Cryptic goings on…

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[This post is adapted from a book I’ve just finished, which will be published sometime next year by Thames and Hudson.]

Nematodes are typically small animals that to the naked eye look very much alike; however, these creatures are fantastically diverse – at least on a par with the arthropods in terms of species diversity. Currently, around 24,800 nematode species have been formally described, but it’s fair to say this is the tip of the veritable ice-berg. Almost 20 years ago it was suggested there could be as many as 100 million species of nematode out there. This colossal estimate was based on an extrapolation using regional data, a technique that is beset with all manner of pit-falls. 100 million has since been revised down to around 1 million, which is quite a climb down; nonetheless the nematodes are nothing other than an incredibly speciose group of animals.

At face value nematodes lack the charisma of larger animals, so there are very few biologists who have made it their life’s work to understand them. Those that do have been rewarded with glimpses of the incredible diversity of these animals, an example of which is the complex menagerie of nematode species that dwell in the guts of large, tropical millipedes. Some SEMs (false coloured) of these nematodes can be seen below:

Parasitic nematodes from the guts of tropical millipedes (clockwise from top left: Rhigonema tomentosum, Carnoya sp., Carnoya fimbriata and Heth sp.). All images are courtesy of David J. Hunt and are false coloured SEMs.

The SEMs of the four species in the composite image above go to show how morphologically diverse the nematodes can be, but it’s their ecology we are interested in here. These four species are parasitic in the gut of their millipedes hosts where they help themselves to gut secretions, intestinal cells and the host’s food.  Nothing odd about that – there are loads of nematodes species that spend their lives in the guts of larger animals. What makes this example particularly fascinating is that these nematodes are not alone. Also in the gut of the host are other, related nematodes (Zalophora spp. see SEM below) that pursue and eat the parasitic species. What I love about this example is what it says about hidden diversity and the huge variety of ecological interactions that are out of sight and out of mind. The gut of the millipede host is a microcosm – an ecosystem in its own right – inhabited not only by a huge range of microorganisms, but also an array of other animals that are found nowhere else. Beyond the basic predator-prey interactions played out between these nematodes there is so much that we have no idea about. Does each of the parasitic species depend on a different part of the host’s gut? What adaptations do the parasitic nematodes have for evading the predatory nematodes? Can the juvenile predatory nematodes maximise their chances of getting into a millepede infested with their prey?

These hidden layers of complexity are everywhere in the living world, so it is no surprise that we have so far only scratched the surface of the Earth’s biodiversity.

The mouth of Zalophora deinostoma. The three cuticularized, jawlike structures inside the mouth slice up the prey nematodes that are hunted in the gut of the millipede host (SEM courtesy of David J. Hunt).

Further reading

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