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Amami Guntō NP, Kagoshima
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At about 70 hectares, Sumiyo is Japan’s second-largest mangrove forest, after Iriomote’s. It spreads where the Sumiyo and Yakugachi rivers meet the sea.
Tiny soldier crabs sometimes gather in groups of hundreds, sift organic matter from sand, and leave little pellets behind. Unlike most crabs, they walk straight ahead—and spiral-burrow when startled.
At low tide, a mudskipper can sit on exposed mud because it stores water in its mouth, keeping its gills supplied with oxygen while it’s out of the water.
In Sumiyo, fishers’ wives turn local catch—including assorted fish once rarely used—into tsukiage, island-style fried fish cakes. Their group was founded in 2006.
The mangrove’s breathing roots shelter young Ryukyu ayu from birds and larger fish. This island fish’s wild population survives on Amami Ōshima; it disappeared from Okinawa Island in the 1970s.
Sumiyo is a northern outpost for mangroves: Japan’s Environment Ministry identifies the local trees as species growing near the northern limit of their global range.
Some Sumiyo mangroves begin growing before they fall: their seeds sprout on the parent tree and develop for about a year, then drop as young plants ready to take root.
The mangroves are fed by the mountains: the Sumiyo and Yakugachi rivers carry soil and nutrients from Amami’s subtropical rainforest down to the estuary.
A 3-day low-tide survey recorded 21 kinds of snails and crustaceans here. One leaf-litter snail, classified as Endangered I in Kagoshima, was found at every sampled mangrove site and recorded in the prefecture only on Amami Ōshima.
Conservation reaches upstream: in rivers connected to the estuary, people have loosened compact spawning gravel and improved fish passage to help sea-going Ryukyu ayu complete their life cycle.
A male Okinawa white fiddler crab does its eating with the small claw, not the oversized one. At Sumiyo, these crabs emerge from their mud burrows when the tide goes out.
