Brooding behaviour and cost of brooding in small body size brachyuran crabs

Abstract

Constraints in the capacity to brood, specifically supplying oxygen to aggregated embryo masses, is one of the mechanisms invoked to explain the association between small body size and brooding exhibited by marine invertebrates. We hypothesized that brooding costs may affect the capacity to supply oxygen to the brood, therefore constraining brooding to small body size. We ana- lyzed female brooding behaviour and patterns of oxygen availability in embryo masses of females carrying early and late stage embryos in 2 species of small size crabs: Pisoides edwardsi and Acan- thocyclus gayi. These species are between 3 and 10 times smaller than previously studied crab spe- cies. The costs of incubation were estimated by comparing oxygen consumption of nonbrooding and brooding females in each crab species. The same brooding behaviours reported for large body size species were performed by the 2 small species studied. However, behavioural patterns throughout embryo development were different between small and large body size species. Oxygen availability (% air saturation) never showed limitations in embryo aggregations in the 2 small crab species, in contrast to results from previous studies on large crabs. Another clear difference between brooders with small and large body size is the cost of oxygen provision, as no differences in oxygen consump- tion of brooding and nonbrooding females were found in the 2 small species. In contrast, oxygen con- sumption of brooding females is twice that of nonbrooding females in crabs of large body size. Our results provide direct evidence for the effects of adult size on the costs of providing oxygen to the brood, suggesting that brooding at large body sizes may not be related to the capacity to hold a brood, but to the capacity to provide oxygen to the embryos, and to the associated cost. Further, we discuss the effect of embryo size on female brooding behaviour and brooding costs.