The Caribbean king crab Damithrax spinosissimus, a phyletie giant and the largest crab in the western Atlantic, was used to explore the costs of brooding, in terms of reproductive investment, in marine invertebrates. Given (i) the theoretical allometry between gamete production and brooding surfaces and (ii) increased physiological costs with increased brood mass, it was expected that larger broods from larger crabs should suffer higher mortality and brood loss than smaller broods from smaller crabs. It was also expected that smaller females should allocate disproportionably more resources to egg production than larger females. In D. spinosissimus, fecundity in females carrying early and late embryos varied, respectively, between 5.170 and 26.024 eggs/crab (mean ± SD = 16.569 ± 4.899) and between 1.966 and 26.906 eggs/crab (15.147 ± 7.003). In disagreement with expectation, females did not experience brood loss during embryo development. Egg mortality in females carrying early and late eggs varied, respectively, between 0% and 69% (4.71 ± 15.37) and between 2% and 100% (27.17 ± 33.99). Also in disagreement with expectations, egg mortality did not increase disproportionably with parental female body size. Reproductive output varied between 10.45% and 43.79% (34.02% ± 7.4) of crab body dry weight. Finally, in agreement with expectations, the slope (b = 0.54 ± 0.17) of the line describing the relationship between brood and parental female dry weight was significantly less than unity. Overall, most of the results disagree with the notion that the allometry of gamete production and increased physiological costs with increased brood size explain the association between brooding and small body size in marine invertebrates. Comparative studies on the reproductive investment of brooding species pertaining to monophyletic clades with extensive disparity in body size are warranted to further our understanding about the evolution of brooding in marine invertebrates.