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Report:
Mesophotic Coral Ecosystems of Pohnpei, Federated
States of Micronesia
Sonia J. Rowley1, T. Edward Roberts2, Richard R. Coleman3, Heather L. Spalding4,
Eugene Joseph5, Mae Ki Dorricott6
1Department of Geology and Geophysics, University of Hawai‘i at Mānoa, Honolulu, HI, USA
2Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University, Townsville,
QLD, Australia, and Australian Institute of Marine Science, Townsville, QLD, Australia
3Hawai‘i Institute of Marine Biology, University of Hawai‘i at Mānoa, Kāne‘ohe, HI, USA
4Department of Botany, University of Hawai‘i at Mānoa, Honolulu, HI, USA
5Conservation Society of Pohnpei, P.O. Box 2461, Kolonia, FM 96941
6The University of the West of England, Bristol, UK
Corresponding author: Rowley SJ; srowley@hawaii.edu
Table of Contents
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Abstract ...................................................................................................................................................... 2
Introduction ................................................................................................................................................ 2
Research History ........................................................................................................................................ 4
Environmental Setting ............................................................................................................................... 4
Habitat Description ................................................................................................................................... 9
Biodiversity ............................................................................................................................................... 11
Macroalgae ............................................................................................................................................ 11
Anthozoans ............................................................................................................................................ 14
Scleractinia ........................................................................................................................................ 16
Octocorallia ....................................................................................................................................... 17
Sponges .................................................................................................................................................. 21
Fishes .................................................................................................................................................... 22
Elasmobranchs ...................................................................................................................................... 25
Other Biotic Components ..................................................................................................................... 27
Threats and Conservation Issues ........................................................................................................... 30
Conclusion ................................................................................................................................................ 33
Acknowledgements ................................................................................................................................. 33
References ................................................................................................................................................. 34
____________________________________________________________________________
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Abstract
The mesophotic coral ecosystems (MCEs) of the Senyavin Islands (Pohnpei Island, and
neighboring atolls Ant and Pakin) in the Federated States of Micronesia have received little
research attention until recent years. These vibrant, environmentally dynamic ecosystems
harbour a reservoir of biodiversity, with species and interactions new to science. Depths of ≥90
m have up to 20ºC annual variance. A strong El Niño event in 2016 resulted in a bloom-forming
cyanobacteria smothering the upper MCEs of Pohnpei (25–60 m). Conditions persisted into
2017 with extensive coral bleaching and reef degradation with associated smothering by bloom-
forming cyanobacteria and algae in the shallows. We discovered that reef health could be
determined by the depth transition between scleractinians and gorgonians, further revealing
areas for conservation management. Reef health is also heavily influenced by the continuously
growing marine resource exploitation and terrestrial runoff. Furthermore, the $30 million
international fishing industry and its associated consequences are contributing towards the
decline in health of Pohnpei reefs. The lush reefs of previous times, therefore, are being
replaced by barren reefs, smothered by invasive algae, filamentous cyanobacteria, or crustose
coralline algae at the atolls. Resource management strategies developed by Pacific Island
cultures over hundreds of generations face significant challenges in the modern world. It is
recommended to: 1) develop community-based conservation strategies towards sustainable eco-
tourism, which generates incentive and alternative measures to prevent illegal fishing practices
(as successfully demonstrated in Indonesia, see www.misoolfoundation.org); these include local
rangers, protection enforcement, and local community education; 2) annual monitoring and
research on specific reefs throughout the Senyavin Islands from 0-150 m depth, and 3) a
reduction to eventual ban of international fisheries throughout the FSM. In summary, it is
essential that local communities and the government act now if the reefs of the Senyavin
Islands are to recover and survive.
Introduction
The Federated States of Micronesia (FSM) is one of five independent sovereign nations
within Micronesia proper, and consists of four states: Yap, Chuuk, Pohnpei, and Kosrae
(collectively, the Caroline Islands; Fig. 1a). The FSM exclusive economic zone extends
3,000,000 km2 with a total land area of 702 km2 (Buden and Taboroši 2016). Thus, the insular
FSM landmass provides opportunities for evolutionary innovation and novelty through atoll and
island isolation, and also stepping stones of connectivity for many taxa. Pohnpei Island
(hereafter Pohnpei) is the largest of the FSM islands (Buden and Taboroši 2016), as well as the
principle island of Pohnpei State. A further eight outer atolls Pingelap, Mwoakilloa (Mwokil*),
Ant (Ahnd*), Pakin, Sapwuahfik (Ngetik*), Oroluk, Nukuoro (Madalama*), Kapingamarangi
(Kirinidi*), and Minto Reef, a largely submerged atoll with only a 1.8 m high sand bar breaking
the surface, constitute the state [*most commonly used names from Motteler (2006)]. Pohnpei
and its neighboring atolls Ant and Pakin make up the Senyavin Islands (Fig. 1b) and will herein
be the primary focus of this report (see Rowley et al. 2019).
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Fig. 1. Location map of (a) the Federated States of Micronesia, also known as the Caroline Islands. (b)
The Senyavin Islands consisting of the island of Pohnpei and the atolls Ant and Pakin. Inset shows the
Senyavin Islands on the world map. Map and inset (b) by RR. Coleman.
Mesophotic coral ecosystems (MCEs)—historically referred to as the ‘Twilight zone’—
extend from ca. 30–150 m depth (Hinderstein et al. 2010) and are among the most diverse yet
most unexplored realms on the planet. MCEs are primarily characterized by strong attenuation
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gradients in light, temperature, and wave action, and often, high nutrient deep-water upwelling.
Characterization of such ecosystems addresses questions of biodiversity, resilience, refugia, and
priority conservation. In particular, the volcanic islands and atolls of the equatorial Pacific
exhibit an increase in biodiversity and persistence of forms that would typically be selected
against on the more diverse communities of continental regions and adjacent archipelagoes
(Simberloff 1974).
Research History
Research on the MCEs of Pohnpei State is sparse, with what little conducted mostly
confined to shallower depths (<30 m) (e.g., Rhodes et al. 2005, 2008, 2014a, 2014b; Golbuu et
al. 2008; Muir and Wallace 2016). Muir and Wallace (2016) reported low-light, ‘deep-water’
scleractinian coral assemblages in the shallow (10–20 m depth) lagoon of southwest Pohnpei
(Fig. 2). In 1988, ichthyologists Randall Kosaki and Richard Pyle conducted an exploratory
deep reef expedition to Pohnpei and Ant Atoll. These preliminary dives to depths of up to 85 m
yielded new fish geographic records (Apolemichthys griffisi (CARLSON AND TAYLOR, 1981),
Chaetodon burgessi ALLEN AND STARCK, 1973, Centropyge multicolor RANDALL AND WASS,
1974) and observations of unidentified new species (RL Pyle, pers. comm.). More recently,
from 2014 to the present, the University of Hawai'i at Mānoa and collaborators have conducted
research expeditions annually within the region (Rowley 2016). These expeditions have yielded
new fish species at mesophotic depths (Copus et al. 2015; Anderson and Johnson 2017) along
with ecological surveys of key MCE benthic components such as fishes (Bridge et al. 2016),
scleractinian and gorgonian corals, as well as monitoring of key environmental variables.
Environmental Setting
Pohnpei (6º52’ N, 158º13’ E) is an 8.7 million year old volcanic island (Rehman et al.
2013) with an area of 362 km2. It is fringed by a discontinuous barrier reef that consists of
twenty-five basaltic and coral islets enclosing an inner lagoon with an area of ca. 69 m2 and up
to 90 m depth in places. A short fringing reef surrounds the southeast portion of the island, with
the outer reef slope that surrounds the island dropping to over 1000 m deep (Ashby 1993;
OceanGrafix, 2008). Pohnpei is the tallest island in the FSM, with Mount Nahnlaud reaching
791 m above sea level, and deep valleys bearing the most extensive native tropical rainforest of
Micronesia (Balick 2009). Waterfalls and headwater streams feed the 2 km wide mangroves (ca.
17% of the island; Balick 2009) and up to 6 km wide mostly low-light and high-sediment
lagoons, which border virtually the entire island (Buden and Taboroši 2016). This combination
of high island and high vegetation density leads to a strong annual orographic precipitation of >
800 cm yr-1; Pohnpei, therefore, is one of the wettest places on earth. It is also one of the most
biodiverse in terms of botanical endemism (Balick 2009); however, comparatively less is known
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Fig. 2. The benthic communities of Pohnpei lagoons: (a) ‘platy’ and branched coral (e.g., Porites rus and
P. cylindrica), and sponge (e.g., Clathria cf. sp.) assemblages across the thermohalocline at 3–5 m depth;
(b) ‘platy’ corals (P. rus) below the thermohalocline ≥8 m depth; (c) Subergorgia sp. gorgonian coral on
lagoon floor at 60 m depth, and (d) Annella sp., colonies at 56 m. Photo credits: SJ. Rowley 2014 – 2016.
