By Susan Milius, Science News
They don’t surf, but caterpillars found only in Hawaii are the first insects known to feed and grow as readily in water as on land.
Other land insects can endure a dunking. And other aquatic ones can survive a dry spell stranded out of water, says evolutionary biologist Daniel Rubinoff of the University of Hawaii at Manoa. But he and a colleague now describe amphibious habits of larvae in 12 new species in the moth genus Hyposmocoma. Young of each species can thrive both underwater in rushing streams and exposed to air on rocks poking out of the water.
Hyposmocoma moths live only in the Hawaiian islands, and most species in the genus spend their caterpillarhood exclusively on land before flitting away as full-grown moths. Yet genetic analyses show that at least three times within the genus, landlubber lineages have independently evolved amphibious caterpillars, Rubinoff and University of Hawaii at Manoa colleague Patrick Schmitz report online March 22 in the Proceedings of the National Academy of Sciences.
Besides introducing some remarkable caterpillars, the work emphasizes the importance of islands in the study of evolution. Isolated mixes of the relatively few kinds of creatures that arrive on islands can come up with novelties unknown elsewhere. “Islands are clearly these crucibles of evolution,” Rubinoff says.
Rubinoff and Schmitz discovered the wet-dry caterpillars as part of wide-ranging surveys of the diversity of forms that Hyposmocoma has evolved in Hawaii. The team collected caterpillars grazing on rocks in streams, seemingly unfazed by changes in the water level. In lab studies, the researchers found that these caterpillars don’t have gills or a natural scuba mechanism of trapping air bubbles. Instead, they appear to get oxygen directly from water. To survive submerged, the caterpillars need fast-flowing waters where they shelter on the downstream sides of rocks and spin tethers to keep from washing away.
Caterpillars in this genus crawl around partly covered by silk-spun cases of a variety of shapes and sizes that they add to as they grow. Species in the newly described amphibious lineages, still awaiting formal scientific names, make cases Rubinoff calls cones, bugles and burritos. Researchers have also found cases in the shapes of cigars, candy wrappers, oyster shells, dog bones and bowties. “We’re running out of names to describe them,” Rubinoff says.
In addition to body form, ways of making a living have also diversified considerably in the roughly 400 Hyposmocoma species already known. (Rubinoff suspects there may be twice that number.) In 2005, Rubinoff described a caterpillar that hunts down and eats snails. Other caterpillars in this genus feed mostly on rotting wood in the manner of termites, which are relative newcomers to Hawaii.
This whole genus has probably been evolving its abundant variation in the Hawaiian islands for roughly 20 million years, Rubinoff and Schmitz estimate. The dating comes from their three-gene analysis of 89 species, calibrated with the geological ages of various islands. All the species the researchers have analyzed so far have been found on only one island, typically in only one locale.
This abundance of locals does not come from splitting species at the smallest excuse, according to Rubinoff. “These species are at least as different as chimpanzees are from us,” he says.
“Interesting, fun and really quite significant,” Steve Jordan of Bucknell University in Lewisburg, Penn., says of the new work. The Hawaiian islands lie particularly far from other land and so offer a natural laboratory for evolutionary novelty. For example, the Megalagrion damselflies Jordan studies include at least one Hawaiian species with larvae that live on land, a habit documented in only one other damselfly. “This is like a fish living out of water,” Jordan says.
Island diversity, however, comes with the downside of island vulnerability. People building and farming can easily overwhelm small bits of island wilderness, and invasive species disrupt the unusual native ecosystems. Jordan says, “Working in Hawaii is horribly exciting for evolutionary biologists, but sobering and sad, too.”