They came without warning - strange jellyfish-like creatures that flooded our oceans and beaches by the millions. At first they seemed a mere curiosity, these alien visitors glowing faintly purple as they pulsed through the waves. Tourists gathered to marvel at their graceful forms, children delighted in their squishy texture. We should have been warier from the start. The first victim was a scuba diver, stung by the tentacles of...