Oxygen Dependence of Visual Physiology and Behavior in Marine Invertebrate Larvae and Its Ecological Implications

Many invertebrates undergo a planktonic larval stage, during which they have a deeper distribution during the day (to 80 m), and then ascend to the surface at night. In regions with eastern boundary currents, such as the Southern California Bight, larvae are exposed to large gradients of both oxygen and irradiance with depth in the ocean, in addition to seasonal variability. Marine larvae of visual species rely on sophisticated eyes for prey capture, predator avoidance, and vertical migration; this vision is very oxygen demanding. The early life stages of marine invertebrates can be vulnerable to changes in ocean conditions, and stress associated with ocean deoxygenation could compromise optimal visual function, fitness, and survival. This research evaluated the effects of reduced oxygen partial pressure (pO₂) on visual physiology, metabolism, and visual behavior in larvae of cephalopods and arthropods, and the potential consequences for their distributions in the ocean. A decrease in pO₂ from 21 kPa (265 μmol kg^-1; surface ocean) to 3 kPa (55 μmol kg^-1) caused retinal function to decline by 60-100% in larvae of the market squid Doryteuthis opalescens, the two-spot octopus Octopus bimaculatus, the graceful rock crab Metacarcinus gracilis, and the tuna crab Pleuroncodes planipes. Retinal function decreased at much higher pO₂ than the critical oxygen limit for metabolism (P_crit) for D. opalescens and O. bimaculatus, indicating visual effects are dissociated from general metabolic decline. Phototaxis behavior decreased with exposure to pO₂ that would cause 50% retinal function in the cephalopod larvae. Using a custom-built sensor package and hydrographic profiles taken in the Southern California Bight (CalCOFI) to measure oxygen and light, limiting conditions for vision were found during the seasonal upwelling periods and in the nearshore environment; ocean deoxygenation could further restrict larval distributions, with consequences for ecosystem health and fisheries.