Nematodes (roundworms) just can't get enough of being parasites. Parasitism has independently evolved at least fifteen times among nematodes, with each of them realising their best lives inside the body of another animal (and for some of them, inside plants). But how did they end up like this? Well, the worm being featured in this post may offer some answers (while raising many others).
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| Left: A bundle of Echinomermella matsi inside of an opened sea urchin. Right: An E. matsi juvenile viewed under a microscope. Photos from Fig. 2 of the paper. |
Echinomermella matsi is a species of roundworm that parasitises sea urchins. It can grow fairly large and is very noticeable, opening up a heavily infected sea urchin is like looking at a bowl of udon noodles. In some urchin populations, these worms can be rather common, and they exert a demanding toll on their hosts, so much so that they can end up castrating the sea urchin as they divert resources away from their host's gonads. But sometimes, mere castration is not enough, and this worm can also shorten the urchin's lifespan.
All this affects the population dynamics of sea urchins, which has flow on effects on the rest of the intertidal ecosystem as sea urchins are often major grazers of kelp and seaweed in such environments. Given the impact that this nematode can have on urchin populations, there is some interest in figure out what makes this worm ticks, and this study took a genomic approach to that question.
It has previously been thought that Echinomermella and similar parasitic nematodes might be marine relatives of mermithid nematodes, which are a family of roundworms that infect insects on land. But subsequent studies have found that their resemblance is merely superficial. While like the mermithids, they can take up a lot of room in their host's body cavity, that is hardly a unique characteristic. Turns out, Echinomermella and related worms seem to belong in a group of nematodes that have all independently become parasites of marine invertebrates.
So how does our urchin-dwelling friend fit into all this? The researchers' analyses found that Echinomermella belong to a larger group of nematode worms called Plectida which contains both parasitic and free-living species, with some species found in the sea and others in freshwater. The parasitic species within that group infect all kinds of marine organisms, even single-celled foraminiferans. The closest living relative of Echinomermella is a nematode called Trophomera, a parasitoid worn that lives in a range of different deep sea invertebrates including crustaceans and bivalves.
Since Echinomermella is evolutionarily close to both free-living and parasitic worms, that can provide us with some clues as to how it evolved to be parasitic. One of the hypothesis is that the ancestor of Echinomermella and other parasitic plectids started out as marine nematodes. Their long slender bodies would allow them to squeeze into all kinds of tight spaces, including the insides of other animals. And because the body cavity of marine invertebrates contains fluid which is almost at the same solute concentration as seawater, it seems natural for those worms to squeeze in there to escape from the bustle of the outside world. Once they're in, they're protected from potential predators and other sources of danger, all while bathing in delicious nutrient-rich fluids.
There are multiple paths to parasitism, many of which have been explored and mastered by nematodes. In a dangerous and uncertain world, perhaps shacking up in the cozy wet insides of other creatures is the best way to live when you are just a worm.
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