"Slow" marine animals show their secret life under high magnification. Corals and sponges are very mobile creatures, but their motion is only detectable at different time scales compared to ours and requires time lapses to be seen. These animals build coral reefs and play crucial roles in the biosphere, yet we know almost nothing about their daily lives.
Swimming crabs are characterized by the flattening of the fifth pair of legs into broad paddles, which are used for swimming. This ability, together with their strong, sharp claws, allows many species to be fast and aggressive predators.