Animal collective behaviours are regarded as emergent properties that result from local interaction between individuals in a swarm. Then we discern between individual movements and swarm behaviour. Dynamic swarm, however, might manage both of them in parallel. Swarm of soldier crabs seemed to change individual behavioural rules in water-crossing behaviours. We conducted experiments where the regular oscillations by soldier crabs resulted from collective movements triggered by cohesion. Then we focus on effects from a collective to individuals and vice versa. We demonstrate bidirectional characteristics that swarm of soldier crabs have.