There are many talented creatures in the Animal Kingdom, from swimmers and flyers to species with extreme hearing and vision, but Wen-San Huang from the National Museum of Natural Science, Taiwan, wants to add rafting to the list of extraordinary animal achievements. He knew that flightless insects, and even some small mammals, can wash up on distant shores in just a matter of days, having clung to pieces of vegetation that are washed out to sea and carried over hundreds of kilometres by strong ocean currents. However, success is only assured if the plucky seafarers are capable of surviving immersion in dehydrating saltwater. Huang also realised that insects may undertake these perilous voyages at different life stages, either as eggs or as larvae. As flightless Pachyrhynchus weevils have dispersed over great distances and successfully colonised some Pacific islands, Huang and Hui-Ying Yeh tested the resilience of one member of the family to saltwater.
‘These weevils [Pachyrhynchus jitanasaius] cannot be caught using traps, so we look for them in the field and catch them in trees with nets’, says Hui-Yun Tseng, who travelled to Green Island off the coast of Taiwan with Jung-Ya Hsu to collect three pairs of the elusive beetles. Then, Yeh painstakingly collected the beetles’ minute eggs and installed them in tiny perforations in fish poison tree (Barringtonia asiatica) leaf veins to develop, first into larvae and then into adults. Once Yeh had established the P. jitanasaius colony in the laboratory, she immersed all three beetle life stages in water – ranging from fresh to brackish (16.5‰ salinity) and full seawater (35‰ salinity) – for 3, 7 and 14 days to see how the insects faired. Sadly, none of the adults survived for more than 1.5 days – even in freshwater – with most dying within the first 24 h, making it unlikely that the adults could endure such perilous crossings. However, the larvae were hardier – 25% surviving 7 days of immersion in seawater – and many went on to develop successfully into adults. But the weevil eggs were the most robust, with more than 80% hatching, regardless of the salinity.
So how well would the larvae survive when burrowed within pieces of ocean-going fruit? Having implanted one weevil larva per fish poison tree fruit, Yeh installed a wave-making machine in a tank in the lab and allowed the fruit to bob about in the simulated ocean for 6 days – the length of time that it would take for a fruit to travel from the Babuyna Islands, off the Philippines, to the Japanese Yaeyama Islands. And, in an even more audacious experiment, Huang and Yeh cast some of the larvae-laden fruit adrift in the Kuroshio current off Taiwan for 6 days to find out whether any of the insects would survive. Remarkably, two of the larvae emerged alive from the lab-bound fruit and the team was even more impressed when, after retrieving the drifting fruit from the Pacific, they discovered two more survivors, both of which went on to develop into adults.
‘Our study provides the first empirical evidence that P. jitanasaius larvae can survive oceanic rafting inside fish poison tree fruit’, says Huang, who suspects that the tough waterproof fruit, packed with buoyant air pockets and nutrition, could provide the ideal mode of transport for pioneering weevil settlers.