The missing link: tidal-influenced activity a likely candidate to close the migration triangle in brown shrimp Crangon crangon (Crustacea, Decapoda)

Life cycle closure for species inhabiting areas with daily varying currents but directed net water transport requires specific behavior to minimize losses due to advection of passive drifting life stages. Variations in swimming activity of different-sized Crangon crangon (15-65mm total length) were therefore monitored under constant laboratory conditions immediately after being caught in the German Wadden Sea. Activity of shrimps of different sizes, caught at different seasons, always peaked at times corresponding with ebb tide in the habitat from where they were taken. This behavior was maintained for several days if no external stimuli were present but shifted to night activity if a light-dark cycle was provided. The observed behavior/activity pattern was included in a coupled hydrodynamic and individual-based model (IBM) and the shift in the location of a shrimp cohort was monitored over time. Performance of ebb tide activity not only allowed the shrimps to reach the preferred deeper winter and spawning areas but also allowed them to migrate against the dominating current from eastern nurseries to more western located spawning areas. Passively drifting larvae released at these locations and later larval and juvenile stages that perform flood tide transport can reach the nurseries again. This links the nurseries and adult spawning grounds and closes the migration triangle.