They Aren’t Just for Counterillumination: How Ocular Photophores Illuminate Locomotory Adaptations of the Midwater Teuthoids, Chiroteuthis calyx and Galiteuthis phyllura (Mollusca: Cephalopoda)

The counterilluminating ventral ocular photophores of Galiteuthis phyllura and Chiroteuthis calyx appear to continuously orient ventrally regardless of body orientation. Because both species have a round pupil in a round eye, there is no visual method of monitoring eye movement in comparison to photophore movement. Eye tissue sections elucidate the mechanical relationship between the photophore and the eye, which is fused in both species and thus provides a visual point of reference for eye movement on an otherwise radially symmetrical eye. Using image analysis of in situ still images taken from the ROV Ventana, the angular displacement between the photophore and the body axes is derived to show the behavioral extremes of mobility in these midwater forms. Chiroteuthis calyx has an angular rotation range of +65/-102 and Galiteuthis phyllura has an angular rotation range of +71/-68. As compared to previously published laboratory-derived data of both benthic and midwater species, these data show a greater degree of flexibility and movement, indicating a lesser degree of three-dimensional movement in benthic organisms.