Full time mothers: daily rhythms in brooding and nonbrooding behaviors of Brachyuran crabs

Abstract

Daily rhythms in activity have been reported for marine and terrestrial organisms, including brooding behaviors which supply oxygen to the embryos of marine invertebrates. Considering that oxygen is a limiting factor in embryo masses of Brachyuran crabs and that female crabs play a critical role in supplying oxygen to the embryos, we studied and compared daily patterns of (1) brooding and nonbrooding behaviors in brooding females, (2) behaviors in nonbrooding females, and (3) oxygen provision to the embryos in a Xanthid crab ( Homalaspis plana). We also experimentally evaluated whether daily patterns of nonbrooding behaviors were related to food intake. Behaviors were identified using video recordings and correlated with oxygen analyses in the center of the egg masses by use of microoptodes. According to PO 2 recordings abdominal flapping was identified as the single, most important behavior supplying oxygen to the center of the egg mass, maintained at a constant rate during both day and night. Furthermore, the lack of a daily pattern in oxygen availability in the center of the embryo mass is consistent with the lack of daily pattern in abdominal flapping. In contrast, locomotor activity (walking) and maxilliped movements remained unrelated to PO 2 oscillations and showed a pronounced diurnal pattern, increasing during the night. This strong diurnal rhythmicity in walking behavior decreased as embryos developed. The frequency of locomotor activity and egg ventilation increased as embryo development progressed. The increase in egg ventilation throughout embryo development may be a response to the increase in embryonic oxygen demand during development. The change in locomotor activity was unrelated to feeding activity, but may serve to make the ventilation process more efficient. These findings emphasize that oxygen is a crucial factor during early development, affecting the normal rate of development of embryos. In consequence, females permanently provide oxygen to the brood despite daily cycles in other behaviors.