[4] As of 2018[update], H. japapigu is only known to be found in Japan, including Kashiwa-jima Island, Sukumo Bay; Kushimoto, Kii Peninsula; Osezaki, Izu Peninsula; the Izu Islands of Miyake and Hachijo; Sagami Bay; and Chichi-jima, Ogasawara Islands.

[2] It is the size of a jellybean, and its coloration is made for hiding in algae-covered reefs, clinging to soft corals while feeding on plankton.

[6] In 2013, after completing his PhD on the biology of the Bargibant's and Denise's pygmy seahorses, Richard Smith went to a fish biology conference in Okinawa in 2013, after which he photographed the Japanese pygmy seahorse on several dives off of Hachijo-jima, one of the Izu Islands about 180 miles from Tokyo.

[7] The species description was published by Short and colleagues in a 2018 issue of ZooKeys; it was based on one female holotype, a male and a female paratype, and two photographs of additional specimens.

The specific epithet comes from its colloquial Japanese name: Japan Pig or Japapigu.