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of its marine fauna and flora. Moreover, the high land, rich vegetation and marine resources
have, at least in part, resulted in Pohnpei having the highest population density in the FSM (over
35,000 inhabitants). Pohnpei’s natural resource use continues to be steeped in traditional
cultural practices, yet the ongoing pressures of!upland disturbance and its associated runoff,
domestic and international fisheries (Rhodes et al. 2014b), and climate change pose great threats
to Pohnpei, its inhabitants, and reef communities.
The atolls Ant and Pakin have a steep bathymetry similar to that of Pohnpei, but are not
exposed to the same freshwater or agricultural runoff. Ant Atoll (6º45' N, 158°00' E) is eight
nautical miles southwest of Pohnpei and consists of 13 islets with a land area of 1.16 km2, and
an open lagoon of 46 km2 (Ashby 1993). It is the largest of Pohnpei’s outlying atolls with an
open channel, Tauenai Passage, to the south into the lagoon. Ant Atoll is privately owned and
sparsely populated (ca. 8–10 inhabitants), although has had a history of fluctuating residency
primarily due to the production of copra from the husk of coconuts until the 1970s. Pakin Atoll
(7º02' N, 157°47' E) is situated 18 nautical miles west of Pohnpei, has 16 islets, a land area of
0.68 km2 and a largely closed lagoon of 8.85 km2 that can only be accessed by small boats at
high tide through a narrow opening on the southwestern side. Pakin has a population of ca. 50–
80 inhabitants primarily on the islet Nikalap, which is on the northwestern side of the lagoon.
MCEs are typically characterized by strong attenuation gradients in environmental
variables such as water clarity and wave surge. Light attenuation is due to the absorption,
reflection, and scattering of light by sediment, dissolved organic matter, and phytoplankton.
Therefore, the greater the concentration of particulates in the water column results in an
increased attenuation in light availability for photosynthesis. The euphotic zone is the depth
range whereby photosynthesis can take place, with the subsurface irradiance defined as the
percentage at which light has penetrated. Water clarity1 at the outer reefs of Pohnpei had a mean
diffuse attenuation coefficient of Ko 0.043±0.003 m-1 compared to the less turbid atolls at Ko
0.038 ± 0.002 m-1. Mean optical depths2 at the midpoint of the euphotic zone (10% subsurface
irradiance), were at the upper mesophotic at 42 m, 222± 17 SE µE m-2 s-1 for Pohnpei and 56m,
212 ± 25 SE µE m-2 s-1 at the atolls. At 1% subsurface irradiance (lower limit of the euphotic
zone) was at 95 m, 29±3 SE µE m-2 s-1 for Pohnpei and ca. 110m, 23µE m-2 s-1 for the atolls,
with a 1.4% subsurface irradiance at 100 m, 29±6 SE µEm-2 s-1 for the latter. Here the water
clarity on the outer reefs of Pohnpei and neighboring atolls is slightly greater at the atolls, and
comparable with other high water clarity regions (e.g., Hawai’i; Pyle et al. 2016; Spalding et al.
2019).
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1!In!Pohnpei,!water!clarity!(light!penetration)!was!measured!using!a!calibrated!spherical!(4π)!quantum!
sensor!(LI-193SA!and!logger!LI-1500,!LI-COR,!USA),!which!measures!the!photon flux!of!photosynthetically!
active!radiation!(PAR:!400–700!nm)!in!μmol!photons!m-2!s-1.!Light!profiles!(n=17,!in!2017)!were!taken!over!a!
depth!gradient!of!0–100!m!and!recorded!every!1–5!m.!
2!Light!attenuation!values!are!independent!of!water!depth!and,!therefore,!are!defined!as!optical!depth!(ς)!to!
make!them!comparable!with!other!locations.!
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Fig. 3. Mean daily temperature from the outer reef of southwest Pohnpei Island at 10 m increments to
130 m depth from August 2015–July 2016. Missing depths (20, 50, 100, 110, 140 m) in the series were
due to faulty or lost loggers. Data were taken every 30 minutes using Tidbit® v2, Onset® for
approximately 11 months (Figure compiled by S.J. Rowley and D. Barshis, www.mesophotic.org, can be
reused under CC by license)
The MCEs of the Senyavin Islands are not benign environments, particularly in
comparison to the shallow-water coral reefs. In addition to irradiance, temperature variability
across bathymetry is likely a ‘first-order’ determinate of MCE health and distribution (see
Kleypas et al. 1999). Pohnpei’s MCEs are thermally dynamic environments (Fig. 3–4), a pattern
that has also been shown at other Indo-Pacific locations, e.g., Palau (Wolanski et al. 2004; Colin
et al. 2017), and Hawai‘i (Pyle et al. 2016; Spalding et al. 2019). Over a three-year period,
thermographs (Tidbit® v2, Onset®) placed at 10 m increments from 10–140 m depth at Pohnpei,
Ant, and Pakin revealed significant diurnal and seasonal fluctuations. Diurnal temperature
variances were typically over 10ºC at depths greater than 60 m (Fig. 3 and 4b–c). In March
2016, temperatures soared dramatically bringing the El Niño to a close (Oceanic Niño Index
[ONI]: Very Strong El Niño; Fig. 3). Shallow-water communities from the Porites LINK, 1807,
micro-atolls of the lagoons to the scleractinian reefs began to bleach, and the majority of reefs
eventually became smothered in algae by 2017 (see section Macroalgae). A succession of
cyanobacteria and invasive algae smothered communities to a depth of 68 m at Pohnpei from
2016 to 2017. Temperatures at 130 m depth ranged from a minimum of 10.5ºC to a maximum of
29.2ºC yet continued to fluctuate ca.10ºC within any single day irrespective of the mean. Such
thermally challenging environments have been suggested to be responsible for the “biologically
depauperate communities” observed at mesophotic depths of Palau (Wolanski et al. 2004). A
similar reduction in biodiversity also characterizes depths of ≥130 m at Pohnpei Island proper.
However, rich octocoral communities (see Octocorallia) flourish at ≥130 m off Ant and Pakin
Atolls, which are equally thermally challenged. Such observations suggest that temperature is
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Fig. 4. Diurnal dissolved oxygen (black lines) and temperature (blue lines) taken on the western reefs of
Pohnpei Island over an eight-day period 18–25 August 2017 at: (a) 30 m, (b) 90 m, and (c) 130 m depths.
Tidal influence indicated (a) during a period of high cloud (cloud icon) and no cloud but during high
spring tides (red arrow; days 4–7). Data were taken every 30 seconds using HOBO U26, Onset® (Figure
by S.J. Rowley, www.mesophotic.org, can be reused under CC BY license)
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not the primary variable that determines biological community structure at mesophotic depths,
at least for the islands and atolls of Micronesia. Over three years the diurnal, seasonal, and
annual temperature variance ranged up to 20ºC in a single day at 90 m depth. These
considerable thermal oscillations may likely be due to internal waves (e.g., Wolanski et al.
2004) coupled with a Rossby wave deepening the thermocline, and a rapid increase in
temperature as it passed through Micronesia in 2016 (Colin et al. 2017).
Oceanic islands and atolls are exposed to strong water currents and wave action and the
Senyavin Islands are clearly no exception. Water flow has not been quantified in Pohnpei;
however, temperature data thus far demonstrate high-energy hydrodynamic regimes with the
influence of two different water bodies: oceanic and shallow reef. This can also be seen through
the non-linear relationship between water temperature and dissolved oxygen (Fig. 4). Shallow
reef temperatures followed the diurnal influence of the sun’s irradiance. Tidal influence was
exerted during spring tides similarly observed with dissolved oxygen: high values at high tide
with correspondingly low values at low tide (days 4–7; Fig. 4a). With increased depth, values
appeared progressively independent of the diurnal cycle (influence from the warmth of the sun),
with isotherm vertical displacement likely due to internal waves (Wolanski et al. 2004). The
variation in dissolved oxygen (30 m, 4.5–7.1 mg L-1; 90 m, 5.2–7.6 mg L-1; 130 m, 3.4–9.4 mg
L-1; Fig. 4), with depth was likely due to an increase in bacterial respiration feeding on sinking
organic matter (Talley et al. 2011). Chlorophyll values peaked at depth (90 m, average 0.64 µg
L-1, max. 2098.5 µg L-1) compared to shallow (30 m, average 0.87 µg L-1, max. 127.5 µg L-1;
fast CTD profiler, Valeport) implying that biological productivity is greatest at mesophotic
depths irrespective of the close proximity to mangrove areas at Pohnpei. Salinity varied little at
shallow (30 m, 34.2 ± 0.1 PSU) and mesophotic (90 m, 34.4 ± 0.5 PSU; fast CTD profiler,
Valeport) reefs. Ongoing monitoring across a depth gradient from <10–150 m or more of key
environmental variables (including water flow, pH, nutrients, and carbonate saturation state)
will be necessary to develop a better understanding of MCEs, and enable elucidation of the
biological success of MCE benthic communities in such dynamic environments.
