Upstream migration of amphidromous shrimp in the Luquillo Experimental Forest, Puerto Rico: Temporal patterns and environmental cues

Amphidromous shrimp play a large role in freshwater stream ecosystems in the neotropics. The magnitude, timing, and proximate cues directing upstream migration of post-larval shrimp has received little study despite its importance in understanding recruitment of juveniles into adult populations. I combined 6 months of field observations with controlled artificial stream experiments to: (1) quantify the upstream migration of 3 genera of amphidromous shrimp into the Espiritu Santo watershed of northeastern Puerto Rico, identifying patterns in timing and magnitude; (2) identify environmental factors that affect migratory timing; and (3) experimentally test the potential physical and chemical factors that cue migratory behavior. Migration rates of all shrimp were evenly distributed throughout the night with an average migration rate of 2,187 shrimp/day. Xiphocaris elongata, and especially Macrobrachium spp., migration rates peaked in the late summer, wet season. In contrast, Atya spp. migrants were present in large numbers throughout the entire study period. Regression models predict slightly less than 30% of the variation in Atya spp. migrations, with migration highest on dark nights and following high flow events. Artificial stream experiments indicate that Atya spp. are positively rheotactic and changes in abiotic factors, such as chemical signals may affect migration. Regression models also predict approximately 30% of the variation in Xiphocaris elongata migrations, with models explaining approximately 75% of the variation in Macrobrachium spp. migrations. Xiphocaris elongata migrate primarily on nights with low stream discharge and are also affected by abiotic factors such as chemical signals. Macrobrachium spp. migrates on clear nights in late summer when stream discharges are low. Both Atya spp. and Xiphocaris elongata post-larvae can chemically detect fish predators in artificial streams and alter their behavior to avoid predation risk.