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Kerama mesophotic (2021)

about Kerama mesophotic

Figure 1. (a) The FIFISH V6 Plus and water sampler used in the initial experiments. The sampler was mounted on the top of the drone and was capable of collecting approximately 0.5 L of seawater. (b, c) Researchers monitoring the onboard display while assessing whether the underwater drone could be controlled in fast-flowing mesophotic environments. The drone and the vessel were connected by a 150 m cable. (d) Water sampling was performed using a syringe-based mechanism. Successful sampling was confirmed by the release of air bubbles originally trapped inside the sampler, demonstrating that seawater could be collected approximately 1 m above the target mesophotic coral reef. (e) Transferring collected seawater from the sampler into a plastic container aboard the vessel.

The waters of the Kerama Islands are renowned for their exceptional clarity and are often referred to as “Kerama Blue.” Although ocean currents vary among locations within the archipelago, they are generally strong enough to provide an ideal opportunity to test whether an underwater drone equipped with a water sampler can be controlled from a vessel and used to collect seawater from mesophotic coral reefs. To evaluate this possibility, we first conducted trials using two FIFISH V6 Plus mini underwater robots—one equipped with the water sampler for sample collection and the other used to observe the movements of the sampling vehicle (Figure 1). The sampler operates using a syringe-based mechanism, and successful collection is indicated by the release of air bubbles from inside the sampler (KE-Video1–4). These experiments demonstrated that the system was capable of collecting seawater from mesophotic coral reef environments. The current system has since been upgraded to the FIFISH W6 Plus platform, together with improvements to both the sampling device and battery performance. For details regarding the development and refinement of these research technologies, please refer to Okada et al. (2024), listed in the related publications below.

We then collected seawater samples from 24 sites (six regions) in the Kerama Islands, ranging from the surface to depths of 80 m, in order to investigate whether coral environmental DNA could be detected and whether coral community composition changes with depth.

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Reference(s)

Nishitsuji, K., Nagahama, S., Narisoko, H., Shimada, K., Okada, N., Shimizu, Y., Satoh, N. Possible monitoring of mesophotic scleractinian corals using an underwater mini-ROV to sample coral eDNA , R. Soc. Open Sci. 11: 221586. , 2024 . [Link]
== 沖縄・座間味諸島周辺の水深20〜80mでmini-ROVにより採取した海水のeDNAメタバーコーディング解析を行い、少量（約0.5L）の海水から属レベルで中深度サンゴ群集（MCEs）を把握できる可能性を示しました。
（OISTニュース: サンゴ調査に革命を：水中ドローンで准深海の環境DNA解析が可能に）
岡田修宏・清水悠紀・長濱晋一郎・西辻光希・成底晴日・佐藤矩行水中ドローンと環境DNA法を駆使した準深海サンゴ生態系の探索 , 日本サンゴ礁学会誌　26：81−88 , 2024

Point Number	Point Name	Latitude/Longitude	Sampling Depth	Dominant Genus	Percentage
A0	A. Kuba West SF	26.17423, 127.22865	0 m	Acropora (ミドリイシ属)	69.69 %
A40	A. Kuba West 40m	26.17423, 127.22865	40 m	Acropora (ミドリイシ属)	54.23 %
A50	A. Kuba West 50m	26.17423, 127.22865	50 m	not examined
B0	B. Kuba Northwest SF	26.18219, 127.233	0 m	Acropora (ミドリイシ属)	85.34 %
B9	B. Kuba Northwest 9m	26.18219, 127.233	9 m	Acropora (ミドリイシ属)	46.16 %
B20	B. Kuba Northwest 20m	26.18219, 127.233	20 m	Hydnophora (イボサンゴ属)	25.26 %
B28	B. Kuba Northwest 28m	26.18219, 127.233	28 m	Fungia (シタザラクサビライシ属)	52.09 %
B40	B. Kuba Northwest 40m	26.18219, 127.233	40 m	Dipsastraea (キクメイシ属)	71.09 %
B50	B. Kuba Northwest 50m	26.18219, 127.233	50 m	Anacropora (トゲミドリイシ属) / Montipora (コモンサンゴ属)	99.89 %
C0	C. Kitahama Beach SF	26.20615, 127.28625	0 m	Acropora (ミドリイシ属)	89.58 %
C15	C. Kitahama Beach 15m	26.20615, 127.28625	15 m	Acropora (ミドリイシ属)	52.68 %
C28	C. Kitahama Beach 28m	26.20838, 127.28927	28 m	Leptoria (ナガレサンゴ属) / Platygyra (ノウサンゴ属)	50.07 %
C42	C. Kitahama Beach 42m	26.20804, 127.29432	42 m	Acropora (ミドリイシ属)	74.23 %
D0	D. Kerama West SF	26.23519, 127.33255	0 m	Acropora (ミドリイシ属)	82.36 %
D20	D. Kerama West 20m	26.23537, 127.33365	20 m	Acropora (ミドリイシ属)	53.27 %
D40	D. Kerama West 40m	26.2355, 127.34677	40 m	not examined
E0	E. Shiru East SF	26.21689, 127.31063	0 m	Acropora (ミドリイシ属)	72.51 %
E20	E. Shiru East 20m	26.21694, 127.31143	20 m	Porites (ハマサンゴ属)	78.61 %
E41	E. Shiru East 41m	26.21708, 127.31375	41 m	Pachyseris (リュウモンサンゴ属)	67.18 %
E50	E. Shiru East 50m	26.21113, 127.32035	50 m	Porites (ハマサンゴ属)	44.32 %
F0	F. Jijigatama SF	26.2231, 127.25013	0 m	Acropora (ミドリイシ属)	37.92 %
F20	F. Jijigatama 20m	26.2231, 127.25013	20 m	Porites (ハマサンゴ属)	58.66 %
F41	F. Jijigatama 41m	26.22145, 127.25472	41 m	Acropora (ミドリイシ属)	76.61 %
F80	F. Jijigatama 80m	26.23482, 127.24683	80 m	Porites (ハマサンゴ属)	100 %

