INTRODUCTION
Located in the Central Pacific over 800 km from the Hawaiian Islands, 1,450 km from the Line Islands, and 2,500 km from the Marshall Islands, Johnston Atoll is the most remote atoll on Earth. Despite its remoteness, numerous expeditions have targeted the atoll in order to explore and characterize its marine biodiversity (reviewed by Amerson & Shelton, Reference Amerson and Shelton1976; Lobel & Lobel, Reference Lobel, Lobel, Riegl and Dodge2008). In 1880, a vessel of the North Pacific Guano Company collected 27 fish species from Johnston, including five undescribed species, which were subsequently described by scientists at the United States National Museum (Smith & Swain, Reference Smith and Swain1882). In 1923, a partnership between the US Navy, the Bishop Museum and the Bureau of the Biological Survey (now US Fish and Wildlife Service) led to a series of expeditions to the north-western Hawaiian Islands, Wake Atoll and Johnston Atoll. As part of these expeditions, several scientists visited Johnston Atoll in 1923 aboard the US Navy minesweepers ‘Tanager’ and ‘Whippoorwill’ (Amerson & Shelton, Reference Amerson and Shelton1976; Olson, Reference Olson1996). These two minesweepers returned to the atoll in 1924 along with other US navy ships. Although no scientific personnel joined that expedition, biological specimens were collected for the Bishop Museum (Amerson & Shelton, Reference Amerson and Shelton1976). Descriptions of the biota of Johnston Atoll, obtained as part of these efforts by the Bishop Museum to study the biodiversity throughout the Central Pacific, were later published in separate papers focusing on invertebrates (Edmondson et al., Reference Edmondson, Fisher, Clark, Treadwell and Cushman1925; Clark, Reference Clark1949), fish (Fowler & Ball, Reference Fowler and Ball1925), insects (Bryan, Reference Bryan1926; Chilson, Reference Chilson1953) and birds (Olson, Reference Olson1996).
In the 1960s and 1970s, the Smithsonian Institution led the Pacific Ocean Biological Survey Program, a research programme that aimed to increase the knowledge about various islands throughout the Central Pacific. As part of the programme, several biologists visited Johnston Atoll between 1963 and 1973, and although the major focus was on seabirds, other biota were also studied and inventoried (Amerson & Shelton, Reference Amerson and Shelton1976). Throughout the 1960s, the University of Hawaiʻi conducted a series of studies to measure the effects of dredging on the reef ecosystems of Johnston Atoll, and in particular on the prevalence of ciguatera around the atoll (Brock et al., Reference Brock, Jones and Helfrich1965, Reference Brock, Van Heukelem and Helfrich1966; Buggeln & Tsuda, Reference Buggeln and Tsuda1969).
In 1983, the Hawaiʻi Undersea Research Laboratory brought its manned submersible ‘Makaliʻi’ to Johnston in order to study the deep waters surrounding the atoll. A total of 35 submersible dives were performed to maximum depths of 500 m, four of which were dedicated to geological studies (Keating, Reference Keating1985), and the remainder centred on biological investigations (Randall & Ralston, Reference Randall and Ralston1984; Agegian & Abbott, Reference Agegian and Abbott1985; Randall et al., Reference Randall, Lobel and Chave1985; Ralston et al., Reference Ralston, Gooding and Ludwig1986; Chave & Mundy, Reference Chave and Mundy1994; Chave & Malahoff, Reference Chave and Malahoff1998). In 2000, the Bishop Museum coordinated a major effort to inventory the shallow-water (<30 m) marine biota of Johnston as part of a survey of non-indigenous marine species of the atoll (Coles et al., Reference Coles, DeFelice and Minton2001). Following these efforts, the US National Oceanic and Atmospheric Administration (NOAA) has regularly visited Johnston since 2004, in order to further characterize and monitor the shallow-water (<30 m) algae, fish, corals and other invertebrates of the atoll (Brainard et al., Reference Brainard, Maragos, Schroeder, Kenyon, Vroom, Godwin, Hoeke, Aeby, Moffitt, Lammers, Gove, Timmers, Holzwarth, Kolinski and Waddell2005; NOAA, 2006; Lobel & Lobel, Reference Lobel, Lobel, Riegl and Dodge2008; Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010).