Habitat Description
The Senyavin islands all possess extensive MCE habitats. Pohnpei is a remnant of a
shield volcano with shear drop-offs on the outer barrier reefs particularly on the western side of
the island (Spengler 1990). The exposed windward/eastern outer reef also has steep drop-offs, as
well as low-relief reefs with little coral cover in areas of very high wave and current exposure.
The inner lagoon supports vast mangroves and thriving coral reefs loosely characterized as low
light, ‘platy assemblages’ where the dominant coral form is platy, yet, can include variable
branching forms (Muir and Wallace 2016). Towards the outer lagoon outlets or passes, depths
can reach over 60 m with strong currents transporting high-nutrient runoff from the mangrove
areas. These deeper lagoonal habitats typically support gorgonian coral, algal, and sponge
dominated hard substratum and sediment subject to significant bioturbation by benthic
invertebrates (Fig. 2c). Research has primarily been conducted on the western outer reefs. These
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Fig. 5. Geomorphology of the shallow to mesophotic reefs at: (a, d, g, j, m) west and southwest Ant
Atoll, (b, e, h, k, n) east and northeast Pakin Atoll, (c, f, i, l, o) and west side of Pohnpei Island. Each
vertical image series represents the geomorphology at the contrasting locations for (a, b, c) ≤ 5 m, (d, e,
f) 10 – 30 m, (g, h, i) 60 – 80 m, (j, k, l) 100 ≤ 110 m, (m, n, o) 140 – 160 m. Photo credits: SJ. Rowley
2014 - 2017.
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sites are characterized by shallow reef flats of low relief that typically extend to no more than 50
m out to a reef crest at 12–17 m depth (Fig. 5f). A steep wall or escarpment extends to 70 m
where the gradient reduces and then becomes a continuous and precipitous escarpment from
110–115 m depth. Buttresses are often present at ca. 70–90 m (Fig. 5l), attracting concentrated
communities of invertebrates, mainly gorgonians and black corals, as well as fishes (Rowley,
pers. obs.).
The atolls Ant and Pakin are characterized by near-vertical outer reef drop-offs on the
western and southwestern sides. The eastern and northeastern sides have a sloping bathymetry
from 3–30 m extending 80 m outwards whereby the slope drops steeply with increasing depth to
a distinct wave-cut ledge/notch at 100 to 110 m (Fig. 5k). Thereafter, is a steep wall with
vertical fissures perpendicular to the ledge (Fig. 5n) similarly observed in other tropical regions
(Locker et al. 2010). These features are full of azooxanthellate invertebrates: mainly octocorals,
antipatharians, sponges, bryozoans, ascidians, hydroids, crinoids, and the occasional benthic
ctenophore (Rowley, pers. obs.). Small fish species and juveniles seek refuge amidst the
branches of the gorgonian octocorals and antipatharians. Furthermore, at these deeper depths
small patches of algae are present such as the CCA, Palmophyllum-like taxa, as well as very
small (~3-5 cm2) patches of an unknown Chlorophyta that is commonly observed.
Biodiversity
Macroalgae
The shallow-water flora from Pohnpei and Ant Atoll is diverse compared to other small
similar-sized islands and atolls, and consists of 133 Rhodophyta, 82 Chlorophyta, 26
Phaeophyta, and 3 Magnoliophyta, for a total of 244 species (Hodgson and McDermid 2000;
McDermid et al. 2002). Biogeographic investigation of the shallow marine flora revealed that
Ant Atoll and Pohnpei have a large number of widespread and Indo-Pacific species, but very
few probable regional endemics (McDermid et al. 2002). However, DNA-based species
delineations of several macroalgal genera from island archipelagos in the Philippines (Payo et
al. 2013), Hawaiian Islands (Spalding et al. 2016), and New Caledonia (Vieira et al. 2014) have
shown extensive cryptic species diversity and fine-scale endemism. Although the use of
molecular analyses would be needed to further refine and test these findings from Pohnpei and
Ant Atoll, these collections provide a baseline for broad morphological comparisons with the
mesophotic flora.
Macroalgae from 5 to 140 m depths were haphazardly recorded with photographs and
video to document macroalgal abundance at Pohnpei, Ant, and Pakin from 2014 to 2017.
Species were identified to the lowest possible taxon based on images and preserved
representative collections (n=9). Although detailed molecular and morphological analyses are
needed to properly assess the diversity of the MCE flora, general trends in comparison to the
shallow flora and from year-to-year, particularly between 2016 (Very Strong El Niño) and 2017
(Weak La Niña), were possible for abundant macroalgae. From 2014 to 2016, macroalgae were
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Fig. 6. Mesosphotic macroalgae from the northeastern side of Pakin Atoll in 2016. (a) A large
Avrainvillea sp. with the coral Porites sp. at 50 m depth. (b) Benthic macroalgae at 130m with the green
algal crust resembling Palmophyllum sp., and (c) a prostrate, irregularly branched green alga.
Intermittent sediment in both images largely consisted of dead Halimeda segments (white). Ca represents
nongeniculate coralline algae overgrowing dead Porites, and Pa represents Palmophyllum sp. (Photo
credits: S.J. Rowley, 2016, www.mesophotic.org, can be reused under CC BY license)
generally in low to moderate abundance in comparison to corals. Small patches of a mat-
forming Cladophora sp. were observed at 30 m. Occasionally, large individuals of Avrainvillea
sp. (ca. 20–25 cm in height with clear concentric lines) would be observed only at Pakin Atoll
growing on the surface of live colonies of Porites sp. at 50–60 m depth (Fig. 6a). At this depth,
nongeniculate (non-articulated) coralline algae and scattered clumps of an erect Halimeda sp.
were observed growing at the base of plate corals (e.g., Porites sp. and Acropora sp.). By 130 m
depth, nongeniculate corallines were abundant on vertical surfaces free of sedimentation, and
interspersed with a closely adhering green algal crust (Chlorophyta) resembling Palmophyllum
sp., and a prostrate, irregularly branched green alga (Fig. 6b–c). Horizontal surfaces were
covered in cascading sediment composed of dead Halimeda J.V.LAMOUROUX, 1812 segments,
foraminifera (see section Other Biotic Components), with occasional small patches of non-
geniculate coralline algae.
In the summer of 2016, after the March Very Strong El Niño the upper MCEs of
Pohnpei experienced a cyanobacteria bloom smothering the reef to a depth range of 25–60 m
(Fig. 7g), although not observed on the adjacent atolls. However, drastic changes were further
observed in the MCE flora in 2017 at both Pohnpei and the atolls, with an increase in bloom-
forming cyanobacteria, macroalgae, and non-geniculate corallines. In shallow water from 5 to
30 m depths, a shift from low-moderate abundance in previous years (Rowley, unpubl. data) to
high abundance of Caulerpa racemosa (FORSSKÅL) J. AGARDH, 1873, Microdicyton DECAISNE,
1841, Dictyosphaeria cavernosa (FORSSKÅL) BØRGESEN, 1932, a conspicuous orange
cyanobacteria, brown-coloured filamentous diatoms, and Halimeda spp. dominated the reefs,
overgrowing corals and other available substratum (Fig. 7a–f). White, bleached patches of
Halimeda were observed inside Halimeda spp. beds and draperies, possibly due to sexual
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Fig. 7. Macroalgae from shallow to mesophotic depths during 2017: (a) southwest Pohnpei at 5 m depth,
(b) filamentous diatom mats at 5 m, Ant Atoll, (c) start of Microdictyon beds at 5 m, Ant Atoll that
extend to 50 m depth; (d) bleached Halimeda patches, 5 m, and (e) Caulerpa racemosa, 20 m depth at
western Pohnpei. (f) Halimeda and nongeniculate coralline algae dominated reefs at 30 m depth, Pakin
Atoll. (g) Bloom-forming cyanobacteria at 30 m, Pohnpei in July 2016, and (h) a shift to Cladophora cf.
sp. at the same reef patch in August 2017. (i) Juvenile corals Seriatopora hystrix and Siphonogorgia cf.
with other juvenile invertebrates and corallines at 60 m depth, Pohnpei in August 2016, and (j) the same
reef and depth dominated by a Cladophora cf. sp. in August 2017 (Photo credits: S.J. Rowley, 2016–
2017, www.mesophotic.org, can be reused under CC By license)
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reproductive events, a disease outbreak, or other unknown causes (Fig. 7d). From 30–60 m
depths, a dark-green mat of Cladophora cf. sp. (Fig. 7h–j) and Halimeda spp. covered at least
50% of available substrate, with non-geniculate coralline continuing to be abundant. By 120 m,
macroalgal cover was low with no bloom-forming macroalgae observed, and appeared similar to
the reefs observed from 2016.
