The influence of reproductive state on cardiac parameters and hypoxia tolerance in the Grass Shrimp, Palaemonetes pugio

1. In many crustaceans, female reproduction represents a time of increased metabolic demand. Palaemonetes pugio are typically hypoxia tolerant; but the energetic demands of reproduction may compromise their ability to tolerate hypoxic conditions. Given the correlation between cardiac output and metabolic demand, we used cardiac output (CO) to measure differences in metabolic demand in the life-history stages of P. pugeo. 2. We hypothesized that (1) the cost of egg production would result in an increased CO for gravid females compared with non-gravid females; (2) those females that were both ovigerous and gravid would have an additional metabolic demand due to brooding behaviour (pleopod fanning) and hence an even greater CO; and (3) hypoxia tolerance would decrease with increasing reproductive demand. To test these hypotheses, we compared cardiac output across three reproductive states and at decreasing water oxygen tensions. 3. Ovigerous females had significantly greater pleopod fanning frequency than non-ovigerous females at all oxygen tensions. Additionally, ovigerous/gravid females had significantly higher cardiac output at all oxygen tensions than gravid only or non-gravid females. 4. Changes in cardiac output indicate that females became more sensitive to environmental oxygen tension with increasing reproductive demand. Non-gravid females were able to maintain cardiac output down to 15 mm Hg O-2, whereas gravid and ovigerous/gravid females maintained cardiac output down to 50 mm Hg and 75 mm Hg O-2, respectively. 5. These differences in CO suggest that metabolic demands of females change with reproductive state and, while gravid and ovigerous/gravid females appear more sensitive to low oxygen tensions, they are able to physiologically tolerate low environmental oxygen conditions.