Balancing Body Size and Respiratory Surface: A Morphometric Analysis of the Caribbean King Crab (Maguimithrax spinosissimus) and Caribbean Spiny Lobster (Panulirus argus).

The respiratory organs of water-breathing ectotherms (WBEs), typically in the form of gills, extract oxygen from the water. The size and shape of respiratory organs are vital to the efficiency of oxygen diffusion and often unique to each species. Oxygen concentrations in water are < 1%, far lower than in the atmosphere, so WBEs have evolved gill morphologies that maximize their oxygen uptake but are also shaped by their life history and the environment. Gill surface area is related to the body mass of an individual, scaling as a power-law function whose exponent, based on theory and empirical evidence, generally lies between 0.6 and 0.9. However, nearly all estimates of gill surface area are based on 2-dimensional rather than 3-dimensional measurements, assuming that gill thickness is negligible and unimportant to respiration in accord with the physics of oxygen diffusion. This study aimed to develop methods to measure gills in three dimensions and then convert 2-dimensional gill surface area to the 3-dimensional gill surface area. The body mass and gill surface area scaling relationship was then determined for two decapod crustaceans with contrasting life histories: the Caribbean Spiny Lobster (Panulirus argus) and the Caribbean King Crab (Maguimithrax spinosissimus). We found a positive relationship between gill lamellae thickness and body mass for both species, which could be beneficial or detrimental to larger, oxygen-limited WBEs depending on the balance between oxygen diffusion through the additional surface area and the dynamics of oxygen diffusion into a larger volume. The 2-dimensional scaling exponent between gill surface area and body mass was 0.626 for lobsters and 0.779 for crabs, compared to their 3-dimensional scaling exponents (0.809 and 0.702, respectively). These are the first 2D and 3D body mass and gill surface area scaling exponents determined for these species and will set the basis for future physiological research.