As a result of the multitude of scientific expeditions that have surveyed Johnston, there is ample information available on the marine biodiversity found in the shallow waters (<30 m) of the atoll (reviewed by Coles et al., Reference Coles, DeFelice and Minton2001; Lobel & Lobel, Reference Lobel, Lobel, Riegl and Dodge2008). Additionally, numerous records exist on the flora and fauna found in the deeper waters (100–500 m) surrounding Johnston Atoll (Randall & Ralston, Reference Randall and Ralston1984; Agegian & Abbott, Reference Agegian and Abbott1985; Randall et al., Reference Randall, Lobel and Chave1985; Ralston et al., Reference Ralston, Gooding and Ludwig1986; Chave & Mundy, Reference Chave and Mundy1994; Chave & Malahoff, Reference Chave and Malahoff1998). However, as in many regions around the world, little is known about the marine biodiversity found between these two depth ranges. This intermediate depth range hosts mesophotic coral ecosystems (MCEs), which are light-dependent coral reefs found below the depth limits of conventional SCUBA diving (>30 m) that extend to the deepest portion of the euphotic zone, which may be over 150 m in some oceanic regions with high water clarity like Johnston (Maragos & Jokiel, Reference Maragos and Jokiel1986; Kahng & Maragos, Reference Kahng and Maragos2006; Kahng et al., Reference Kahng, Garcia-Sais, Spalding, Brokovich, Wagner, Weil, Hinderstein and Toonen2010). The fixed upper depth limit of MCEs (30 m) corresponds to the depth limits of conventional SCUBA diving and does not represent a static ecological boundary (Kahng et al., Reference Kahng, Copus and Wagner2014). As a result, shallow-water (<30 m) and mesophotic reefs (>30 m) can be quite similar in the distributions of their flora and fauna, particularly close to the depth boundary between these two ecosystems.
The only information on the mesophotic biodiversity of Johnston Atoll is derived from a limited number of surveys for reef fish at depths between 25 and 75 m (Kosaki, Reference Kosaki1989; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991). The purpose of this study was to survey this historically under-surveyed depth range at Johnston, in order to characterize and quantify the mesophotic flora and fauna of the atoll. Additionally, this study sought to compare the mesophotic biodiversity between Johnston Atoll and the Hawaiian Archipelago, because numerous previous studies have noted strong faunal and floral similarities between these two geographical areas (Gosline, Reference Gosline1955; Buggeln & Tsuda, Reference Buggeln and Tsuda1969; Bailey-Brock, Reference Bailey-Brock1976; Grigg, Reference Grigg1981; Grigg et al., Reference Grigg, Wells and Wallace1981; Randall et al., Reference Randall, Lobel and Chave1985; Maragos & Jokiel, Reference Maragos and Jokiel1986; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991; Coles et al., Reference Coles, DeFelice and Minton2001; Maragos et al., Reference Maragos, Potts, Aeby, Gulko, Kenyon, Siciliano and VanRavenswaay2004; Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010). Due to these similarities, Johnston Atoll has been considered an important stepping stone for marine organisms reaching the Hawaiian Archipelago (Grigg, Reference Grigg1981; Grigg et al., Reference Grigg, Wells and Wallace1981), an interpretation that is supported by both genetic (Rivera et al., Reference Rivera, Kelley and Roderick2004, Reference Rivera, Andrews, Kobayashi, Wren, Kelley, Roderick and Toonen2011; Timmers et al., Reference Timmers, Andrews, Bird, deMaintenton, Brainard and Toonen2011) and oceanographic studies (Kobayashi, Reference Kobayashi2006) that demonstrate strong connectivity between these two regions.
