The porcellanid crab Petrolisthes armatus, with a known geographic distribution covering a wide range of latitudes, was selected to conduct a comparative study on egg production and reproductive output between two populations from Pacific and Atlantic coasts. Ovigerous females were collected between September and November 2005 in Punta Morales, Pacific coast of Costa Rica (n = 137) and from March 2005 to July 2006 in Araçá region, São Paulo, Brazil (n = 46). The mean size of females was statistically different between both populations with larger females from Brazil than Costa Rica (7.9 ± 1.31 and 6.6 ± 0.95 mm CW, respectively). Likewise, egg production was significantly different in both populations and started in Pacific Costa Rica at a smaller female size than in Brazil. Specimens from Pacific Costa Rica produced roughly three times more eggs than equally sized females from Brazil, while newly extruded embryos were larger in Brazil than in Costa Rica (0.045 and 0.039 mm 3 , respectively). Egg volume increased during embryogenesis by 112.8 and 164.5% in Costa Rican and Brazilian populations, respectively. The egg water content increased steadily in both populations; however, eggs produced in Brazil contained always more water than those from the same developmental stage in Costa Rica. Average reproductive output (based on dry weight) was substantially lower in Brazil (0.031) than in Costa Rica (0.065). Our results confirm an impressive intraspecific plasticity of reproductive features in an intertidal decapod. The observed phenotypical variability might be related to local environmental conditions as well as to the location of the studied population concerning its geographic range of distribution.

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Pagurus exilis (Benedict, 1892) is an endemic South Atlantic hermit crab with a distribution ranging from Rio de Janeiro State, Brazil, to Buenos Aires Province, Argentina. The present study analyzed the reproduction of two populations at the extremes of this geographical distribution, and compared their reproductive period, fecundity, and changes in egg size and egg volume during the incubation period in order to assess the variability over this latitudinal range. Hermit crabs were collected monthly over a 2‐year period. In total, 108 females were analyzed for Brazil (44 non‐ovigerous and 64 ovigerous), and 141 for Argentina (87 non‐ovigerous and 54 ovigerous). Reproduction in Brazil occurs year‐round, with peaks in the fall and winter seasons; in Argentina reproduction occurs only in spring and summer. The Brazilian ovigerous females were significantly larger than the Argentina ones (Brazil: SL = 5.33 ± 1.45 mm; Argentina: SL = 4.15 ± 0.52 mm; P < 0.001). The fecundity was 1447 ± 831 eggs (317 to 2885) in Brazil and 987 ± 711 eggs (114 to 2665) in Argentina, with a trend towards higher fecundity in Brazil. Eggs in the Argentina population were larger than those in Brazil for all the three stages investigated, and no changes in egg volume were found during egg development for both populations. The reproductive traits of the two populations showed some important differences, which may reflect adaptations to local environmental conditions, demonstrating a high plasticity of reproductive features of the species in Brazilian and Argentine waters. The strategy adopted by the Argentina population involves a lower production of larger eggs compared to the population in Brazil; this lower production is associated with reproduction in cold‐water regions.