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These guys are stunning! They are covered in photophores! Another distinguishing feature is that the left eye is bigger than the right eye, although on smaller specimens, it is sometimes harder to tell.
Just woke up today and thought about eye differential in Histioteuthis was a good pondering point.
Here some literature which discusses the importance of it: Function of the Dimorphic Eyes in the Midwater Squid Histioteuthis dofleini
"The squid Histioteuthis dofleini, like other members of the family
Histioteuthidae, has a large left eye and a small right eye. The large eye points in a
dorsal posterior direction while the squid typically orients at an oblique angle with
the arms downward. The large eye, as a result, points vertically upward. The small
eye appears to be directed ventrolaterally. This squid occurs primarily at depths of
500 to 700 m during the day where it is exposed to low levels of downwelling light.
Presumably the large eye utilizes this faint downwelling light while the smaller eye
utilizes bioluminescent light" - Young, 1975