MATERIALS AND METHODS
All dive surveys were performed using closed-circuit rebreathers on a research expedition to Johnston Atoll aboard the NOAA ship ‘Hiʻialakai’ in the summer of 2013 (HA-13-01). Dive sites were chosen using historical charts, as well as new multibeam data collected by the NOAA ship ‘Hiʻialakai’. Chosen areas contained steep vertical drop-offs and hard substrate at depths between 32 and 78 m. A total of eleven dive sites were surveyed around Johnston Atoll (Figure 1). During each survey, one diver identified and counted all large, conspicuous, diurnally-active fish to the lowest possible taxonomic level along a 25 × 2 m belt transect (Kane et al., Reference Kane, Kosaki and Wagner2014). A second diver took photographs of the benthos at 25 randomly selected points along the transect using a 0.5 m2 photoquadrat. Upon transect completion, both divers collected macroalgae, sponges and corals that could not be identified in situ as time permitted. No crustose coralline algae or turf algae were collected due to time constraints. Collected samples were photographed in situ, placed into separate bags and preserved for later identification by taxonomy experts. Additionally, both divers recorded the presence of fish and macrobenthic species that were not captured during the transect itself, and documented observations with video vouchers. Benthic cover was determined from photoquadrat images with the aid of Coral Point Count with Excel extensions software (CPCe) using 100 random points per image (Kohler & Gill, Reference Kohler and Gill2006). For this purpose, all macrobenthic organisms were identified to the lowest possible taxonomic level from photoquadrat images, using identifications of collected specimens by taxonomy experts where available. All species of the calcifying green alga Halimeda that were identified from collected specimens were grouped during the photoquadrat analysis, due to difficulties of differentiating individual species in photographs. Additionally, all identified organisms were classified post hoc based on their geographical distribution as being restricted to: (1) circumtropical or circumtemperate waters; (2) the Indo-Pacific; (3) the Indo-Pacific but not found in Hawaiʻi; or (4) Johnston Atoll and Hawaiʻi (Gosline, Reference Gosline1955; Randall et al., Reference Randall, Lobel and Chave1985; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991).
Fig. 1. Map showing the location of the eleven sites that were surveyed at mesophotic depths (32–78 m) as part of surveys of the flora and fauna off Johnston Atoll. Areas in black show the four emergent land features of Johnston Atoll.
RESULTS
Macrobenthos
A total of 31 macrobenthic species in seven phyla, 15 orders and 21 families were identified from mesophotic depths, including one antipatharian coral (Stichopathes sp.), one zoanthid (Palythoa caesia), three sponges (Dragmacidon sp., Prosuberites sp. and unidentified niphatid), two macroalgae (Neomeris vanbosseae and Halimeda distorta) and one squat lobster (Babamunida debrae) which were not previously recorded from Johnston Atoll (Table 1). Macrobenthic species identified to species level included 15 species of cnidarians, nine species of macroalgae, three species of echinoderms and one species of squat lobster (Table 1). With the exception of site 10, which was mostly covered by crustose coralline algae (58.0% ± 22.7%), sponges (25.5% ± 15.9%) and corals (16.5% ± 20.6%), all other sites were dominated by sand, turf algae and crustose coralline algae, which collectively accounted for 68.8–93.4% of the substrate (Figure 2). Macroalgae were generally the next most abundant macrobenthic group, and covered between 4.0 and 25.3% of the substrate, with the exception of site 10 where no macroalgae were recorded (Figure 2). Coral cover was generally low (0.56–4.56%), with the exception of sites 4 and 10, which had coral covers of 13.36% (±27.6%) and 16.50% (±20.6%), respectively (Figure 2). The most abundant macrobenthic species were the sponge Prosubirites sp. at site 10 (17.19% ± 14.1%), the green macroalga Caulerpa bikinensis at site 9 (13.72% ± 18.0), and the scleractinian corals Acropora cytherea (10.72% ± 28.5%) and Montipora capitata (9.07% ± 16.7%) at site 4 and 10, respectively (Table 1). Of the 31 macrobenthic species that were identified, 17 (54.8%) are also found throughout other Indo-Pacific locations including Hawaiʻi; five (16.1%) have widespread circumtropical or circumtemperate distributions, three (9.7%) are found throughout the Indo-Pacific but not in Hawaiʻi, three (9.7%) have unknown distributions, two (6.5%) have a distribution restricted to Hawaiʻi and Johnston Atoll, and one (3.2%; Caulerpa bikinensis) has a distribution restricted to Micronesia, French Polynesia and Johnston Atoll (Table 1).
Fig. 2. Relative cover by macrobenthic group at the eleven sites that were surveyed at mesophotic depths (32–78 m) off Johnston Atoll.
