It is usually easy to determine the gender of most pet crabs by considering their colors, claws and other anatomical characteristics. Scientists can look for internal differences to determine gender, but that's not feasible for hobbyists. However, as captive propagation is beyond the skills of most hobbyists and the husbandry of both sexes is identical, determining the gender of your pet is not a necessity.
The males and females of some crab species bear different colors -- especially on their claws. For example, red-clawed crab (Perisesarma bidens) males have larger, redder claws than females do. This sexually dimorphic trait is also common among several commercially harvested food species, such as blue crabs. Blue crab (Callinectes sapidus) males have blue claws, while the females' claws bear red tips. Male Halloween crabs (Gecarcinus quadratus) are usually more brightly colored than females are and appear red rather than orange.
Almost without exception, male crabs attain larger sizes than female crabs do. This happens because females have to devote more resources to reproduction than males, which reduces the available resources for growth. While this is not helpful for determining the gender of immature animals, it is safe to identify exceptionally large specimens as males. Despite these differences in overall size, females often have abdomens that are larger than those of similar-size males because females carry unfertilized eggs internally.
The males of most crab species have larger claws than females do, although the difference is often subtle. Presumably, this adaptation enhances the males' ability to fight with conspecifics for breeding rights or territory. Additionally, the males of some species wave their claws as a form of sexual advertisement. The males of some species, such as fiddler crabs (Uca sp.), feature only one enlarged claw each.
If you gently flip your crab on its back, you will see a triangular section of the shell that points toward the crab's head. Called the telson, this part of the shell contains the reproductive organs of the crabs. In most species, males have elongated V-shaped telsons and females have larger, broader telsons. While this criterion is very helpful for determining many commercially harvested species, the difference can be subtle. For example, the telson shape of Halloween crabs does not differ as markedly as occurs in some other species.
Although they are close cousins to true crabs, hermit crabs actually form the superfamily Paguroidea. Accordingly, they are anatomically different from true crabs. It is theoretically easy to distinguish male hermit crabs from female hermit crabs. Females have two small holes located on the first segment of their last pair of walking legs, called gonopores; their presence confirm a specimen is a female. However, hermit crabs are usually reluctant to emerge from their shells far enough for you to search for the gonopores. Never attempt to extract your hermit crab from his shell, as doing so may cause serious injury or death. Instead, be patient and wait for the crab to change shells, at which time you can easily investigate the legs.