Kaijū (song)

On August 8, 2024, it was unveiled that Sakanaction would perform the opening theme of the anime adaptation for Orb: On the Movements of the Earth.^[2] The next month, on September 5, the official trailer for the anime was uploaded, announcing the opening theme's title, "Kaijū".^[3] At the first show of the band's tour Sakanaquarium 2025: Kaijū in Sagamihara on January 25, 2025, the band's vocalist, guitarist, and songwriter Ichiro Yamaguchi revealed that the song was just completed the day before the concert.^[4] The song was released on February 20, 2025; the band's first anime-affiliated work in their career and their first music in three years since "Shock!" in 2022.^[5]

According to Yamaguchi, "Kaijū" is focused on "how to make [the song] as abstract and persuasive as possible" rather than "easy-to-understand, concrete music,"^[6] and mixed Sakanaction's musical style with the worldview of Orb: On the Movements of the Earth.^[7]

In Japan, "Kaijū" debuted at number two on the Oricon Combined Singles Chart for the date issued March 3, 2025, behind Sakurazaka46's "Udagawa Generation".^[8] It also topped the Digital Singles Chart, selling 28,497 units in its first week, surpassing Sakanaction's previous record, "Wasurerarenai no", with 22,034 units.^[9] "Kaijū" entered the Billboard Japan Hot 100 on February 26, 2025, at number two, reaching 26,512 downloads and 8,284,152 streams in its first week.^[10] The song also debuted atop the Download Songs,^[11] and number three on the Streaming Songs charts,^[12] before reaching the new peak at number two the next week with 9,942,536 streams, a 120% increase, the highest among that week's top-100 songs.^[13] Additionally, "Kaijū" debuted at number one on the specific-genre Hot Animation chart.^[14]

A collaborative music video for "Kaijū" with Orb: On the Movements of the Earth was uploaded on February 20, 2025, the same day as the single release, which is available for a limited time of three months. Directed by Yusuke Tanaka, the music video features footage from the anime, from the first to 21st episode, the last episode to be aired before the video's release.^[15]

The official music video, also directed by Tanaka, premiered on March 15, following the anime's season finale.^[16] It depicts Yamaguchi carefully holding an unknown object, shaped like an egg, through a dimly lit underground metro station. Many of the people he runs into in the video try to steal the object from him, sometimes by force, or by manipulation, such as seduction and hypnosis. The video ends with Yamaguchi dropping the egg to an unidentified woman, before being pulled inside the station's wall.^[17]

Before the release, Sakanaction debuted the performance of "Kaijū" on the NHK television special Sakanaction Yamaguchi Ichiro "Chi" o Fukameru on October 28, 2024.^[18] The band performed the full-length version of the song for the first time at their concert tour Sakanaquarium 2025: Kaijū and later included it on the tour's setlist.^[19] Yamaguchi performed "Kaijū" live on his YouTube channel on February 20, shortly after the single's release.^[20]