit I gene (COI) were performed following protocols described by Arango et al. (2019) using BOLFishF1/BOLFishR1 primers. 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 (Kearse et al. 2012). Our genetic dataset contains sequences of only one mitochondrial DNA marker. Therefore, we did not attempt to perform a phylogenetic reconstruction, and genetic distances are uncorrected. We used all available Chromis COI sequences from GenBank and the Barcode of Life databases for genetic comparisons.

In addition to Chromis abadhah, three other species of Chromis have an overall pearly white body with black markings in life: Chromis axillaris (Bennett, 1831), Chromis pelloura Randall & Allen, 1982, and Chromis woodsi Bruner & Arnam, 1979. All are mesophotic, having only been recorded below 30 m depth. Chromis axillaris is the most widely distributed, being recorded in various locations between the western Indian Ocean and the western Pacific at depths of 60 to 100 m (Bruner and Arnam 1979; Allen 1991). Chromis pelloura is found in the Red Sea at depths of 30 to 80 m, and is very similar in color and morphology to C. axillaris (Randall and Allen 1982). In addition to the black bar covering the caudal peduncle (also in the previous two species), the distal half of the anal fin of C. woodsi (found at 50 to 175 m depth in the coastal western Indian Ocean) also is solid black (Bruner and Arnam 1979). Chromis abadhah differs from all of these by lacking a black bar on the caudal peduncle, having 13 dorsal spines (all others have 14), and by a distribution apparently restricted to the Maldives.

Recently, Tang et al. (2021) revised the taxonomy of Chromis and reassigned several species to the genera Azurina and Pycnochromis. All mesophotic species remained in Chromis stricto sensu, and several authors have pointed out that most of them have 14 dorsal-fin spines (Pyle et al. 2008; Allen and Erdmann 2009; Tea et al. 2019). Chromis abadhah is unusual among mesophotic species by having 13; therefore, the number of dorsal-fin spines does not appear to diagnose a deep-reef clade, and a comprehensive molecular analysis that includes more species is needed to better understand the relationships among shallow and deep lineages.

We are grateful to JD Fong for pictures and x-rays of the type specimens and D Pitassy for providing the USNM 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 B Greene, 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 in all expeditions. Research and collecting permits were provided by the Maldives Ministries of Fisheries Marine Resources and Agriculture and of Environment, Climate Change and Technology. This is a collaborative research initiative with the Maldives Marine Research Institute, and we especially thank MMRI Director S Naeem for her support.