Influence of hypoxia on cardiac functions in the grass shrimp (Palaemonetes pugio Holthuis)

Crustaceans frequently encounter hypoxic water and have evolved a variety of compensatory mechanisms to deal with low O₂ conditions. Typically, large decapod crustaceans attempt to maintain cardiac output by increasing stroke volume to compensate for the hypoxia-induced bradycardia. Grass shrimp (Palaemonetes pugio), small hypoxic tolerant decapod crustaceans, were used to investigate cardiac responses to hypoxia in a smaller crustacean using videomicroscopy and dimensional analysis techniques. In addition, these techniques were compared to the more established dye dilution technique for calculation of cardiac output. No significant difference was found between the two methods for determining cardiac output in grass shrimp. Cardiac parameters (heart rate f(H), stroke volume V(S), and cardiac output V(b)) were monitored in grass shrimp exposed to progressive hypoxia (P(O₂) s = 20, 13.3, 10, 5.3, and 2 KPa O₂). Shrimp exhibit a cardiac response to hypoxia that is atypical when compared to larger crustaceans. Cardiac output was maintained until water P(O₂) fell below 10 KPa O₂. This maintenance of V(b) is consistent in both large and small decapods, however the mechanism differs. In grass shrimp, V(S) was P(O₂) dependent and declined significantly while f(H) increased significantly when P(O₂) was reduced to 13.3 KPa O₂.