Table 1. Species of macrobenthic organisms recorded during mesophotic surveys off Johnston Atoll. *, new record for Johnston Atoll; off transect, species recorded outside of transect and therefore benthic cover data were not collected; †, percentage cover at transect with the highest value; ‡, mean percentage cover of all transects ± standard deviation.
Fish
A total of 99 fish species in 29 families were identified during mesophotic surveys, including 22 species which had not previously been recorded from Johnston Atoll (Table 2). Additionally, we recorded Centropyge fisheri, a species with an uncertain previous sighting at Johnston Atoll (Randall et al., Reference Randall, Lobel and Chave1985). In terms of number of individuals, Chromis verater, Pseudanthias randalli, Acanthurus olivaceus, A. thompsoni, Centropyge nahackyi, Parupeneus multifasciatus, Pseudocheilinus evanidus and Sufflamen bursa were most common (Table 2). With the exception of A. thompsoni, which was only recorded at one site, this group of fish was also most widespread, being recorded from seven (Chromis verater) to three (Pseudanthias randalli) of the surveyed sites. Among species that were less abundant, Canthigaster jactator, Parapercis schauinslandii, Centropyge loricula, Bodianus albotaeniatus, Chaetodon tinkeri, Ctenochaetus hawaiiensis, Zanclus cornutus and Acanthurus dussumieri were widespread on the surveyed mesophotic reefs, being recorded at three or more of the surveyed sites. Of the 99 fish species that were identified, 74 (74.7%) are widely distributed throughout the Indo-Pacific including Hawaiʻi, 18 (18.2%) are restricted to Johnston Atoll and Hawaiʻi, six (6.1%) have widespread circumtropical distributions, and one (1.0%; Pseudanthias randalli) is found throughout the Pacific but not in Hawaiʻi (Table 2).
Table 2. Fish species recorded during mesophotic surveys off Johnston Atoll. *, new species record for Johnston Atoll; off transect, species recorded outside of transect and therefore abundance data were not collected; †, abundance at transect with the highest value; ‡, mean abundance of all transects ± standard deviation; NR, depth not recorded.
DISCUSSION
In contrast to the shallow (<30 m) and deep-water (100–500 m) marine biodiversity of Johnston, which has been extensively surveyed (reviewed by Coles et al., Reference Coles, DeFelice and Minton2001), only limited surveys have been performed at mesophotic depths surrounding the atoll (Kosaki, Reference Kosaki1989; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991). This study thus represents the first dedicated effort to inventory and quantify the mesophotic flora and fauna at Johnston Atoll. We recorded a total of 130 species from mesophotic depths, most of which (76.4%) have also been found during previous shallow-water (<30 m) surveys off Johnston Atoll (Tables 1 & 2). This indicates that the mesophotic flora and fauna of Johnston is not composed of a specialized group of species, but rather consists of a subset of species that is also found in shallow waters (<30 m). The only species which have not been found in previous surveys off Johnston Atoll include one black coral, one zoanthid, one squat lobster, two macroalgae, three sponges and 22 fish (Tables 1 & 2). With the exception of the three sponges, which have unknown geographical distributions (Table 1), all other species are commonly found in shallow-water reefs throughout the Indo-Pacific (Hoover, Reference Hoover2006; Huisman et al., Reference Huisman, Abbott and Smith2007; Randall, Reference Randall2007). Thus, it is likely that these species have been missed in previous shallow-water surveys at Johnston, and are not restricted to mesophotic depths at the atoll. This interpretation is consistent with most mesophotic surveys in the Hawaiian Islands, which indicate that the mesophotic flora and fauna are composed mainly of the same species, albeit in different proportions, that are also found in shallow waters (Brock & Chamberlain, Reference Brock and Chamberlain1968; Parrish & Boland, Reference Parrish and Boland2004; Rooney et al., Reference Rooney, Donham, Montgomery, Spalding, Parrish, Boland, Fenner, Gove and Vetter2010; Kane et al., Reference Kane, Kosaki and Wagner2014). However, in some Hawaiian locations, the mesophotic benthic flora and fauna appears distinct from shallow-water (<30 m) communities (Kahng & Kelley, Reference Kahng and Kelley2007; Spalding, Reference Spalding2012).
