Some crustaceans show variations of their reproductive biology within their geographical distribution, and knowledge about such variations is important for the comprehension of their reproductive adaptations. This study compared two populations of the fiddler crab Uca uruguayensis from two locations on the south-western Atlantic coast: Ubatuba Bay, São Paulo, Brazil and Samborombón Bay, Buenos Aires, Argentina. The population features analysed were the body size variation (carapace width = CW) and the size at the onset of sexual maturity (SOM) in order to test the hypothesis that the size at SOM, should be the same in relative terms (RSOM), independently of the latitudinal position. In the Brazilian population the CW ranged from 4.18 to 11.60 mm for males and 3.90 to 9.80 mm for females, and in the Argentinean population from 3.60 to 14.10 mm for males and 2.85 to 12.00 mm for females. In the Brazilian population the SOM was 7.1 (RSOM = 0.58) and 5.9 mm CW (RSOM = 0.57) for males and females, respectively, and in the Argentinean population it was 7.0 (RSOM = 0.42) and 6.75 mm CW (RSOM = 0.53) for males and females, respectively. This fact is probably related to a great plasticity in the life history features of Uca uruguayensis under different environmental conditions.

The genetic relationship between fiddler crab Uca crassipes populations from the continental coast, continental islands, and oceanic islands in the west Pacific was investigated using 1039 bp (base pairs)-long combined 12Sr-RNA–16Sr-RNA sequences and a 504-bp mitochondrial DNA control region. The combined 12Sr-RNA–16Sr-RNA sequences indicated that the Vietnamese population, located along the continental coast, and the Chichi-jima population, which is located on an oceanic island north of the Northern Mariana Islands, formed different clades than populations from the other Ryukyu Islands and Moorea Island. Conversely, the Ryukyu Islands and Moorea Island populations exhibited a close genetic relationship, although the mtDNA control region indicated significant differentiation between the Ryukyu Islands and Moorea Island populations. The isolated Vietnam and Chichi-jima populations exhibited higher genetic diversity in the control region than the other populations.

A specimen of West African fiddler crab, Uca tangeri, was collected in October 2011 on the north-east coast of Gran Canaria (Canary Islands). This species is frequent along the shores of the east Atlantic from Portugal to Angola, but is very rare in the Canary Islands. There is only a previous record from 1971, but that did not report the place where it was found in the archipelago.

This study was carried out on the behaviour of two species of sympatric fiddler crabs on Abi estuary, Bandar Abbas, Iran. Species differences in activity level, grooming, burrowing, muddy structure construction and feeding were all related to their habitat and food source. The more common Uca annulipes lives in low muddy areas along the edges of small streams while Uca sindensis lives in small patches on the higher areas with sandier sediment. At low tide, both species spent most of their time feeding, while at high tide they retreated into their burrows. Both species frequently walked while feeding and interacted aggressively. There were similarities in the feeding behaviour of these species: they fed at the same times of the tidal cycles, they were more likely to engage in courtship and mating behaviour during February–September and were least aggressive at this time. In both species, crabs excavated mud balls from their burrows and placed them away from their burrow entrances. There were, however, also differences between the species. Uca annulipes constructed mud hoods at the entrance to their burrows while U. sindensis did not; and the species differed in feeding rate.

The relative growth of the fiddler crab Uca cumulanta was studied, primarily to determine the size at the onset of sexual maturity for a mangrove population in the estuary of the Patitiba River, Paraty, Rio de Janeiro state, Brazil. The evaluation of the morphological sexual maturity of U. cumulanta was performed using the allometric technique. The relationships that most precisely indicated the size at onset of sexual maturity were carapace length (CL) vs propodus length for males and CL vs abdomen width for females. Males and females are mature at 5.25 and 4.75 mm CL, respectively. The remarkable ontogenetic changes observed in the allometric growth of the male major cheliped and the female abdomen, indicate that growth of these structures is closely connected to the timing of sexual maturity. The relative size at onset maturity obtained for this species was 0.68 and this index was compared to that seen in other species in the genus.