As would be expected, genera that were described as dominant in the shallow flora, such
as the brown algae Dicytota and Padina (Hodgson and McDermid 2000), were not observed at
mesophotic depths. However, both the shallow and mesophotic flora contained similar species
and genera, such as Halimeda spp., Microdicyton sp., Dictyosphaeria cavernosa, and
Avrainvillea sp. (Hodgson and McDermid 2000). The majority of Rhodophyta described from
shallow water included turf and diminutive algae, making comparisons with the MCE flora
difficult without extensive collections. Additional samples analyzed with molecular and
morphological analyses are needed to properly characterize the MCE macroalgal community in
Pohnpei. Nevertheless, the current information suggests that this assemblage can experience
dramatic and unexpected shifts in algal abundance, with a recent (2017) algal bloom event from
shallow to MCE depths. The persistence and causes of this phase-shift may likely, in part, be
due to the very strong El Niño of 2016, with subsequent succession in dominant taxa. Moreover,
eutrophication (excessive influx of nutrients from land runoff) at Pohnpei and increased fishing
pressure, which removes all the grazers, herbivores, and detritus feeders, further encourages
algal overgrowth. Future research directions should focus on extensive collections for molecular
analyses of the MCE flora, including non-geniculate and turf algae. Even though ecological
benthic transect imagery data has been collected (Rowley, unpubl. data), lack of funds and
personnel render conclusions wanting. Nonetheless, continued monitoring of sites through time
would determine temporal changes in community structure and abundance. This coupled with
ecophysiological studies including top down and bottom up processes is required to elucidate
the drivers of algal blooms from the shallow reefs to MCE depths.
Anthozoans
Coral is essentially a polyphyletic term, defined by Cairns (2007): “as those Cnidaria
having continuous or discontinuous calcium carbonate or horn-like skeletal elements.” It is
estimated that over 5080 coral species are currently described, 66% of which are found at depths
below 50 m (Cairns 2007). Of this 66%, azooxanthellate (corals without the photosynthetic
dinoflagellate symbiont Symbiodinium FREUDENTHAL, 1962) octocorals constitute 75% of the
species found at depth with a rate of species discovery unlikely to reach an asymptote in the
near future. Nearly 42% of the scleractinians currently described occur at depth (below 30 m),
with new coral species being discovered at mesophotic depths (e.g., Randall 2015).
It is important to note that other coral groups are well represented on the MCEs of
Pohnpei, most notably the Antipatharia and Zoanthidea. However, no research has been
conducted on these taxa within the region.
!
15
Fig. 8. (a) Depth ranges of scleractinian coral species. Each bar represents the depth range of a species,
arranged by the mid depth of the range. The X-axis represents depth (m), and Y-axis is each species. (b)
Species richness of scleractinian corals over depth. Species richness accumulation estimates at each of
nine, 5 m wide depth bins. The red line represents the projected trend of species richness over depth by
fitting a Huismann-Olf Fresco model (Jansen and Oksanen 2013; Jansen et al. 2017). Grey dots represent
999 permutations of species accumulation estimates at a common sample size of 72 individuals. Red line
represents trend of richness over depth (Figure by S.J. Rowley, www.mesophotic.org, can be reused
under CC By license)
!
16
Scleractinia
Pohnpei and the surrounding atolls of Ant and Pakin support a variety of benthic
habitats, and a corresponding diversity of coral communities. In particular, the outer edges grade
steeply to deep depths, in clear oligotrophic waters, which are well suited to the development
and maintenance of photosynthetic MCEs, with high levels of light penetration into deeper
waters. Additionally, the lagoonal patch reefs provide low-light habitats in shallower waters,
creating a ‘shallow mesophotic-like’ community in some locations (Muir and Wallace 2016).
Surveys of the reef-building zooxanthellate coral species over depth (0–45 m) were
conducted in August 2017, using up-slope point count transects (sensu Roberts et al. 2016).3 Of
the 160 species recorded, 56 were present at depths below 30 m (Fig. 8a). Half of these species
(28) were also represented in the shallowest 15 m of the water column, although this number
represented ca. one-quarter of the 117 species recorded in the top 15 m. Only 15 species were
solely recorded from the mid depths (15–30 m). These results suggest that the deeper water
MCE communities are a partially nested subset of the shallow communities. This has significant
implications for the vertical connectivity of these communities, and the potential for MCE
communities to act as refugia for the shallow-water communities (Bongaerts et al. 2010;
Bongaerts and Smith 2019).
To assess the changes in species richness over depth, species accumulation curves were
generated for each depth bin (n=9), with 1000 repetitions completed at each depth. The resulting
values were used to demonstrate the species richness trend over depth (Fig. 8b), and fitted with
a Huismann-Olf Fresco model (Jansen and Oksanen 2013; Jansen et al. 2017), which fits one of
seven ecologically relevant shapes to the data. The most commonly chosen model fit after 100
permutations of the model was a gradual monotonic decline with depth (Fig. 8b), although the
trend showed no continued decline beyond ca. 20 m depth.
Severe bleaching episodes in 2016 and 2017 impacted the shallow regions of the reefs,
and the effects were visible up to and beyond 30 m in some cases. Despite this, some regions
showed little effect of the bleaching, in particular one site populated heavily by plating
Acropora hyacinthus (DANA, 1846) colonies, which all appeared to be the result of a single
recruitment pulse. The extent of the variability in the damage caused by the repeat bleaching
events was greatest in the shallow depths (15 m), which is likely responsible for the retention of
a surprisingly large species pool at these depths. The variety of habitats provided around the
island of Pohnpei and the associated atolls has maintained a source pool of colonies in spite of
the bleaching damage, which could allow for a rapid recovery. Clear, oligotrophic water
conditions on the atolls mean that the shallow coral communities were entombed in crustose
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
3!Transects!were!run!from!45!m!to!the!surface,!with!count!stations!located!in!nine!depth!bins!monitored!at!5!
m!depth!increments.!In!total,!1,236!individual!colonies!were!recorded!from!a!depth!range!of!0!to!45!m,!
representing!160!nominal!species.!Where!species!could!not!be!confidently!identified!in!the!field,!they!were!
photographed,!and!temporary!working!names!given!to!unclear!taxonomic!units.!A!proportion!of!these!
species!with!working!names!are!likely!to!result!in!novel!species!descriptions,!or!new!geographic!records.!
!
17
Fig. 9. Mesophotic coral communities at Pohnpei lagoon and Kimbe Bay, PNG, showing a similar
composition of species. (a) Acropora pichoni and, (b) A. tenella, both at a depth of 18 m in Pohnpei
lagoon. (c) Kimbe Bay coral communities at a depth of 35 m (Photo credits: T.C.L. Bridge, 2016–2017,
www.mesophotic.org, can be reused under CC By license)
coralline algae (CCA) on their death, without the colonisation of filamentous algae, or other
benthic megafauna likely to inhibit coral recruitment. This phenomenon has created a
substratum that is extremely well suited to coral recruitment, as corals are known to recruit onto
CCA ( Harrington et al. 2004) and more complex substrates (Hata et al. 2017). When combined
with the retention of the regional species pool, and the extensive MCE habitats, the recovery
potential of these reefs is highly encouraging, and worth continuing to monitor.
Amongst the coral habitats found around Pohnpei, the ‘shallow mesophotic-like’
communities found in the lagoon of Pohnpei at 15–30 m depth are particularly intriguing. These
habitats have been described in the literature (Muir and Wallace 2016; Fig. 9a–b), and the
species composition of these communities is in keeping with those found at upper mesophotic
depths (i.e., 30–40 m) in other Indo-Pacific locations, for example Kimbe Bay in Papua New
Guinea (Fig. 9c). Notably, the species found at both Kimbe Bay and the shallow lagoon of
Pohnpei [such as Acropora pichoni WALLACE, 1999 (Fig. 9a), and Acropora tenella!(BROOK,
1892); Fig. 9b] were not found in the upper 45 m on the outer barrier habitats, suggesting that
they either exist further down the slope, or have extensive dispersal ranges and particular
environmental requirements. Pursuing a quantitative analysis of the deeper (depths greater than
45 m) regions of these habitats will provide answers to some of these questions.
Octocorallia
The subclass Octocorallia comprises the orders Alcyonacea (soft corals and gorgonians),
Helioporacea (blue coral), and Pennatulacea (sea pens). The zooxanthellate blue coral Heliopora
coerulea PALLAS, 1766 is the only extant octocoral that produces a massive aragonite
!
