Fecundity selection depends on local environmental conditions among sympatric populations of two Amazonian stream-shrimp species

Abstract In most ectotherms, females are larger than males. Darwin suggested that this was because larger females have a fitness advantage: they had more space and resources to bear more offspring, an idea known as fecundity selection. However, evidence for a strong fecundity-size relationship among populations is contradictory and may depend strongly on local environmental conditions. Here, we tested for divergence in the fecundity-size relationship in shrimp reproductive females along small-scale environmental gradients in a tropical rainforest. We sampled 235 reproductive females of two endemic shrimp species (Macrobrachium inpa and Macrobrachium amazonensis) in 50-m stream reaches across a 64-km2 area in Central Amazonia. Total body size of individuals was measured and all eggs were counted. Our results showed that M. inpa reproductive-female body length was significantly different between stream catchments, while M. amazonensis body length was significantly associated with a stream-area gradient. Furthermore, the fecundity-size relationship for M. inpa varied along a water-velocity gradient, while that of M. amazonensis varied along a substrate-type gradient. These patterns could be explained by either local adaptation, plasticity, or both. Regardless, they show that the expression of fecundity selection varies even across relatively small spatial extents and along subtle environmental gradients within a tropical rainforest.