The two species examined in this study: (a) an adult Histioteuthis heteropsis (ROV image), (b) a semi-transparent juvenile H. heteropsis showing the differently sized and shaped eyes, (c) the left and right side of a juvenile Stigmatoteuthis dofleini, and (d) an adult S. dofleini (ROV image).
Still frames of Histioteuthis heteropsis from in situ ROV video. (a) Examples of an adult (i) and a juvenile (ii) in the J-pose posture. (b) Sequential images (i–iii) of a ‘ratcheting’ turn in the Straight Arms posture. The sequence shows the starting position (i), twisting of the mantle relative to the head (ii), and the ending position (iii). (c) A juvenile (i) and adult (ii) with unpigmented large left lenses, and an adult (iii) with a yellow-pigmented left lens.
Histioteuthid eye and body axis orientations (left, solid white lines) relative to a vertical axis (left, dotted white lines). (a) Absolute eye orientations, with 0° indicating an eye oriented directly upward and 180° directly downward. Orientations of the larger, left eye are plotted on the right (grey) and the smaller, right eye on the left (black) to show where the eyes are directed relative to one another. (b) Absolute body axis orientations with 0° indicating a vertical, tail-up position and 180° representing a vertical, tail-down position. Observations are shaded by posture observed at first sight of the animal: J-pose (black), Straight Arms (grey), or unknown/unseen (unfilled). (c) A cross section through both eyes of a histioteuthid showing the approximate field of view for each eye (shaded) given an orientation of 45° for the large left eye and 120° for the small right eye. Adapted with permission from Young . (Online version in colour.)
Depth distributions of H. heteropsis for (a) juveniles and (b) adults. Darker portions of bars indicate true counts of individuals, while lighter portions correct for total ROV effort in each depth bin over the period studied. Black points indicate the true counts of H. heteropsis that could not be identified to life stage. Juveniles occurred significantly shallower than adults (Welch Two Sample t-test, p < 0.0001).