18
Fig. 10. Gorgonian corals across bathymetry. (a) Viminella sp., at Ant Atoll, 65 m depth, (b) Subergorgia
sp., populating the underhangs and small caves at Ant and Pakin Atoll, 12m depth, (c) and inset (d)
Melithaea sp., in crevices and underhangs, 10 m depth at Pohnpei. (e) Annella reticulata at 90 m depth,
Pohnpei. This species typically spans the full depth range (5 to > 150 m). The zooxanthellate gorgonians
(f) Rumphella sp., at 20 m, and (g) Briareum sp., at 38 m depth at Pakin Atoll. Both taxa span the
shallow to upper mesophotic depth range. (h) The depth-generalist genus Astrogorgia, which spans the
full bathymetric range, 75 m, Pakin Atoll. (i) Paracis sp., at 110 m, Pohnpei, and (j) Acanthogorgia sp.,
125 m at Pakin Atoll. Both morphospecies are specific to the lower mesophotic, although species within
the genera can be found across the full depth range. Deep-reef specialists (k) Heliania sp., at 106 m, and
(l) Parisis at 95 m at Pohnpei. Both genera increase in abundance and diversity with increased depth
(Photo credits: S.J. Rowley, 2014–2017, www.mesophotic.org, can be reused under CC By license)
!
19
exoskeleton. Branching forms of this species are particularly abundant on the south to
northwestern side of the atolls Ant and Pakin, extending into the upper mesophotic (≤45 m
depth). Laminar colonies were only present in the shallow waters (≤5 m depth), which may
suggest the presence of two (cryptic) species partitioned by depth; branching forms at depth and
laminar forms in the shallows. Distinct lineages reflected by growth form have been shown in
H. coerulea, yet are not segregated by depth (Yasuda et al. 2014). Whatever the case, neither
morphotype present in the Pohnpei region was affected by the temperature anomalies or algal
overgrowth described in this chapter. Sea pens, such as those within the genus Virgularia
LAMARCK, 1816 were also observed at mesophotic depths; however, no known research has
been conducted on this group throughout the region. It is the gorgonian corals within the
Alcyonacea that has received the most research attention, and thus will be the primary focus.
MCEs within the Indo-Pacific are typically dominated by a diverse array of gorgonian
(sea fan) octocorals (Rowley 2014). Of the 65 genera and 15 families currently recorded at
mesophotic depths throughout the Indo-Pacific (see Sánchez et al. 2019), 35 genera and 11
families were present within the Senyavin Islands, and comprised all higher order groups (total
of 897 specimens collected during four expeditions 2014–2017; see also Rowley 2016). The
majority of taxa are found at depths below 70 m, consisting of 32 genera within 11 families.
This is in contrast to gorgonians on the shallow reefs (<30 m), where there are 16 genera within
eight families. However, 12 genera within seven families occur throughout the full bathymetric
range (1–157 m; Rowley 2016).
In August and September 2017, quantitative surveys were conducted as described by
Roberts et al. (2016); see section Anthozoans: Scleractinia).4 Within the narrow band of the
upper mesophotic (30–60 m), 38 morphospecies of gorgonians, and 18 genera within nine
families were present. At this ‘transitional’ depth, the majority of taxa were those spanning the
full bathymetric range such as Viminella GRAY, 1870 (Fig. 10a). In the shallows (<30 m) dense
communities of a sciophilous (shade tolerant) Subergorgia GRAY, 1857 populated the caves and
under overhangs (Fig. 10b). However, only two morphospecies were specific to the shallows.
Both morphospecies were within the azooxanthellate genus Melithaea MILNE EDWARDS, 1857,
and also appeared sciophilous, being found exclusively under overhangs and crevices (Fig. 10c–
d). Only two zooxanthellate gorgonians were present at mesophotic depths at Pakin Atoll,
Rumphella BAYER, 1955 (up to 61 m; Fig. 10f) and Briareum BLAINVILLE, 1834 (up to 75 m;
Fig. 10g), most likely due to the water clarity at this site. Whether such colonies are
reproductively viable is unknown. These were single observations of each taxon, both typically
at depths shallower than 40 m (Rowley, pers. obs.).
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
4!Survey!methods!were!adapted!for!gorgonian!corals!and!for!a!greater!depth!range!whereby!point!count!
transects!were!run!upslope!every!10!m!from!140!to!10!m!depth.!A!total!of!109!morphospecies!were!
identified!from!the!11!surveys!conducted!at!Pohnpei!(n=5),!Ant!(n=3)!and!Pakin!(n=3).!Of!the!1565!
individual!colonies!surveyed,!1056!were!at!depths!below!70!m,!which!is!in!contrast!to!the!shallow!reefs!(<30!
m)!that!had!a!total!abundance!of!148!colonies.!In!the!upper!mesophotic!(30–60!m)!a!total!of!361!individual!
colonies!were!present.!
!
20
Fig. 11. Species richness of gorgonian and scleractinian corals over depth (5–140 m) at (a) all study sites
within the Senyavin Islands (Pohnpei Island and Ant and Pakin Atolls); (b) the atolls Ant and Pakin, and
(c) Pohnpei Island. Species richness accumulation estimates were at each 5 m wide depth bins. The red
line represents the trend of scleractinian richness over depth (every 5 m from 5–45 m). The blue line
represents the trend of gorgonian richness over depth (every 10 m from 10–140 m). Coloured dots
represent 999 permutations of species accumulation estimates for gorgonians (blue) and scleractinians
(red) (Figure by S.J. Rowley, www.mesophotic.org, can be reused under CC By license)
!
21
Specific to the lower depths were morphospecies mostly within the Plexauridae (n=27;
genera=8), particularly within the genus Paracis KÜKENTHAL, 1919 (Fig. 10i), as well as
Ellisellidae (n=14; genera=5; Fig. 10k), and Acanthogorgiidae (n=12; genera=3; see Fig. 10j).
Less diverse, yet characteristic taxa that generally increase in diversity and abundance with
increased depth are those within the Keroeididae (n=3; genus=1), Primnoidae (n=2; genera=2),
and Parisididae (n=2; genus=1; see Fig. 10l). In total, 19 morphospecies occurred throughout the
bathymetric range, most notably Annella spp. (Fig. 10e), Acanthogorgia spp., and Astrogorgia
spp. (e.g., Fig. 10h). Nevertheless, all such ‘depth generalists were typically more abundant with
depth.
Changes in species richness across depth between Pohnpei and the atolls, were
assessed.5 Notably, gorgonian diversity increased with depth a pattern that has been reported
previously (Rowley 2018). Their high overall diversity may well be a consequence of a
continuously dynamic environment (Connell 1978) at depth, particularly with regard to
temperature. Conversely, the hermatypic scleractinian corals, the primary benthic space
competitors throughout the shallow reefs, follow a depth trajectory consistent with light
availability. The transition between these two benthic groups, gorgonians, and scleractinians,
within the Senyavin Islands occurs at ca. 60 m depth (Fig. 11a). However, at Pohnpei the depth
transition is at ca. 45 m (Fig. 11b). This shallower transition depth at Pohnpei, compared to the
atolls, may be due to a combination of high relief walls and the effects of overfishing at many
sites (Fig. 11c). Therefore, characterizing the transition from a photosynthetic community to a
non-photosynthetic filter-feeding community (i.e., scleractinians to gorgonians with increased
depth) may assist with reef health assessments and resilience to anthropogenically-induced
environmental change (Knights et al. 2017).
Sponges
Sponges (Phylum: Porifera) are a highly complex, diverse group, and the oldest living
metazoans on earth (Van Soest et al. 2012). Nevertheless, sponges are notoriously difficult to
identify due to intra- and interspecific variability in response to environmental factors such as
hydrodynamics, light, and turbidity (Van Soest et al. 2012). Initial observations and surveys of
the diversity and distribution of sponges on shallow and mesophotic outer reefs throughout the
Senyavin Islands reveal an increase in diversity and abundance with increasing depth. The
shallow portions of the reef (5–10 m) are low in sponge diversity and abundance. Nonetheless,
sponge diversity increased with increasing depth, particularly on the topographically complex
outer steep reefs with undercut walls and overhangs, where the more delicate species could be
found. Encrusting taxa such as Siphonodictyon BERGQUIST, 1965, Clathria SCHMIDT, 1862, and
Hyrtios erectus (KELLER, 1889), were prevalent in the shallows within the shadows of the
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
5!Species!richness!was!assessed!using!the!methods!described!in!the!Anthozoans:*Scleractinia!section,!
whereby;!species!accumulation!curves!were!generated!for!each!depth!bin!(n=14),!with!1000!repetitions!at!
each!depth.!Values!were!then!used!to!illustrate!the!trend!of!species!richness!over!depth!(Fig.!11),!and!fitted!
with!a!Huismann-Olf!Fresco!model!(Jansen!and!Oksanen!2013;!Jansen!et!al.!2017).!Species!accumulation!
curves!of!gorgonians!and!scleractinians!were!then!compared.!
