This is a Japanese Devilray Mobula mobular, not a manta. The North Pacific form of this species (which this individual is) has long been called Mobula japanica but has recently been lumped with the Atlantic taxon M. mobular.
@DaLilFishie Thanks. I was going to look up what species it was later but never got round to it. I saw this species again just recently as well at the Okinawa Churaumi Aquarium (along with about three or four other manta / devilfish species).
@DaLilFishie I don't recall - I had a bad visit to this aquarium! But at Okinawa (the best aquarium by a long way that I visited while in Japan) I saw alfredi, birostris, mobular and thurstoni.