Mesophotic sea corals

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.

The first experiment using an underwater drone equipped with a water-sampling device was conducted in the highly transparent waters of the Kerama Islands, where we were able to confirm the effectiveness of this approach (see "Mesophotic Reefs – Kerama Islands"). During that survey, video footage of the reef captured by the monitoring camera aboard the vessel allowed us to roughly infer the coral genera inhabiting the mesophotic reef where the drone was deployed. eDNA analyses subsequently identified coral genera. However, without reliable taxonomic identification of the corals present, there remained some uncertainty regarding the validity of this approach. To address this issue, it was necessary to deploy the underwater drone at mesophotic reef sites where coral specialists had already confirmed and identified the resident coral populations through direct visual surveys, and then to verify that eDNA collected from those locations accurately reflected the same coral genera. Fortunately, a research team led by Dr. Saki Harii and Dr. Frederic Sinniger of the University of the Ryukyus maintained mesophotic coral research sites off Motobu, Okinawa Island. At the relatively shallow Shigeo Reef (~40 m depth), the genus Seriatopora was known to dominate, whereas at the deeper Shigeo Reef (~55 m depth), the genus Alveopora was dominant. With the cooperation of Drs. Harii and Sinniger, we conducted water-sampling surveys at these two mesophotic reefs on April 25, 2023. The underwater drone used in this study was a FIFISH W6 Plus equipped with two water samplers, each capable of collecting 500 mL of seawater (Figure 1).

An aerial view of the coast of Onna Village (Fig. 1A) shows that an “Ino” (reef flat: a shallow area enclosed by coral reefs in the Okinawan language) extends for approximately 32 km from the northeastern side of Cape Zanpa to Busena Marine Park (light blue area). The central part of the Ino near Maeganeku consists largely of sandy bottom, where mozuku seaweed aquaculture is practiced, representing one of the many benefits provided by reef ecosystems. Although the scenery varies from place to place, many areas feature a rocky reef zone at the outer edge of the Ino (visible as the light-brown area), where coral reefs typically begin. Beyond this zone, the seafloor gradually deepens, with the dark-blue area corresponding to depths of approximately 10–200 m. Farther offshore, the seabed drops steeply into the deep ocean, shown in light blue. Within the reef lagoons and rocky reef areas, one can observe large colonies of branching corals, table corals, and many other genera, including Acropora and Montipora. Furthermore, the “Blue Cave” at Cape Maeda and the coral reefs beneath Cape Manzamo are among Okinawa’s most famous diving destinations. How deep do the shallow coral reefs along the coast of Onna Village extend? To investigate this question, we conducted a series of mesophotic coral eDNA surveys, from below Cape Zanpa to Maeganeku in autumn 2022, from Maeganeku to the vicinity of Cape Manzamo in spring 2023, and from Seragaki to Busena in autumn 2023. Although the overall eDNA analysis is still ongoing, we briefly introduce results from northwestern Cape Manzamo site using footage obtained with the underwater drone.

In contrast to the west coast of central Okinawa Island, including the Onna coast, where no large islands occur nearby and the coastline is relatively open and linear, the central and southern parts of the east coast are dotted with sizeable offshore islands. From south to north, these include Kudaka, Tsuken, Hamahiga, Henza, Miyagi, and Ikei Islands. Between these islands, particularly off Henza Island, broad and relatively gentle rocky reef habitats extend across the seafloor. Unlike the west coast, where coral reefs generally continue from shallow waters to depths of around 40 m before transitioning to rubble fields and sandy bottoms at approximately 50 m depth, we hypothesized that mesophotic coral reefs might exist in this area. To investigate this possibility, surveys were conducted from June 4 to 6, 2025.