NUI Maynooth researchers have discovered that a large group of creepy-crawlies are more closely related to crabs than they are to other bugs

HERE’S A slightly disturbing thought: the vast majority of animals on the planet are classified as arthropods – a major grouping that includes spiders.

It’s a confusing assemblage of creatures, but scientists at NUI Maynooth have used new techniques to help settle an old question about how the relationship lines should be drawn. These suggest that the centipedes and millipedes should now shuffle away from the spiders and over to the section of the arthropod tree where the crabs live.

The arthropods – animals with jointed legs – make up around 80 per cent of animal biodiversity, according to Dr Davide Pisani from NUI Maynooth, and grouping species isn’t straightforward. “Arthropods are this big group of animals – insects, crustaceans, spiders, mites, scorpions – and we are talking about all ecosystems: marine, terrestrial, forest. They fly, they swim, they do everything,” says Pisani, who along with Dr James McInerney heads the molecular evolution and bioinformatics unit at NUI Maynooth.

“As with all of these types of extremely successful groups, there tends to be something that makes them very plastic so they can adapt, and it means they can come in all shapes and forms, which makes them difficult to classify.”

One niggling bone of contention has been where to put the myriapods – the group that contains the centipedes and millipedes. For many years they were classified into a grouping of allsorts based on their physical shape, explains Pisani, but then along came DNA studies and the new findings seemed to go against the grain.

“The suggested an arrangement where the centipede and millipede were supposed to be clustered together with the spiders and their allies,” he says. “This was going against all previous studies on morphology. People couldn’t understand it because it doesn’t make any sense – there is no obvious connection which should put together the spider and scorpion with the centipede and millipede.”

A puzzling factor that threw doubt on in this association was that centipedes and millipedes have jaw features called “mandibles” that are different from those of spiders spiders, notes Pisani. “The structure of the head is completely different.”

The question mark over these strange bedfellows remained for more than a decade, and even Pisani’s earlier genetic work had pointed to the unconventional link with spiders.

But now a new study involving five centres – NUI Maynooth, University College London, Dartmouth College, Université de Montréal and London’s Natural History Museum – has used new techniques that cast a different light, and they have just published the findings in Proceedings of the Royal Society B.

By combining datasets on arthropod morphology together with detailed analysis of around 200 genes and regulatory molecules called microRNAs, the researchers found evidence that the many-legged centipedes and millipedes were more closely related to crabs than they were to to spiders.

One of the factors in previous genetic studies that had been throwing the answer towards the spiders was that they were looking at genes that evolve quickly.

In contrast, the current study used next-generation genetic sequencing and looked at genes that evolve slowly and offer better historical markers, explains Pisani, whose group received funding from Science Foundation Ireland for the research.

In addition, the study discovered new genetic markers for particular groups of arthropods and again the pattern was to cluster the centipedes and millipedes with the crustaceans rather than the spiders, says Pisani: “We had three lines of evidence pointing to the same result.”

The findings offer a clearer picture of how the arthropods are organised and can set the scene for more informed questions about the origins of biodiversity, according to Pisani.

“Before you start asking any sort of questions about adaptation or how the group could become that successful, you need to have a clear understanding of the evolutionary relationships because this information is used as a background for other types of studies,” he says. “This is a key point in the study of evolution and biodiversity, and considering the current biodiversity crisis, that is quite important.”