Motion cues tune social influence in shoaling fish.

Social interactions have important consequences for individual fitness. Collective actions, however, are notoriously context-dependent and identifying how animals rapidly weigh the actions of others despite environmental uncertainty remains a fundamental challenge in biology. By exposing zebrafish (Danio rerio) to virtual fish silhouettes in a maze we isolated how the relative strength of a visual feature guides individual directional decisions and, subsequently, tunes social influence. We varied the relative speed and coherency with which a portion of silhouettes adopted a direction (leader/distractor ratio) and established that solitary zebrafish display a robust optomotor response to follow leader silhouettes that moved much faster than their distractors, regardless of stimulus coherency. Although recruitment time decreased as a power law of zebrafish group size, individual decision times retained a speed-accuracy trade-off, suggesting a benefit to smaller group sizes in collective decision-making. Directional accuracy improved regardless of group size in the presence of the faster moving leader silhouettes, but without these stimuli zebrafish directional decisions followed a democratic majority rule. Our results show that a large difference in movement speeds can guide directional decisions within groups, thereby providing individuals with a rapid and adaptive means of evaluating social information in the face of uncertainty.

Platelet activation and erythrocyte lysis during brief exposure of blood to pathophysiological shear stress in vitro.

BACKGROUND: Interaction of von Willebrand factor (VWF) with circulating platelets is the trigger for thrombosis in a region of arterial stenosis. These events are typically studied in vitro under conditions where platelets adhere to a VWF-coated surface. Our approach assesses platelet responses in the absence of adhesion. OBJECTIVE: To characterize extent of platelet activation and erythrocyte lysis in an artificial stenosis model. METHODS: Whole blood is perfused through a length of polyetheretherketone tubing that includes an artificial stenosis, comprising narrow-bore (89-381 µm) tubing. Secretion of [14C] serotonin and hemoglobin release was measured to evaluate platelet activation and hemolysis respectively at various perfusion rates and different stenosis dimensions. RESULTS: Platelet activation and erythrocyte lysis increased progressively with increasing perfusion rate and decreasing stenosis diameter; the length of the stenosis had negligible influence. Modest platelet activation (5-10% secretion of [14C] serotonin) occurred without significant erythrocyte lysis under a limited range of perfusion conditions (4-6 mL/min flow through a 127 µm stenosis). CONCLUSIONS: Our experimental approach mimics conditions in severe arterial stenosis or a mechanical heart valve. It could be a valuable aid in the development of novel drugs to treat arterial thrombosis and in the design of heart valves.