02/11/2013 08:42 am ET

Parasitic Nematode, Or Roundworm, Called Most Numerically Abundant Animal On Earth

By Jennifer Frazer

The next time you find yourself becoming mosquito chow, remember this video:

This is Strelkovimermis spiculatis — a parasitic nematode, or roundworm — casually escaping from an unlucky, soon-to-be-expired mosquito larva. The way this larva twitches as the nematode slithers out is gut-wrenching. You can still see the poor larva’s vitals pumping even after nematode evac is complete and the larva’s torn cuticle gapes, dooming it to die. It appears nematode-infested mosquito larvae get a bonus microtaste of the Aztec human sacrifice experience.

You may also think humans own the planet. You’d be wrong. Worms like the one in this video are Earth’s animal overlords; nematodes are the most numerically abundant animals on Earth. They’re not just a slim majority. Four out of every five animals on Earth is a nematode.

Nathan Augustus Cobb, a nematologist working for the U.S. Department of Agriculture, famously put it this way in 1915:

In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.

No organism on Earth is exempt. According to Colin Tudge in “The Variety of Life”, it has been suggested that one of every two animal species on Earth has at least one parasitic nematode species that lives only with it. To cite but one example, here’s a vivid image by Alex Wild of a nematode-filled ant from Belize. “I can’t imagine what an insect infected with a mermithid nematode must feel,” he writes. “Scaled to human size, a mermithid would be at least as intrusive as an anaconda coiled among the intestines.”

At the other end of the spectrum, the largest known nematode is Placentonema gigantissima, which can reach around 30 feet long in the placentas of its host, sperm whales. I managed to find a purported photo of this beast; I particularly like the fourth bullet point in its caption. And I wonder: how does this nematode get from host to host? Surely sperm whales are not like dogs, if you know what I mean, and they don’t exactly have to worry about washing their hands.

In the world of plants, root-knot nematodes crawl into roots and release chemicals that swell the roots with tumors, sapping the plant’s strength aboveground. At the heart of these tumors, female nematodes suck the juice from grotesque, multinucleate “giant cells”. They grow fat off this rich provender, swelling in turn into their own grotesque balloons before squeezing a slimy mass of eggs from their enormous backsides. Their impact on us is not small; the American Phytopathological Society estimates plant-pathogenic nematodes are responsible for 14% of crop losses worldwide.

Closer to home, the nematode diseases of hookworms, river blindness, and elephantiasis plague humans, while “worms” are major cat and dog parasites. Consider the recent shocking news that every two days, the dogs of Bristol, U.K., produce in their 8 tons of poop a population of roundworm eggs — easily preventable by having dogs wormed — equivalent to the number of humans on Earth (that’s 7 billion, for those of you keeping track at home). These worms can cause blindness in humans who accidentally ingest them after handling dirt. Bad owners. BAD.

Toxocara canis, the dog roundworm. I would really rather not think about what went into the creation of this photograph. Creative Commons Joel Mills. Click for license and source.

Not all nematodes are parasites. Some work for their daily bread like the rest of us, foraging for food in water or soil. There are predatory nematodes who use their piercing stylets to attack and devour protists or other microbes. Here’s the predatory nematode Pristionchus pacificus stabbing and then sucking the life from the lab rat nematode Caenorhabditis elegans. If you look, you can see the gulps of C. elegans’s vital fluids flowing rhythmically into P. pacificus’s gullet*.

Other nematodes are saprophytes, getting by on whatever tasty morsels they can scavenge. Water, soil, plants, and fungi are lousy with them. Rotten apples teem with nematodes in the tens of thousands. This author was somewhat disturbed in college biology lab to find one feisty nematode hanging out on an otherwise delectable-looking apple slice viewed under the microscope. “We unintentionally eat and drink enormous numbers of roundworms in our lifetimes,” my college biology helpfully, if nauseatingly, explains.

Soils and sediments are particularly rich in nematodes. They compose 90% of life forms in ocean sediment. There are so many nematodes in soil that entire groups of fungi have dedicated themselves to trapping and feasting upon them, as David Attenborough explains here:

Attenborough doesn’t mention it, but in addition to the celebrated inflatable noose, fungi have evolved a wide variety of nematode traps and snares. Some sport sticky traps; others weave nets.

As for the hero (villain?) of our first video, because Strelkovimermis attacks a wide range of mosquito larvae, scientists have taken an interest in it as a potential biocontrol agent. Mosquito-borne diseases kill a lot of humans, so the killing-skills of this little worm could be life-saving for a lot of impoverished people.

S. spiculatus eggs can remain dormant and even dehydrated for months. But hatching at the wrong time comes with a steep price: if the newly-hatched larvae fail to find their very own mosquito home within 24-48 hours, they will die. Scientists from the Center for Vector Biology at Rutgers University hypothesized that eggs must have a very accurate way of telling if mosquito larvae are around. But what is it about mosquito larvae, they asked, that stimulates the eggs of Strelkovimermis to hatch?

Was it the scent of a mosquito larva? Or the way they squirm?

So they exposed Strelkovimermis eggs to water previously exposed to mosquito larvae, and to what they charmingly called “artificial larvae”. Translation: a three-millimeter snippet of iron wire coated in hot glue, dropped into water, and persuaded to dance not by a Smith & Wesson, but by an ordinary magnetic stirrer.

They found that while the smell of larvae worked fairly well in stimulating eggs to hatch (33% hatched in larvae-exposed water vs. 12% for plain control water), artificial larvae by themselves had little effect (17% hatched). But spiking larvae-scented water with a wriggling fake mosquito amplified the effect of smell alone and persuaded over half — some 60% — of the eggs to hatch. Real mosquito larvae, of course, worked best of all, but only a little better than the imitation: about 73% of these hatched. That’s a pretty impressive feat for a basic combination of eau de larva and a few seconds with a hot glue gun.

Interestingly, the authors concluded that the present strategy of releasing juvenile nematodes in search of a host for mosquito control is justified in temporary pools, where mosquitoes only breed when they are filled. But in ponds or other permanent water bodies, they concluded the current strategy is wrong. Since juveniles only live for a day or two and will die if they don’t chance on a host, while dormant eggs can survive for months and perform host “surveillance” without any help from us, they concluded it makes much more sense to release dormant eggs, adults, or even pre-infected mosquito larvae. Pretty much anything but infective juveniles. Go science!

Whatever their modus operandi, nematodes like Strelkovimermis live everywhere on Earth you can conceive of. Even rock is not imperious to them. Nematodes have been fished out of crevices in South African gold mines almost two miles deep. According to the USDA scientist Cobb, one species of nematode has been described exclusively from felt German beer coasters.

Since nematodes lack wings, we might presume there is only one refuge: the sky.


*As the author of the video notes, P. pacificus possesses the same number and configuration of nerve cells as C. elegans. Yet it possesses predatory behavior and C. elegans does not. The author of the embedded video is interested in why that might be.


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