|Photo by Haseeb Randhawa & Ken Miller here
Meet Mermis nigrescens, a nematode worm which arrived in New Zealand inside European earwigs (Forficula auricularia). Mermis nigrescens is a species which had been known since 1842, and it is likely that it might have even been discovered earlier than that in 1766, but was mistakenly identified as Gordius - a genus of hairworm - which despite superficial resemblance and a similar life cycle, belongs to a different phylum. Its host, the European earwig, first arrived in New Zealand during the 19th century, but it was only recently noticed that these insects have also brought along a parasite from their original home range.
|Photo of earwig host with M. nigrescens from this paper
So how do we know this parasite is an introduced species and isn't one that the earwig had simply acquired in its new home?
Molecular analysis looking at three genetic markers from M. nigrescens showed that the closest relatives of these particular nematodes are found in Canada - M. nigrescens appears to be well-travelled, and is found all over the world. But it does not seem to be as abundant in Dunedin, New Zealand as seen elsewhere in world. In the population that was examined for this study, only 19 out of the 198 earwigs examined were infected with M. nigrescens, whereas a study in Tasmania, Australia found the parasite in half of the earwigs examined, and it was even more common in Ontario, Canada where infection prevalence reached 63%. It is currently unknown why M. nigrescens does not seem to be as abundant in New Zealand as it is elsewhere in the world, though it could just be something about this particular bunch of earwigs and that there are more heavily parasitised earwig populations elsewhere in New Zealand.
But where did the M. nigrescens population in New Zealand originate? While its closest living relatives are found in Canada, did it arrive to New Zealand from there? After all, the original home of M. nigrescens is Europe, so the Canadian population was not native to that region either. The missing piece of this puzzle is genetic sequences of M. nigrescens specimen from its original range in Europe, which might resolve where this newly discovered New Zealand population originated - from Europe or elsewhere. For all we known, this supposedly widespread species may in fact be composed of a complex of closely-related cryptic species, with each species found in a different region of the world.
Just goes to show that even in the common earwig, there are natural history secrets waiting to be revealed.
Presswell, B., S. Evans, R. Poulin, and F. Jorge. 2015. Morphological and molecular characterization of Mermis nigrescens Dujardin, 1842 (Nematoda: Mermithidae) parasitizing the introduced European earwig (Dermaptera: Forficulidae) in New Zealand. Journal of Helminthology 89: 267-27