!
22
Acropora spp. dominated portions of the reef. Other notable shallow reef taxa include those
within the Demospongiae genera Cinachyrella WILSON, 1925, Coscinoderma CARTER, 1883,
and Hymeniacidon BOWERBANK, 1858. As the topography shifts at the reef crest to the high
relief walls particularly characteristic of the western reefs, sponges become larger with a
branching morphology. For example, species within the genera Cymbastela HOOPER AND
BERGQUIST, 1992 and Cribrochalina SCHMIDT, 1870 were increasingly observed between the
depths of 50–100 m. Furthermore, at depths ≥90 m Niphates DUCHASSAING AND MICHELOTTI,
1864, as well as lithistid sponges were also present, the latter previously noted in Palau (Colin
2009). A variety of morphospecies were observed from shallower depths past the reef crest in
the low-light environments of the under the overhangs and small caves. Taxa spanning the
shallow and mesophotic depths included those within Dactylospongia BERGQUIST, 1965,
Spongia LINNAEUS, 1759, and Luffariella THIELE, 1899, as well as the previously noted
Clathria, Cribrochalina, and Cymbastela.
These preliminary observations on the Porifera assemblages of the Senyavin Islands
across bathymetry suggest two patterns. Firstly, that they appear to correspond with that of the
azooxanthellate gorgonians of the region, reflecting a reduction of irradiance (also observed in
Palau: Colin 2009; Kelly and Bell 2016). And secondly, that there may be little difference in
sponge assemblages between the outer reefs of Pohnpei and the atolls, Ant and Pakin.
Fishes
From 2014–2016, a total of 473 fishes have been collected from the waters of the
Senyavin Islands for taxonomic, biogeographic, and population genetic research (by Brian D.
Greene, RL Pyle, Joshua Copus, and Richard R. Coleman; see Rowley 2016). The majority of
these fishes are from mesophotic depths with at least 11, collected below 90 m depth that are
new species to science. These include the recently described Luzonichthys seaver COPUS,
KA'APU-LYONS, AND PYLE, 2015,!Grammatonotus xanthostigma and Grammatonotus pelipel
ANDERSON AND JOHNSON, 2017 (Fig. 12).
Recent research on the island of Pohnpei (and neighbouring Ant Atoll), investigated fish
assemblages across depths and found patterns related to trophic position and highlighted
patterns that are specific to the Central Pacific region. Pohnpei supports over 650 species of reef
fishes (Allen 2005; Goldberg et al. 2008). To document the shift from a shallow to mesophotic
reef fish community, we conducted visual and roving-video surveys of reef fishes along a
gradient (0–130 m) using closed-circuit rebreathers.
Trophic position has previously been found to be a distinguishing feature between
shallow and mesophotic-associated fishes (Bejarano et al. 2014; Kane and Tissot 2017), with
few herbivores at mesophotic depths, a trend that remained consistent in Pohnpei (Coleman et
al. 2018). At 30 m trophic assemblages were indistinguishable from other depths; however,
shallow (<30 m) and deep (>30 m) assemblages were found to be significantly different from
each other (PERMANOVA, Pseudo-F=4.43, p <0.001; Coleman et al. 2018). These
!
23
Fig. 12. Specialist mesophotic fishes from both Pohnpei and Ant Atoll. (a) Luzonichthys seaver COPUS,
KA'APU-LYONS, AND PYLE, 2015, at 90–100 m depth, (b)!Grammatonotus xanthostigma ANDERSON AND
JOHNSON, 2017, at 142 m depth, and (c) Grammatonotus pelipel ANDERSON AND JOHNSON, 2017, with
types from 136–151 m depth (Photo credits: B.D. Greene, 2014–2015, www.mesophotic.org, can be
reused under CC By license)
!
24
observations corroborate 30 m as the depth at which shallow assemblages begin to shift to
mesophotic assemblages, which has also been observed in the South Atlantic (Rosa et al. 2016),
Hawai‘i (Kane and Tissot 2017), and the Red Sea (Brokovich et al. 2008). Concordance of a 30
m transitional depth across different ocean basins provides further support that this is a global
phenomenon.
Trophic position was also found to be associated with a species ability to specialize in
shallow or mesophotic depths. Our results indicated that the probability of a deep-specialist
being a planktivore ranged from 33–61%, and the probability of a shallow-specialist being an
herbivore was high ranging from 91–99% (Coleman et al. 2018). Food availability of herbivores
is directly related to light intensity, which decreases with depth. Algal communities are known
to thrive as deep as 268 m allowing for the potential for food availability for herbivores in
MCEs (Littler et al. 1985). However, as depth increases the algal community composition
changes considerably (Littler et al. 1985; See Macroalgae section) leading to a decline in
grazing pressure (Brokovich et al. 2010). Furthermore, very little macroalgae were observed
deeper than 30 m at the time of study (2014; Coleman and Rowley, pers. obs.) in contrast to
subsequent years. Alternatively, planktivores can potentially thrive at any depth and it has been
suggested that upwelling increases food availability in MCEs via primary production for
planktivores (Leichter and Genovese 2006; Leichter et al. 2013).
Although trophic position can influence the depth distribution of fish species, an
assessment of Hawaiian fish assemblages found that it accounted for only 33% of the variation,
with additional factors explaining distribution across depth (Kane and Tissot 2017).
Morphological characteristics (Bridge et al. 2016), habitat availability (Brokovich et al. 2008),
and the physical and geological environment (Kahng et al. 2010) have been identified as other
potential drivers influencing the depth distribution of fish assemblages. Taxonomic grouping
has also previously been used to predict species distribution. However, highly variable patterns
among distant regions suggest taxonomic grouping is not a sufficient predictor of inhabiting
MCEs. Snappers (Lutjanidae) were associated with mesophotic depths in both Pohnpei and the
Red Sea. Additionally, surgeonfish (Acanthuridae) were found to be the dominant family at
shallow depths in both regions, as well as the South Atlantic (Rosa et al. 2016). However, we
also observed conflicting patterns in damselfish (Pomacentridae) and wrasses (Labridae)
distributions between regions. Red Sea damselfish were associated with shallow habitat due to
the presence of branching coral, which served as shelter from predation (Brokovich et al. 2008).
Conversely, in Pohnpei, damselfish were associated with deeper depths, an environment where
branching corals are rare. Red Sea wrasses were associated with deeper depths and thought to be
outcompeting planktivorous damselfish for space, whereas in Pohnpei we found that wrasses
were more associated with shallow depths. It is not immediately clear what the drivers are that
are facilitating the disparity between these regions. Nonetheless, identifying these
inconsistencies will help to illuminate and explain unknown drivers that influence species
vertical distributions.
!
25
The upper boundary of mesophotic communities is likely to shift in response to a
changing climate. Although the impacts to shallow and MCEs remain unknown, identifying the
mechanisms that drive species depth distributions will help us to understand how species will
respond to these events. At the family and trophic level, we found that there is little overlap
between shallow and MCE fish assemblages in Pohnpei, and this pattern limits the utility of
MCEs to act as a refuge from disturbances from which shallow reefs can be replenished.
Management strategies must take into account the distinction between shallow and mesophotic
ecosystems in the development of future protective measures.
Elasmobranchs
The subclass Elasmobranchii BONAPARTE, 1838 within the class Chondrichthyes
HUXLEY, 1880 (cartilaginous fish), comprises sharks, rays, skates, and sawfish. The oceanic
islands and atolls of the Pacific attract migratory taxa, as well as provide ideal habitat for
species migrating locally between the atolls and islands, as well as vertically. Relatively little is
known about the sharks and rays of the Senyavin Islands. What has been documented is
primarily associated with bycatch from the industrial tuna fisheries (Hutchinson et al. 2015),
with young silky sharks, Carcharhinus falciformis (BIBRON, 1839), constituting 95% of the
entire elasmobranch bycatch (Lawson 2011; Rice 2013). From 2014 to 2017, the abundance of
sharks on the reefs of Pohnpei has dramatically declined (Rowley, pers. obs.). Nevertheless, at
both Ant and particularly Pakin Atolls, large numbers of juvenile black tip reef sharks and white
tip reef sharks populate the shallow reefs. Such taxa are reliant on coral reefs as habitats with
the detrimental effects – direct and indirect – of anthropogenic impacts and climate change
posed as great threats (Field et al. 2009; Lindfield et al. 2016; see section Threats and
Conservation Issues). At mesophotic depths of >50 m depth, sharks are frequently present (Fig.
