Stick Insect Eggs Can Ride The Seas

[Another version of this story first appeared on Cosmos Magazine Online]

If you live in Taiwan and would like your children to travel to Japan, how might you? Your short wings cannot fly, and your six legs paddle like needles. Faced with such odds, the stick insect, Megacrania tsudai, an insect that cannot travel far as an adult, has evolved eggs that could float across the seas.

The bright green M. tsudai lives on the coasts of Taiwan and the islands of the Ryukyu Archipelago, Japan. And the coast is a perilous home. M. tsudai females drop their eggs onto the lower leaves and ground, where tides could wash the eggs away. Every year, typhoons pound the coasts.

Megacrania tsudai adult, Taitung, Taiwan. [credit: Shipher Wu @Flickr]

Megacrania tsudai adult, Taitung, Taiwan. [credit: Shipher Wu @Flickr]

The eggs of M. tsudai however, have adapted to the onslaught of the seas. Spongy structures that improve buoyancy are embedded in the shells of M. tsudai eggs. The eggshell is also lined with a crystalline layer that could prevent water from seeping in.

In 1998, Ushirokita reported that M. tsudai eggs could hatch while floating in seawater. Ushirokita proposed that M. tsudai eggs could disperse across the sea. M. tsudai researchers have been contemplating this hypothesis since.

Shun Kobayashi, entomologist at the University of Ryukyus, tested Ushirokita’s hypothesis in a recent study. Kobayashi and his colleagues collected more than 900 M. tsudai eggs and floated them on seawater for different periods, up to 365 days. They found that floating in seawater did not dampen the hatching rates of eggs.

However, the longer the eggs floated in seawater, the slower they developed. On average, eggs that had not floated in seawater hatched after four months, whereas those had floated in seawater for at least three months took another 10-14 days longer. M. tsudai egg develops slower in seawater likely because the embryo breathes less. M. tsudai eggs have fewer respiration tubes than other stick insect eggs; while floating, two-thirds of the egg surface remains submerged, further restricting the embryo’s gaseous exchange.

[from Woods Hole Oceanographic Institute; Neptune's Web]

[from Woods Hole Oceanographic Institute; Neptune’s Web]

Slower egg development could give the eggs time to cross the sea. To disperse from Taiwan to Ryukyu Archipelago, M. tsudai eggs would have to ride on the Kuroshio Current that flows from The Philippines north to Taiwan and Japan. Kobayashi estimated that the trip would take days to months.

Kobayashi plans to use genetic analysis to “clarify the relationships between M. tsudai distributed on some islands”. This may also reveal the dispersal route of M. tsudai.

Chia-Chi Hsiung, curator at the Lyman Museum, McGill University, has been studying Megacrania for two decades. “I [have] always believed that the eggs dispersed to Japan through the ocean from Taiwan,” says Hsiung. “I am glad that the Japanese scientists did the research to support it.”

Sea-traveling aside, stick insects can boast of other interesting egg dispersal methods. The eggs of many stick insects look like seeds and possess extensions called capitula; capitula resemble edible structures called elaiosomes found on seeds. Elaiosomes attract ants that carry the seeds back to their nest; likewise, ants also cart stick insect eggs with capitula into their nest where the eggs are protected from parasites.

The sea-traveling ability of M. tsudai eggs is rare, not only because this is the first of such record among stick insects, but more so because the insects have failed to dominate the open seas. The eggs of M. tsudai show that insects have the potentials to ride the waves.

[In the first 3 mins of the video, David  Attenborough leads us through the stick insect bluffing the ants into protecting their eggs.]

Reference

Kobayashi et al. 2014. Does egg dispersal occur via the ocean in the stick insect Megacrania tsudai (Phasmida: Phasmatidae)? Ecological Research 29: 1025-1032.

Hughes L., Westobdy M. 1992. Capitula on stick insect eggs and elaiosomes on seeds: convergent adaptations for burial by ants. Functional Ecology 6: 642-648.

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