Identification of different physiological functions within the gills and epipodites of the American lobster: Differences in metabolism, transbranchial transport, and mRNA expression

Abstract

Transbranchial transport processes are responsible for the homeostatic regulation of most essential physiological functions in aquatic crustaceans. Due to their widespread use as laboratory models, brachyuran crabs are commonly used to predict how other decapod crustaceans respond to environmental stressors including ocean acidification and warming waters. Non-brachyuran species such as the economically-valuable American lobster, Homarus americanus, possess trichobranchiate gills and epipodites that are known to be anatomically distinct from the phyllobranchiate gills of brachyurans; however, studies have yet to define their potential physiological differences. Our results indicate that the pleuro-, arthro-, and podobranch gills of the lobster are functionally homogenous and similar to the respiratory gills of brachyurans as indicated by equivalent rates of H+Eq., CO2, HCO3−, and ammonia transport and mRNA expression of related transporters and enzymes. The epipodites were found to be functionally distinct, being capable of greater individual rates of H+Eq., CO2, and ammonia transport despite mRNA transcript levels of related transporters and enzymes being only a fraction found in the gills. Collectively, mathematical estimates infer that the gills are responsible for 91% of the lobster's branchial HCO3− accumulation whereas the epipodites are responsible for 66% of branchial ammonia excretion suggesting different mechanisms exist in these tissues. Furthermore, the greater metabolic rate and amino acid catabolism in the epipodites suggest that the tissue much of the CO2 and ammonia excreted by this tissue originates intracellularly rather than systemically. These results provide evidence that non-brachyuran species must be carefully compared to brachyuran models.