Our surveys indicate that mesophotic reefs off Johnston are covered mostly by turf algae, crustose coralline algae, macroalgae and sand (Figure 2). These groups were also identified as being most abundant in previous surveys on many shallow-water (<18.3 m) reefs off Johnston Atoll (Brainard et al., Reference Brainard, Maragos, Schroeder, Kenyon, Vroom, Godwin, Hoeke, Aeby, Moffitt, Lammers, Gove, Timmers, Holzwarth, Kolinski and Waddell2005; NOAA, 2006). However, previous shallow-water surveys off Johnston have also noted high coral cover on several reefs of the atoll, particularly inside the lagoon, where coral cover can approach 100% (Maragos & Jokiel, Reference Maragos and Jokiel1986; Jokiel & Tyler, Reference Jokiel and Tyler1992; Brainard et al., Reference Brainard, Maragos, Schroeder, Kenyon, Vroom, Godwin, Hoeke, Aeby, Moffitt, Lammers, Gove, Timmers, Holzwarth, Kolinski and Waddell2005; NOAA, 2006). In contrast, coral cover at most of our mesophotic sites was generally low (0.56–4.56%), with the exception of two sites which had 13.36% and 16.50% coral cover (Figure 1). The most commonly recorded corals during our mesophotic surveys were Acropora cytherea, Montipora capitata, Distichopora violacea and Millepora tenera (Table 1). With the exception of Distichopora violacea, these coral species are also dominant on shallow-water (<18.3 m) reefs of the atoll (Maragos & Jokiel, Reference Maragos and Jokiel1986; NOAA, 2006). Additionally, Montipora patula, Pavona spp. and Pocillopora spp. are also dominant on shallow-water (<18.3 m) reefs of Johnston (Maragos & Jokiel, Reference Maragos and Jokiel1986; NOAA, 2006); all species which we did not record during our mesophotic surveys (Table 1). Among macrobenthic species identified during our mesophotic surveys, only the green alga Caulerpa bikinensis, the scleractinian coral Leptoseries hawaiiensis and the black corals Antipathes griggi and Myriopathes cf. ulex were recorded during previous deep-water (>100 m) submersible surveys off Johnston Atoll (Agegian & Abbott, Reference Agegian and Abbott1985; Maragos & Jokiel, Reference Maragos and Jokiel1986; Coles et al., Reference Coles, DeFelice and Minton2001).
The most common reef fish recorded during our mesophotic surveys were Chromis verater, Pseudanthias randalli, Acanthurus olivaceus, A. thompsoni, Centropyge nahackyi, Parupeneus multifasciatus, Pseudocheilinus evanidus and Sufflamen bursa (Table 2). While all of these species have been recorded during previous shallow-water (<30 m) surveys at Johnston, only Parupeneus multifasciatus and Sufflamen bursa are also reported as being abundant on shallow-water (<18.3 m) reefs off the atoll (NOAA, 2006). Among species that were abundant at our mesophotic sites (Table 2), only Parupeneus multifasciatus was also reported as being abundant during previous deep-water (100–365 m) surveys at Johnston Atoll (Ralston et al., Reference Ralston, Gooding and Ludwig1986). In contrast, many of the species we recorded at mesophotic depths have also been recorded during previous deep-water (>100 m) submersible surveys off Johnston, including Carcharhinus amblyrhynchos, Myripristis chryseres, Parapercis schauinslandi, Carangoides orthogrammus, Caranx lugubris, C. melampygus, Chromis verater, Parupeneus cyclostomus, P. multifasciatus, Chaetodon tinkeri, Forcipiger flavissimus, Bodianus albotaeniatus, Acanthurus dussumeri, Naso hexacanthus, Zanclus cornutus and Xanthichthys auromarginatus (Randall et al., Reference Randall, Lobel and Chave1985; Ralston et al., Reference Ralston, Gooding and Ludwig1986; Chave & Mundy, Reference Chave and Mundy1994).
