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Ishigaki Island (2025)

about Ishigaki Island

Ishigaki Island is the third largest island in the Ryukyu Archipelago, after Okinawa Island and Iriomote Island, with an area of approximately 222 km². Ishigaki is roughly pentagonal in shape, with the elongated Nosoko Peninsula and Hirakubo Peninsula extending toward the northeast. Coral reefs are well developed around the entire island, particularly in waters surrounded by Ishigaki Island and Iriomote Island to the west, known as Sekisei Lagoon (with islands such as Taketomi Island and Kohama Island scattered throughout). Highly diverse coral reefs extend across the shallow waters of Sekisei Lagoon. The Shiraho Coral Reef on the southeastern side of Ishigaki is also famous as a habitat for more than 120 coral species, including large colonies of blue coral.

This survey was conducted over four days, from September 25 to 28, 2025, to investigate coral reefs along the entire coastline of Ishigaki Island. Sekisei Lagoon is not included as a target area of this atlas because local coral reef conservation organizations publish annual reports on the lagoon (for example, the Sekisei Lagoon Portal Website).

We conducted eDNA surveys to understand the coral reefs surrounding the entire island and to determine whether differences exist in the coral genera that constitute the reefs in relation to biogeographical factors.

eDNA Analysis-Based Top 10 Dominant Genera

Note

One of the general patterns suggested by the eDNA analysis is that dominant coral genera appear to differ between the western and eastern sides of the island. On the western side (survey sites 1–7, 22–35, and 38–42), Acropora is dominant, whereas on the eastern side (sites 8–21 and 36–37), Montipora is dominant. However, as one moves toward the northern tip, Acropora becomes increasingly abundant (sites 38 and 39), and at sites after circling the tip (40 and 41), Acropora becomes the prevailing genus. Acropora is highly sensitive to seawater temperature, i.e., it is prone to bleaching under rising temperatures, whereas Montipora has higher thermal tolerance. Further investigation of this relationship is planned.

The second finding suggested by this eDNA survey is that at Shiraho Reef (sites 18 and 17), known for their high coral diversity, multiple genera such as Acropora, Montipora, Porites, Dipsastraea, and Pavona appear to coexist at nearly equal densities. In contrast, site 24 is characterized by the site-specific dominance of Porites.

Please also refer to the “Ishigaki Island” section of the Ryukyu Coral Photo Collection.

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