Supply and demand: Mass scaling exponents of metabolism and gill surface area in two tropical decapod crustaceans.

An animal's metabolism is a vital trait for understanding their lifestyle and behavior in their environment. Metabolism scales with mass with an exponent of approximately ¾ for whole organisms across a variety of taxa, but this value alone cannot contextualize whether the animal is being constrained by morphological features. For water-breathing ectotherms, the balance between metabolic demand and supply is vital due to the low concentration of oxygen in water compared to air. Here we calculated the size dependence of the ratio of metabolic rate to gill surface area (the S metric) for both standard (SMR) and maximum (MMR) metabolic rate for two crustaceans: the Caribbean king crab (Maguimithrax spinosissimus) and Caribbean spiny lobster (Panulirus argus). The S metric scaling exponents for SMR and MMR were 0.11 and 0.17, and - 0.003 and - 0.175 for P. argus and M. spinosissimus, respectively. All S metric scaling exponents were not significantly different from 0, indicating that the gills may not be limiting metabolic demand for either species, although these results alone cannot definitely determine oxygen limitation. Crabs had a more negative S metric exponent during MMR suggesting that as size increases, the balance of metabolic supply and demand could become more constrained with a larger mass. It is important to understand if metabolic demand is being constrained by respiratory surface area as it can better contextualize the behavioral mechanisms these animals use in their environment.