Abundances of macrobenthic organisms recorded during our surveys (Table 1), differed from those recorded during mesophotic surveys in the Hawaiian Islands (Rooney et al., Reference Rooney, Donham, Montgomery, Spalding, Parrish, Boland, Fenner, Gove and Vetter2010). The mesophotic benthos at Johnston consists mostly of crustose coralline algae (8.6–58%), sand (0–54%), turf algae (0–49%) and macroalgae (0–21%), with coral cover being low at most mesophotic locations (0.56–4.56%) (Figure 2). In contrast, at similar depths (40–70 m) Rooney et al. (Reference Rooney, Donham, Montgomery, Spalding, Parrish, Boland, Fenner, Gove and Vetter2010) noted that the macrobenthos in the north-western Hawaiian Islands was mostly covered by sand (45–55%), macroalgae (25–40%), crustose coralline algae (~5%), and coral (~2%). While the ultimate drivers for these differences remain unknown, they might be related to differences in the temperature regimes between Johnston Atoll and Hawaiʻi. Johnston is located much closer to the equator (16°N) than the Hawaiian Archipelago (19–28°N) and as a result experiences elevated water temperatures. Previous studies note that sea surface temperatures show little seasonality at Johnston and range between 25 and 27°C (Ralston et al., Reference Ralston, Gooding and Ludwig1986; Boehlert et al., Reference Boehlert, Watson and Sun1992). In comparison, sea surface temperatures can dip as low as 16°C in the north-western Hawaiian Islands (Kane et al., Reference Kane, Kosaki and Wagner2014). Differences in temperature regime have been related to differences in macrobenthic cover within the Hawaiian Archipelago (Grigg et al., Reference Grigg, Polovina, Friedlander, Rohmann, Riegl and Dodge2008; Rooney et al., Reference Rooney, Donham, Montgomery, Spalding, Parrish, Boland, Fenner, Gove and Vetter2010; Spalding, Reference Spalding2012), and might also explain why macroalgal cover is reduced at Johnston Atoll (0–21%; Figure 2) compared to the north-western Hawaiian Islands (25–40%; Rooney et al., Reference Rooney, Donham, Montgomery, Spalding, Parrish, Boland, Fenner, Gove and Vetter2010).
Consistent with previous biogeographical studies (Gosline, Reference Gosline1955; Maragos & Jokiel, Reference Maragos and Jokiel1986; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991; Maragos et al., Reference Maragos, Potts, Aeby, Gulko, Kenyon, Siciliano and VanRavenswaay2004), our mesophotic surveys recorded a near absence of species that are endemic to Johnston Atoll. The scarcity of endemic species has previously been noted among many different taxa at Johnston Atoll, including fish (Gosline, Reference Gosline1955; Randall et al., Reference Randall, Lobel and Chave1985; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991), corals (Maragos & Jokiel, Reference Maragos and Jokiel1986), algae (Buggeln & Tsuda, Reference Buggeln and Tsuda1969; Agegian & Abbott, Reference Agegian and Abbott1985; Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010) and crustaceans (Edmondson et al., Reference Edmondson, Fisher, Clark, Treadwell and Cushman1925; Amerson & Shelton, Reference Amerson and Shelton1976). The only potential endemic species from Johnston Atoll are three unidentified reef fish (Randall et al., Reference Randall, Lobel and Chave1985; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991), the angelfish Centropyge nahackyi (Kosaki, Reference Kosaki1989), the ostracods Parasterope pacifica and Bruuniella beta (Kornicker & Harrison-Nelson, Reference Kornicker and Harrison-Nelson2005), the red alga Neotenophycus ichthyosteus (Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010), the cyanobacterium Borzia elongata (Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010), the gammaproteobacterium Pseudoxanthomonas kalamensis (Harada et al., Reference Harada, Campbell and Li2006) and the three unidentified sponges recorded during this study (Table 1). However, several of these species will likely be found in other locations in the future, either as solitary waifs from Johnston Atoll (Kosaki, Reference Kosaki1989), or as species that were originally described from Johnston Atoll (Fowler & Ball, Reference Fowler and Ball1925; Bryan, Reference Bryan1926), but were subsequently found to have reproducing populations elsewhere in the Indo-Pacific.
