... And the younger ones look like this.
In the reference below, the authors place Moroteuthis japonica and Moroteuthis pacifica into synonymy with M. robusta, and then go on to place all Moroteuthis species into the genus Onykia, considering the true Onykia species to represent juvenile forms of species that belong in the same genus as what has historically been Moroteuthis. Since Onykia is the senior name, it takes precedence, if the species do all belong in the same genus.
Tsuchiya, K., and T. Okutani. 1991. Growth Stages of Moroteuthis robusta (Verrill, 1881) with the Re-evaluation of the Genus. Bulletin of Marine Science, 49(1-2):137-147.