13a & b). Numerous white tip reef sharks, Thunnus obesus, can be observed resting on the
sandy bottoms of the passes between the lagoons and the open ocean (40–60 m depth). Solitary
grey reef sharks, Carcharhinus amblyrhynchos (BLEEKER, 1856), are territorial and are often
observed at upper mesophotic depths (Fig. 13a). Yet, shark aggregations in general are
becoming increasingly rare. In 2015, an aggregation of 50 or more, silky sharks, C. falciformis,
were recorded at Pakin Atoll (Fig. 13b). However, such numbers have not been recorded since,
likely due to population decimation as bycatch by the international purse seine tuna industry
(Lawson 2011; Rice 2013). With greater depths, an increase for aggressive offshore taxa can be
observed, such as black tip [Carcharhinus limbatus!(MÜLLER AND HENLE, 1839)], and silver tip
sharks [Carcharhinus albimarginatus (RÜPPELL, 1837)]. These patterns are curious, but the
latter species are territorial in twos or fours, often-displaying aggressive behavior and posture
(Rowley, pers. obs.).
The rays of Pohnpei, Ant, and Pakin are typically seen on shallow reefs. Manta rays are
commonly observed, particularly within the lagoons and passes of Pohnpei. Here, cleaning
stations, nutrition in the form of fish spawning aggregations, and mates can be found! Two
species of manta ray have been recorded throughout the Senyavin Islands: the giant oceanic
!
26
Fig. 13. Observations of elasmobranchs in the Senyavin Islands at depth. (a) Grey reef shark
(Carcharhinus amblyrhynchos (BLEEKER, 1856); IUCN near threatened) 50 m Pohnpei, (b) a school of
Silky sharks (Carcharhinus falciformis (BIBRON, 1839); IUCN Vulnerable) at 50 m, Pakin Atoll, and (c)
six Blotched Fantail Ray (Taeniurops meyeni (MÜLLER AND HENLE, 1841); IUCN Vulnerable) resting
under a deep buttress, 90 m depth, Pohnpei. Photo credits: SJ. Rowley 014, 2015 and 2017 respectively.
!
27
manta ray Mobula birostris (WALBAUM, 1792), and the smaller resident reef manta ray Mobula
alfredi (KREFFT, 1868). Research is currently underway on their migratory behaviour using
acoustic telemetry tagging (Hartup J., pers. comm.), yet, it is still uncertain whether these
remarkable animals inhabit mesophotic depths at Pohnpei. Other rays, such as the blotched
fantail ray [Taeniurops meyeni (MÜLLER AND HENLE, 1841)] have been observed resting at 90 m
depth in Pohnpei (Fig. 13c). These typically solitary rays were sequentially circling each other,
then returning to their resting place. It is unclear whether this behaviour is reproductive in
nature. Nonetheless, the majority of ray taxa are sensitive to temperature, which may limit their
bathymetric distribution (Jawad 2011). The spotted eagle ray (Aetobatus BLAINVILLE, 1816) is
frequently observed, darting too and from the upper mesophotic depths. Acoustic telemetry
tagging would certainly assist in ascertaining the role, if any, of MCEs in the life history of
these rays. However, rays are also targets for local fishing (Yamauchi and Ota 2012), which
suggests that they should be protected.
Other Biotic Components
The invertebrate taxa at mesophotic depths, other than those within the Hexacorallia
(i.e., Antipatharia and Scleractinia) and Octocorallia (i.e., gorgonians and soft corals), are poorly
known. The MCEs of the Senyavin Islands harbour a remarkable biodiversity of invertebrates;
however, no research other than those taxonomic groups discussed herein has been conducted.
Moreover, the upper mesophotic zone is not immune to the invertebrate predators of the
shallows. The voracious coral predator Acanthaster planci (LINNAEUS, 1758) (commonly known
as the Crown-of-Thorns starfish) is present on the MCEs of Pohnpei (Fig. 14a), although little is
known about them. Numbers of A. planci are observed to decline at depths of ca. 40 m likely
due to lack of prey and the concomitant increase in macroalgae.
Observations suggest an overall increase in biodiversity with increased depth,
particularly on the atolls (Rowley, pers. obs.). Notable taxa include numerous vagile (mobile,
not stalked) crinoids, benthic ctenophores (Lyrocteis KOMAI, 1941), black corals (e.g.,
Antipathes PALLAS, 1766, Cirrhipathes DE BLAINVILLE, 1830, Stichopathes BROOK, 1889),
bryozoans (particularly taxa within the genus Reteporellina HARMER, 1933, and family
Crisinidae D'ORBIGNY, 1853), and a variety of ascidians, flatworms, crustaceans, mollusks, and
hydroids (including the Stylasteridae GRAY, 1847, and the family Aglaopheniidae
MARKTANNER-TURNERETSCHER, 1890). Few holothurians or echinoids were found. Large
oysters similar to those seen in Palau (see Colin 2009) were present at underhangs and deep
ledges from ≥90 m depth (Fig. 14b). Azooxanthellate scleractinians within the families
Dendrophylliidae and Caryophylliidae DANA, 1846 were common throughout the deeper depths,
a pattern also noted at other Indo-Pacific locations (e.g., Marshall Islands: reviewed in Kahng et
al. 2010; Hawai‘i: Wagner et al. 2016).!
Many taxa are in association with other invertebrates, as well as fishes, demonstrating
that this environment most likely reflects fascinating co-evolutionary interactions between taxa
over geological time. For example, numerous individuals of the decapod shrimp Plesionika cf.
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Fig. 14. Other invertebrates at mesophotic depths. (a) Acanthaster planci (LINNAEUS, 1758) (crown of
thorns starfish) at 40 m depth, Pohnpei; (b) Giant oysters present in the underhangs and crevices, 90 m
depth, Pohnpei, and (c) Crinoids commonly observed filter feeding from the high position of the
gorgonian (Muricella sp. pictured), 140 m depth, Pakin Atoll. Photo credits: SJ. Rowley, 2017.
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Fig. 15. Benthic foraminifera Cycloclypeus carpenteri BRADY, 1881 at Pohnpei. (a) In situ individuals at
100 m depth. (b) The edge of the equatorial view of a single specimen. (c) A close-up of the lateral
chambers. Note green colouration depicts the photosynthetic endosymbionts (Photo credits: S.J. Rowley,
2016–2017, www.mesophotic.org, can be reused under CC By license)
flavicauda invariably occur with the deep-reef specialist grouper Cephalopholis igarashiensis
KATAYAMA, 1957 (Greene and Pyle, pers. comm.). Gorgonians, soft corals, and sponges host an
extraordinary array of associate taxa. Gorgonians in particular, provide a protective ‘nursery’ for
juvenile fish species, as well as the pygmy seahorse (Hippocampus RAFINESQUE, 1810), brittle
stars (e.g., Ophiothela VERRILL, 1867), caprellid shrimps, host colour-specific cowries, as well
as occasional high numbers of Ophiothrix MÜLLER AND TROSCHEL, 1840 populating the fringes
of sea fans with no obvious detrimental effects. Motile crinoids also utilize the elevated
positioning of sea fans (Fig. 14c) to capture high nutrients in the water column and possibly
avoid predation from benthic predators (e.g., echinoids; Gorzelak et al. 2012; Donovan and
Renema 2015). With the exception of Pakin Atoll, few large fish can be found at depth, other
than the small, yet diverse, taxa of interest to ichthyologists and the aquarium trade (Fig. 12).
Therefore, with a paucity of predators, filter feeders such as crinoids are likely to be successful
in the MCEs of the Senyavin Islands.
The bathymetric distribution of the Foraminifera D’ORBIGNY, 1826 (hereafter forams)
relates to irradiance, substratum type, nutrient, and hydrodynamic regime (Renema 2018).
Notably, large benthic foram taxa with flat calcareous tests (shell) are found at deeper depths
(Song et al. 1994; Hohenegger et al. 2000). In the Senyavin Islands, forams and Halimeda
segments are the major sediment components, the latter dominating in the shallows. With
increased depth, a shift towards the deep-reef specialist foram Cycloclypeus carpenteri BRADY,
1881 occurs (Fig. 15a). This species is the largest living foram (>100 mm in diameter) and has a
typical depth range of 50–120 m throughout its geographic distribution (Koba 1978; Song et al.
1994; Iryu et al. 1995; Webster et al. 2009; Bridge et al. 2011). C. carpenteri is a free living
benthic foram that is slightly convex, planar, and discoid in shape (Song et al. 1994; Fig. 15b).
This flattened discoid morphology optimizes the surface area for photosynthesis by the
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30
Fig. 16. Threats to shallow reef health in the Senyavin Islands. (a) Healthy coral communities at Pakin
Atoll in 2016, and (b) the same reef covered in coralline algae in 2017. Photo credits: SJ. Rowley, 2016 –
2017.
endosymbionts within its calcareous test. However, this morphology has been shown to reduce
mechanical stability in high water energy environments, particularly in the shallows (Song et al.
