Successful management of fishery resources is dependent on understanding the exploited species' reproductive potential, which, in turn, requires spatially relevant estimates of the size at maturity and the size–fecundity relationship. This information is used by fisheries managers to implement regional maturity-based legal landings sizes and seasonal fishery closures aimed at protecting the spawning stock and maintaining the reproductive output of the fished population at a sustainable level, despite exploitation. Crab fisheries in particular depend on such information, which is often limited, even in long-established fisheries. To address this issue, size–fecundity relationships were obtained for two commercially exploited crabs in waters around the Shetland Islands (60°N 01°W). These species were Cancer pagurus and Necora puber. A statistically significant positive size–fecundity relationship was found for C. pagurus which describes a mean reproduction potential ranging from ~0.78 million eggs in females of 129 mm carapace width (CW) to ~2.4 million eggs in females of 212 mm CW. The positive size–fecundity relationship found for N. puber was not statistically significant, but describes a mean reproduction potential ranging from ~0.16 million eggs in females of 56 mm CW to ~0.19 million eggs in females of 96 mm CW. Morphometric assessments of reproductive capacity in C. pagurus support the notion that positive size–fecundity relationships are attributable to the combined capacities of specific morphological features, each of which contributes to a female's spawning output ability. Statistically significant positive linear relationships were found between the natural logarithms of crab size (CW) and pleopod capacity, sperm plug mass, spermathecae capacity and gonad capacity. Respectively, these structures are crucial for successful egg attachment and incubation, sperm retention and protection, sperm storage and oogenesis.