Despite its extreme geographical isolation, the very low level of endemism indicates that Johnston is relatively well connected with other parts of the Pacific, particularly Hawaiʻi (Grigg, Reference Grigg1981; Grigg et al., Reference Grigg, Wells and Wallace1981; Rivera et al., Reference Rivera, Kelley and Roderick2004, Reference Rivera, Andrews, Kobayashi, Wren, Kelley, Roderick and Toonen2011; Kobayashi, Reference Kobayashi2006; Timmers et al., Reference Timmers, Andrews, Bird, deMaintenton, Brainard and Toonen2011). Kobayashi (Reference Kobayashi2006) used computer simulations and high-resolution current data to identify at least two corridors connecting Johnston Atoll with the Hawaiian Archipelago, an interpretation that is consistent with genetic studies (Rivera et al., Reference Rivera, Kelley and Roderick2004, Reference Rivera, Andrews, Kobayashi, Wren, Kelley, Roderick and Toonen2011; Timmers et al., 2011). Additionally, many other studies highlight the strong floral and faunal similarities between Johnston Atoll and the Hawaiian Islands (Gosline, Reference Gosline1955; Buggeln & Tsuda, Reference Buggeln and Tsuda1969; Bailey-Brock, Reference Bailey-Brock1976; Grigg, Reference Grigg1981; Grigg et al., Reference Grigg, Wells and Wallace1981; Randall et al., Reference Randall, Lobel and Chave1985; Maragos & Jokiel, Reference Maragos and Jokiel1986; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991; Coles et al., Reference Coles, DeFelice and Minton2001; Maragos et al., Reference Maragos, Potts, Aeby, Gulko, Kenyon, Siciliano and VanRavenswaay2004; Tsuda et al., Reference Tsuda, Abbott, Vroom and Fisher2010). These studies report that the flora and fauna of Johnston Atoll are mostly composed of species that are also found in Hawaiʻi, with 91% of all marine species found at Johnston also occurring in Hawaiʻi (reviewed by Coles et al., Reference Coles, DeFelice and Minton2001). Our results are consistent with these previous studies, as 96% of species identified during our mesophotic surveys are also known from Hawaiʻi (Tables 1 & 2). Additionally, this pattern was consistent amongst various taxonomic groups identified during our mesophotic surveys, as 100% of echinoderms, 99% of reef fish, 87% of corals and 78% of macroalgae are also known to occur in Hawaiʻi (Tables 1 & 2). In comparison, previous studies noted that 94% of reef fish species (Randall et al., Reference Randall, Lobel and Chave1985; Kosaki et al., Reference Kosaki, Pyle, Randall and Irons1991), 74% of coral species (Maragos et al., Reference Maragos, Potts, Aeby, Gulko, Kenyon, Siciliano and VanRavenswaay2004) and 94% of algal species (excluding cyanobacteria) of Johnston Atoll are also found in Hawaiʻi (Roy Tsuda, unpublished data). It is currently unknown whether the three unidentified sponge species recorded during this study also occur in Hawaiʻi and elsewhere in the Indo-Pacific. Further investigations will be necessary to determine the taxonomic status and geographical affinities of the sponge species recorded during our surveys.
Coral reef ecosystems below the depth limits of conventional SCUBA diving remain scarcely surveyed worldwide, and particularly in remote locations like Johnston Atoll. This study represents the first dedicated effort to characterize the mesophotic flora and fauna at Johnston Atoll. Our results support the strong connectivity between Johnston Atoll and the Hawaiian Archipelago highlighted by previous studies. Finally, this study adds 30 new records to Johnston Atoll, thereby emphasizing the value of deep-diving technologies in surveying the largest portion of the depth range of coral reef ecosystems (30–150 m), which remains largely unexplored.
ACKNOWLEDGEMENTS
We thank the captain and crew of the NOAA ship ‘Hi‘ialakai’, and J. Leonard, B. Hauk, K. Gleason, G. McFall, D. Pence, J. Copus, K. Lopes, J. Cousins, J. Halonen, B. Prescott, C. Chang and J. Bostick for field support. Special thanks to D. Turner, F. Lichowski and C. Kelley for help with mapping and dive site selection. This work was funded by NOAA's Office of National Marine Sanctuaries, through the Papahānaumokuākea Marine National Monument. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the authors and do not necessarily reflect the views of NOAA or the Department of Commerce.
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