eDNA Analysis-Based Top 10 Dominant Genera

Note

As described above, the combination of an underwater drone equipped with a water sampler and environmental DNA (eDNA) analysis provided a promising approach for exploring mesophotic coral reefs. We therefore collected seawater samples from 24 sites across six regions of the Kerama Islands, ranging from the surface to depths of 80 m, to investigate whether coral eDNA could be detected and whether coral community composition changes with depth. Coral eDNA ZOTU values were successfully obtained from 23 of the 24 sites (sampling itself could not be conducted at Kuba West 50 m because of rough sea conditions). At the single site where coral DNA was not detected (Kerama West 40 m), examination of the drone footage recorded during sampling revealed that the seafloor was covered only by algae and lacked corals. These results gave us confidence that this method can be effectively applied to eDNA analyses of mesophotic coral reefs.

Although the system was limited to collecting only a single water sample during each survey, we nevertheless obtained evidence suggesting that coral community composition may change with depth at certain locations. One such example was the Kuba Northwest site. In shallow water, staghorn corals (Acropora) were the dominant genus. However, as depth increased, the proportion of Acropora decreased, while the mushroom coral genus Fungia and the brain coral genus Dipsastraea became dominant. The Kuba Northwest site faces the open ocean and experiences strong currents, and even this single example suggests that the dominance of coral genera may vary with depth. We hope to conduct follow-up surveys in the future to further investigate this pattern.

In general, the beautiful coral reefs of the Kerama Islands are distributed across relatively shallow rocky substrates. Beyond depths of approximately 35 m, these reefs gradually transition into sandy bottoms, and the common impression is that coral colonies become increasingly scarce and eventually disappear as depth increases further. We therefore sought to determine how deep corals occur in these exceptionally clear waters, including not only stony corals (Hexacorallia) but also so-called soft corals (Octocorallia), with a target depth of 100 m. In practice, simply locating areas within the Kerama Islands where depths of 100 m could be reached proved challenging.

After several exploratory surveys, we succeeded in discovering a soft-coral community dominated by sea fans and gorgonians at a depth of approximately 100 m off the southeastern coast of Kuba Island. Water currents at the site were extremely strong, making it difficult to keep the drone camera stationary. We believe that this discovery would not have been possible without the exceptional skills of drone operator Yuki Shimizu. In recognition of her contribution, we named the site “Yuki Reef.” This represents one of the few documented cases in the Ryukyu Islands where corals, including soft corals, have been investigated at depths approaching 100 m.

ミドリイシ属 ( Acropora ) Anacropora_Montipora

トゲミドリイシ属 ( Anacropora ) / コモンサンゴ属 ( Montipora )

キクメイシ属 ( Dipsastraea ) Fungia

シタザラクサビライシ属 ( Fungia )

イボサンゴ属 ( Hydnophora ) Leptoria_Platygyra

ナガレサンゴ属 ( Leptoria ) / ノウサンゴ属 ( Platygyra )

リュウモンサンゴ属 ( Pachyseris ) Porites

ハマサンゴ属 ( Porites )