In hearts of higher invertebrates as well as vertebrates, the work performed by the ventricle is a function of both rate and contractility.Decapod crustaceans experience a hypoxia induced bradycardia that is thought to result in an overall reduction in cardiac work; however, this hypothesis has not yet been tested and is the primary purpose of this study.In the grass shrimp (Palaemonetes pugio) cardiac pressure and area data were obtained simultaneously and in vivo, under normoxic (20.2 kPa O2) and hypoxic (6.8 or 2.2 kPa O2) conditions and integrated to generate pressure-area (P-A) loops.