Gloomy Nudibranch (Tambja morosa) アオスジリュウグウウミウシ (size 7cm)
The gills look like a crown making this black and blue nudibranch the king of gloom, I guess. Amami 2024
A slightly different angle of the Gloomy Nudibranch. This was on the reef flat by my house. Amami 2024