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 protocol to characterize the mitochondrial phenotypes in anterior (respiratory) and posterior (osmoregulatory) gills. Gill filaments of the shore crab Carcinus maenas were permeabilized with saponin. 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), corresponding to a tightly-coupled, ATP-producing mitochondrial phenotype, in line with their role in sustaining ATP-intensive activities such as osmoregulation and excretion. Our data 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 a functional division of labor within a single organ. In other words, while the two gill types have the same sized mitochondrial "engines", they differ in how that engine is tuned: posterior gills exhibit a Production (ATP-biased) mitochondrial phenotype, whereas anterior gills display a Preservation (leak-biased) mitochondrial phenotype, reflecting contrasting strategies for allocating respiratory capacity. This study provides a mechanistic framework for understanding how mitochondrial plasticity/specialization can contribute to tissue-specific responses in dynamically fluctuating marine environments.