One organ, two engines: divergent mitochondrial phenotypes in anterior and posterior crab gills

Gills are multifunctional organs that integrate respiration with homeostasis, including energy demanding processes such as osmoregulation and excretion. In osmoregulating decapod crustaceans, two spatially segregated gill types differ in function, ultrastructure and membrane composition, as well as in their responses to environmental change. Yet mitochondrial function in crab gills remains poorly characterized. Here, for the first time, we used high-resolution respirometry with a substrate-uncoupler-inhibitor titration (SUIT) protocol to characterize the mitochondrial phenotypes in anterior (respiratory) and posterior (osmoregulatory) gills. For this, gill filaments of the shore crab Carcinus maenas were permeabilized for 30 min in a saponin solution (optimized for each tissue at 25 {micro}g or 5 {micro}g saponin {middle dot} mg-1 gill fresh weight for anterior and posterior gills, respectively). Anterior gills exhibited higher leak control ratios (L/P, L/E), consistent with a leak-dominated mitochondrial phenotype that may contribute to redox balance at expense of maximal ATP yield. In contrast, posterior gills, displayed a higher phosphorylation control ratio and tighter coupling (higher Net P), reflecting a tightly-coupled, ATP-producing mitochondrial phenotype, in line with their role in sustaining ATP-intensive activities such as osmoregulation and excretion. Our results revealed that anterior and posterior gills operate as "two engines in one organ": by quantifying how each gill type partitions respiratory capacity between phosphorylation and leak pathways, this study provides a mechanistic framework for understanding how mitochondrial specialization supports functional division of labour within a single organ and contributes to physiological adaptation to dynamically fluctuating marine environments.