Abstract

Reproductive output (RO) is commonly used as a proxy for reproductive effort (RE) among Brachyuran crabs and other marine invertebrates, and so far no attempt has been made to incorporate the cost of brooding into RE to obtain a more reliable estimate of reproductive investment, nor to compare it along a temperature (latitudinal) gradient. We compared RO among 3 sites (ca. 20, 30 and 40°S, reflecting mean annual sea-surface temperatures of 18, 14 and 10°C, respectively), and the cost of brooding (oxygen provision) at those temperatures, in order to obtain a more reliable pattern of reproductive investment along an extended latitudinal gradient. A total of 187 brooding females of Cancer setosus were collected to estimate RO. Fecundity, volume and weight of the embryos, and embryo losses were also estimated. Laboratory experiments were carried out at the characteristic temperatures of those sites (acclimating females from central Chile) to determine (1) patterns of oxygen provision to the embryos, (2) patterns of brooding behavior, and (3) to quantify the cost of embryo ventilation. Optic fibers, video recording and respiration chambers were used to meet these goals. Results showed that (1) RO, (2) fecundity, and (3) volume and weight of embryos varied with latitude, and that temperature affects (1) period of embryo ventilation (oxygen provision) events, (2) flapping frequency, (3) embryo losses throughout the brooding period, and (4) cost of embryo ventilation per unit of time. While RO increased with latitude, the cost of embryo ventilation decreased with temperature, suggesting a trade-off between investment in eggs and the cost of providing oxygen to the embryos at different temperatures (latitudes). This pattern may not be exclusive to Brachyuran crabs, but may also apply to other marine invertebrates since oxygen limitation during early development occurs in other brooding species.