Marie-Hélène Carignan

Developing a respirometry method for marine invertebrates: a critical step toward assessing physiological plasticity to tropical hypoxia

Marie-Hélène Carignan, Noelle M. Lucey, Mary Collins, Fanny Vermandele, and Piero Calosi

Tropical marine environments are commonly considered stable environments and shelter a great biodiversity of invertebrates’ species. Although they are not expected to undergo the greatest changes in temperature, other parameters will be significantly altered by anthropogenic global change, such as a decrease ocean’s oxygen saturation to hypoxic levels. For instance, Almirante Bay in Panama already experiences frequent and periodic events of extreme hypoxia likely associated with nutrient loading and global warming. The ecological effects of these hypoxic events are beginning to be documented, but no study has yet investigated the role of species’ physiological tolerance in the observed patterns of distribution. Species’ tolerance can be supported by various mechanisms, particularly physiological plasticity which is the ability to alter cells’ metabolism in response to a changing environment. As such, the metabolic rate, defined as the turnover of energy within organisms, can serve as a proxy for an individual’s physiological state when facing suboptimal environmental conditions. Consequently, we developed a method to measure oxygen consumption in two species of brittle stars (Ophiuroidea) occupying dissimilar ranges in Almirante Bay, Ophioderma cinereum and Ophiocoma wendtii, to assess their plasticity when facing severe hypoxic events. We adapted a closed system respirometry method using a non-invasive optical system to determine brittle stars oxygen consumption. To assess species’ metabolic state, we developed a protocol to measure both Routine and Maximum Metabolic rate, in order to determine their Aerobic Scope. Finally, we successfully compared metabolic responses under control and hypoxic conditions to define their level of physiological plasticity.