M. grimaldii was described from a single, small (ca. 38 mm ML) and damaged specimen. Joubin described the squid as being transparent although slightly bluish, presumably because it was young although it possessed integumental photophores.
- Arm suckers with long, pointed slender teeth on distal half of inner ring, "gradually diminishing towards the sides until they disappear at the [proximal] margin" (Joubin, 1985).
- Suckers approximately 0.33 mm in diameter (Joubin, 1985).
- Missing in the holotype.
- Funnel locking-apparatus
- Judging from Joubins illustration (below), a tragus is present and an antitragus is absent.
- Tubercules absent from mantle and elsewhere.
- Rhomboidal (Joubin, 1985).
- Integumental photophores present at least on ventral mantle and dorsal margin of fins (Joubin, 1895).
- Eyelid photophore not reported by Joubin*.
- Measurements not given by Joubin.
This juvenile Mastigoteuthis is not clearly differentiated from the North Atlantic M. schmidti and M. agassizii. The eyes and eyelids, apparently, were intact (see title drawing here) and the animal was transparent. As a result, large eyelid photophores, if present, would probably have been noted by Joubin but we don't know at what size the photophores develop. In both species eyelid photophores about the same size as the integumental photophores could have been overlooked. The mantle extends to the anterior third of the fins. We have placed this species in the synonomy of M. agassizii.
The type locality is near Corvo Island in the Azores at 39°43'N, 33°22'W.