A Diver's Guide to Japan's Nudibranchs & Sea Slugs

Introduction

Japan is one of the world's most densely cataloged regions for nudibranch diving. Across its ~3,000 km from north to south, this site catalogs 2,094 sea slug species (nudibranchs and their relatives) — from cold-water specialists under Hokkaido's drift ice to subtropical reef nudibranchs in Okinawa. Cold, temperate, subtropical, and tropical faunas blend along a north-to-south gradient, and depending on the region, you can find active diving at almost any time of year.

The country's taxonomic legacy is exceptional: Dr. Kikutaro Baba conducted pre-war monographic field work across multiple regions including Sagami Bay, Kii Peninsula, Amakusa, Akkeshi (Hokkaido), and Toyama, and Japanese sea slug taxonomy has remained world-class ever since.

This guide groups Japan's dive destinations into 8 regions, with characteristics, seasonality, signature species, and local operator information for each — built for international divers planning a Japan trip.

Year-round diving — when to come

Japan offers diving year-round if you choose the right region.

Reduced window: Right after autumn typhoons, conditions can stay rough for a few days to two weeks. Autumn overall tends to be the slowest. Check conditions before booking.

Monthly observation distributions per area are available on each area page.

Regional Diving Guide — 8 Regions

Each region links to per-area pages with observation data, popular species, sponsor dive operators, water temperature, suits, and access info.

Quick comparison

1. Hokkaido

Cold-water endemics and Japan's only drift-ice diving (late January–early March, Shiretoko). Drift-ice diving requires a drysuit and a specialist operator — book ahead via the Hokkaido area page.

• Shakotan 53 species
• Rausu 25 species (Shiretoko)
• Hakodate-Usujiri 12 species

Peak: March–May.

For water temperature, suit, and access details, see the Hokkaido area page.

2. Hokuriku — Fukui, Toyama, Ishikawa, Niigata

Sea of Japan coast, home to cold-water specialists and Japan-endemic species. Many sites close in winter due to rough conditions — not divable year-round.

• Echizen 193 species (Fukui)
• Etchu-Miyazaki 70 species (Toyama)
• Oyashirazu 59 species (Niigata)

Peak: March–May.

For water temperature, suit, and access details, see the per-prefecture area pages (Fukui / Toyama / Ishikawa / Niigata).

3. Central Honshu — Izu Peninsula & Sagami Bay

The hub for Tokyo-based divers. Excellent access, and among the highest species and observation counts in Japan.

• Osezaki 653 species (Western Izu)
• Jogashima 504 species (Sagami Bay)
• Tago / Ukishima / Dogashima 483 species (Western Izu)
• Usami 468 species (Eastern Izu)
• Kumomi 453 species (Western Izu)

Peak: November–July.

For water temperature, suit, and access details, see the Shizuoka / Kanagawa area pages.

4. Izu & Ogasawara Islands

Volcanic islands in the open Pacific, struck directly by the Kuroshio. Oceanic-island fauna shaped by warm-current isolation. Ogasawara is a UNESCO World Natural Heritage Site.

• Hachijojima 620 species
• Izu Oshima 271 species
• Ogasawara 236 species
• Miyakejima 187 species

Peak: March–November (divable year-round).

For water temperature, suit, and access details, see the per-area pages (Hachijojima / Izu Oshima / Ogasawara / Miyakejima).

5. Kii Peninsula — Kushimoto

The southernmost reach of Honshu in the Kansai region, under direct Kuroshio influence year-round. Combines Honshu's largest table-coral reef with high sea slug diversity.

• Nanki Kushimoto 438 species
• Shirahama 123 species
• Minabe 107 species
• Koza 81 species

Peak: November–June.

For water temperature, suit, and access details, see the Wakayama area page.

6. Shikoku — Kochi

Kuroshio-influenced waters with southern species rarely seen on mainland Honshu.

• Kannoura 315 species (eastern Kochi)
• Muroto 180 species
• Kashiwajima 92 species (southwest tip of Kochi)
• Kashinishi 86 species

Peak: March–May.

For water temperature, suit, and access details, see the Kochi area page.

7. Kyushu — Amakusa & Amami

A bridge between Honshu and Okinawa. Both northern-limit and southern-limit species can be observed.

• Amami Oshima 533 species (Kagoshima) — mix of mainland-temperate and subtropical species
• Amakusa 466 species (Kumamoto) — inner-bay sandy-bottom sea slug fauna
• Tokunoshima 373 species (Kagoshima)
• Yakushima 167 species (Kagoshima)

Peak: January–May.

For water temperature, suit, and access details, see the per-area pages (Amakusa / Amami Oshima / Kagoshima).

8. Okinawa & Ryukyu Archipelago

Subtropical reefs, divable year-round. Indo-Pacific species dominate. Okinawa Prefecture totals 1,083 species — the largest single-region share on this site.

• Okinawa Main Island (Motobu / North) 530 species
• Okinawa Main Island (Onna / Yomitan) 504 species
• Okinawa Main Island (Chatan / South) 426 species
• Okinawa Main Island (East Coast) 407 species
• Miyakojima 588 species
• Ishigaki & Yaeyama 507 species
• Kerama Islands / Zamami 193 species

Peak: December–August (divable year-round; activity strongest winter through early summer).

For water temperature, suit, and access details, see the Okinawa area page.

Use seaslug.world to plan your dive trip

1. Reverse lookup by species: If you have a target species, search our species pages and use observation records to find regions where it has been recorded
2. Forward search by region: Each area page shows top species, seasonal distribution, water temperature, suit guidance, access info, and sponsor dive shops (including English-friendly operators)
3. Submit your own observations: Photographers visiting from outside Japan are welcome to register sightings (English submissions accepted)