1994). On the MCEs of Pohnpei, fields of C. carpenteri have been discovered at 80–100 m
depth (Rowley, pers. obs.). Such high abundances may be due, in part, to the clear waters and
likely low-energy hydrodynamic regime at some of the sites on Pohnpei. Irradiance and
hydrodynamic regime have been shown to determine the depth distribution of this species, with
less mechanical stability in the high water energy environments of shallow reefs (Song et al.
1994).
Threats and Conservation Issues
The health of the shallow water coral reefs of the Senyavin Islands is being severely
compromised, the signature of which can be seen at deeper depths with increased algal
dominance in the upper mesophotic zone and with a reduction of reef and food fish. The 2016
bloom-signature at depth may act as a predictor of reef health in months to come, particularly
for reefs excessively exposed to multiple stressors. Ocean temperatures have fluctuated over the
millennia, a single stressor that reefs may well have the capacity to recover from in our current
climate, but the addition of multiple stressors such as eutrophication, local and international
fishing exploitation compromise coral reef resilience. The potential thermal tolerance of MCE
taxa may provide insights into the mechanisms of biological success of MCE communities over
geological time. This in turn, may then assist in predictions of which taxa and communities are
likely to persist and perhaps for some taxa may even act as refugia in the advent of global
climate change.
Ant and Pakin Atolls have the greatest abundance and diversity of fishes and
invertebrates of the Senyavin Islands, particularly at depth. The marine environments of these
atolls are considered the most valuable for protection (Allen 2005; Conservation Society of
Pohnpei 2006). Yet, between 2016 and 2017, the coral reefs of Ant and Pakin underwent a shift
!
31
Fig. 17 Conservation and communication in the Senyavin Islands. (a) Mutual sharing of mesophotic
research and local community concerns with the late Sounihrek Pakin and the Pakin community. Photo
by J. Hartup, 2015. (b) Schematic diagram of bottomfish fishing, a favoured fishing practice, targeting
mesophotic depths (typically 40 – 150 m depth) and considered a more sustainable practice in Pohnpei.
Image courtesy of ThisFish, 2017. (c) The Marine Protected Areas throughout the Senyavin Islands,
courtesy of the Conservation Society of Pohnpei.
from vibrant coral reefs to a CCA-dominated reefs (Fig. 16a & b). An increase in scleractinian
corals in the upper mesophotic zone, plus a large abundance of fish taxa at greater depths may,
if protected, re-seed the shallow populations of the atolls. Sharing research findings, footage,
and imagery with local communities (Fig. 17a) has enhanced their understanding of these
valued environments, and has led to the desire to protect such rich biodiverse resources. Bottom
fishing is considered the most skilled and sustainable practice, targeting fishes from 40–140 m
depth (Joseph, pers. obs.; Fig. 17b). Therefore, we present the opportunity to bridge science and
policy through adaptive management strategies (Fig. 17c), making use of MCEs may assist local
community members and chiefs, who are keen to protect their resources. It is unclear if and how
the MCEs of the Senyavin Islands will be either affected by and/or effective for mitigating the
detrimental effects of anthropogenic disturbance. It is also unclear if and how the reefs of
Pohnpei and its sister atolls will recover and persist, but the most powerful tool for change has
been to help communities become aware of the importance of their marine resources (Joseph,
pers. obs.).
The low-lying atolls that populate Micronesia as a whole are extremely vulnerable to sea
level rise (for similar global comparisons see Perry et al. 2018). Local communities are alarmed
at the significant loss of land and increases in water temperature over the last decade (The late
Sounihrek [Chief of] Pakin Atoll, 2016, pers. comm.). The Conservation Society of Pohnpei
!
32
Fig. 18. Threats to shallow and mesophotic reef health in the Senyavin Islands. (a) International factory
fishing vessels, a common site on the picturesque horizon of Pohnpei, (b) a closer view of the purse seine
fishing vessels anchored offshore, and (c) southwest Pohnpei, ~Kehpara at 5 m depth. Images by SJ.
Rowley, 2017 off the outer reef, Sokehs Rock, Pohnpei.
(CSP) was established in 1998, in response to the need to implement newly established
conservation management strategies, and to develop trust, confidence, and build awareness of
the limitations and sustainable use of the natural resources for the local communities. The CSP
work with local communities, international conservation agencies, and researchers to mitigate
the declining marine resources throughout Pohnpei (e.g., CSP 2006; Rhodes et al. 2004, 2008,
2014b; Bosserelle et al. 2017). Yet, what is of immense concern lay anchor in and around
Pohnpei harbour in the form of large purse seine factory vessels (Fig. 18a & b). The cascading
effects of these vessels ultimately destroy the reefs (Fig. 18c), reducing resources for the local
communities by stripping the waters of pelagic fish, and thus, apex predators on the reef (see
also Dulvy et al. 2004). Local fisheries resort to greater reef fish and invertebrate fisheries and
trade, whereby herbivores and detritivores (e.g., Holothurians; Bosserelle et al. 2017), the
cleaners of the reef, are depleted. Moreover, poaching and illegal practices such as night fishing
continue to rise as both fish size and catch are reduced. The coastal fisheries of Pohnpei State
are deemed unsustainable (Rhodes et al. 2014b; Bosserelle et al. 2017). Thus, by permitting
these international fishers, the government of Pohnpei essentially sacrifices the vibrant reefs and
subsistence fisheries of generations before. The local communities do not appreciate the
presence of these vessels. Younger generations frequently approach visiting researchers eager to
find solutions to persuade government officials to amend the 1996 fisheries act, and not let
Pohnpei “get like Hawai’i.” However, the economy of Pohnpei depends on the commercial tuna
fishery targeted by these international fleets (CSP 2006). The FSM receives an annual income of
$30 million in license fees from foreign vessels, which constitutes 70% of the Pohnpei
economy. With the Compact of Free Association (COFA) with the USA ending in 2023, it is
considered highly unlikely that the pelagic fishery license will be revoked. The socio-economic
implications for the Pohnpei communities are vast.
!
33
Conclusion
The research presented here on the mesophotic reefs of the Senyavin Islands, is a
summary of what has and continues to be conducted by the University of Hawai'i at Mānoa and
collaborators since 2014. Annual expeditions have revealed that MCEs are biodiverse, dynamic
environments that provide refuge for certain megafauna, have species-specific bathymetric
distributions, and display remarkable resilience to diurnal fluctuations in temperature. Depth
transitions between functional groups of fish and benthic taxa such as the photosynthetic corals
and filter-feeding azooxanthellate gorgonians are predominantly a function of irradiance.
Taxonomic groups such as Porifera and Foraminifera are highlighted as fruitful areas for future
research. However, the rapid declines in reef health and marine resources have been apparent,
particularly on the shallows reefs. The lush reefs and huge abundances of fishes of previous
times (Turak and DeVantier 2005) are successively being replaced by barren reefs, smothered
by invasive algae, filamentous cyanobacteria, or crustose coralline algae of the atolls. The
surrounding oceanic islands and atolls of Micronesia may act as refugia, representing stepping-
stones of dispersal within and between the regions and bathymetry. Yet, resource management
strategies developed by Pacific Island cultures over hundreds of generations face significant
challenges in the modern world. It is recommended to: 1) develop community-based
conservation strategies towards sustainable eco-tourism, which generates incentive and
alternative measures to prevent illegal fishing practices (as successfully demonstrated in
Indonesia, see www.misoolfoundation.org); these include local rangers, protection enforcement,
and local community education; 2) annual monitoring and research on specific reefs throughout
the Senyavin Islands from 0-150 m depth, and 3) a reduction to eventual ban of international
fisheries throughout the FSM. In summary, it is essential that local communities and the
government act now if the reefs of the Senyavin Islands are to recover and survive.
Acknowledgements
Sincere gratitude is extended to Walter Wilbur, Walter family, and extended family including
Kayem and Boy at the Nihco Marine Park, Pohnpei. A warm Kalangan to all at the
Conservation Society of Pohnpei (CSP), the Office of Fisheries and Aquaculture (OFA), the late
Sounihrek Pakin and his community, and the Hawley family at the Pohnpei LP Gas Distributing
Company, FSM. Special thanks also go to S.M. Stanley, L. Briones, D. Johnson, D. Barshis, K.
Longenecker, R. Langston, B.D. Greene, R.L. Pyle, A. Baird, J. Hartup, A. Malfitani, S.
Lindfield, and many more. SJR was generously supported by; the Association for Marine
Exploration (AME), the Systematic Research Fund (SRF) as supported by the Linnean Society
of London and the Systematics Association, Ocean First Education (formerly Ocean
Classrooms), and the Edmondson Foundation of the Bishop Museum. Poseidon/Cis-Lunar
Technology, and Valeport Ltd., UK generously provided technical equipment support. RRC
would like to thank the Seaver Foundation for their generous support in 2014. MKD would like
to thank the Our World Underwater Scholarship Society, the society’s sponsors and Rolex for
her funding and support during Pohnpei 2017.
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34
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