Kashiwajima

Sukumo · Kochi Prefecture · Japan

Kashiwajima is a small, bridge-connected island off the southwestern tip of Shikoku, Japan's least-explored main island. The Kuroshio Current, the Pacific's western boundary current, sweeps warm tropical water past this rocky coastline, delivering larvae, juvenile fish, and planktonic organisms from the tropics into temperate Japanese waters. The result is a marine biodiversity anomaly, a place where anemonefish and nudibranchs from the Philippines coexist with Japanese bullhead sharks and temperate kelp forests. I arrived in late September when conditions were optimal. Water temperature was a comfortable twenty-five degrees, visibility extended to twenty metres, and the summer's tropical recruits had settled onto the reef. My guide, a quiet man who had been cataloguing Kashiwajima's nudibranchs for fifteen years, carried a slate with the morning's target list. The first dive on the island's southern reef was a masterclass in observation. Within forty minutes and twenty metres of reef, my guide had pointed out seventeen nudibranch species. Chromodoris in electric blue and orange. Nembrotha in green and black. A tiny Jorunna funebris, the panda nudibranch, on a blue sponge. Each discovery was marked on his slate with practised precision. Over two hundred species have been documented here, an extraordinary number for a single site. The deeper reef held different rewards. A pair of pygmy seahorses occupied a gorgonian fan at twenty-two metres, their camouflage so complete that even pointed out they required concentration to see. Japanese spiny lobsters retreated into crevices as my torch beam found them. The rocky substrate was covered in a mosaic of encrusting organisms, sponges, tunicates, and bryozoans creating a living tapestry. On the final dive, at the island's exposed western point where the Kuroshio hits hardest, my guide signalled towards the blue. Two scalloped hammerheads materialised at the limit of visibility, their distinctive head profiles unmistakable as they cruised past the reef edge. They were gone in seconds, swallowed by the blue, but the encounter validated Kashiwajima's claim to be one of Japan's most diverse and surprising dive destinations.