A Pulsing-Mirror Eye in a Deep-Sea Ostracod Parker, Andrew R. Records of the Australian Museum 2023 2023-12-06 75 4 515 517 84NSL Muller 1895 [314,518,749,780] Ostracoda Cypridinidae Gigantocypris Animalia Myodocopida 1 516 Arthropoda genus  Studies of the eye of  Gigantocypristo date have considered only preserved specimens, and their optical apparatus. However, an examination of a whole, preserved animal led to the discovery of four large muscles behind each eye, attached to the near-lateral edges of the reflector, i.e., behind the parabolic part ( Fig. 1). These muscles provided evidence that the mirrors move, prompting an examination of living specimens. In 1999, living specimens of  Gigantocyprissp.were collected by a mid-water trawl off the Cape VerdeIslands during RRS DiscoveryCruise 243. Video recordings were made of several specimens free-swimming in a kreisel tank, including close-ups showing detail of their large eyes. In these recordings, from anterior and dorsal views, the parabolic mirrors of the eyes were observed to flex and pulse. In a resting specimen ( Fig. 2), the eyes could be magnified and observed in detail: the parabolic parts of the mirrors were measured to flex back to a maximum position as shown in Fig. 3Band pulse regularly at a rate of 0.5 cycles per second (n = 28 cycles). The spherical part of the mirrors, in the dorso-ventral (“vertical”) plane, was not observed to move. Ray tracing calculations revealed that when the luminous object is far, the oscillations of the parabolic reflector cause the object to go in and out of focus at the retina, as the reflector is relaxed then “flattened” ( Fig. 3A, B). However, when the luminous object is nearby, the oscillations of the parabolic reflector cause little change to the image focused on the retina ( Fig. 3C, D). This principle was confirmed using a model flexible, parabolic mirror and a laser. Therefore, during a pulse cycle of the retina, a light source nearby will remain detected by the ostracod (appearing always “on”), whereas a light source far away will appear to turn on and off twice per second. The latter light will appear to flicker; a flickering light is more conspicuous than a steady light (Haamedi & Djamgoz, 1996) and hence a distant predator will appear particularly perceptible. In conclusion,  Gigantocyprissp.can distinguish its prey within a field of bioluminescent light sources, while probably requiring less information processing than for rigid lens typeeyes. Such a “pulsing mirror eye” functions in a radically different way to any other eye. Since this eye typeis not evident from preserved specimens, other species with parabolic reflecting eyes, such as the deep-sea amphipod  Scypholanceola(from a similar environment), should be re-assessed while alive. On another note, the transparent window in the carapace of  Macrocypridina castanea(Parker et al., 2019; 2021), was found to have applications in commerce. In a similar manner, examination of the submicron structure of the  Gigantocyprismirror, particularly how it withstands continuous flexing to maintain a flawless mirror, may be relevant to the mirror of the Hubble telescope—a comparable imaging system whose mirror does develop flaws over time.