Lincoln University PhD student Usha Mendis is fascinated by this twist of nature.
She is on a mission to learn as much as she can about the parasitic nematodes – called mermithids – and how they turn spiders into mindless “zombies”.
The mermithids target all kinds of invertebrates, including caddisflies, mayflies, grasshoppers – and large endemic spiders. That poses a big problem for our ecosystem, Mendis said.
So far, she has found three different types of spiders carrying the parasite. She is working to narrow down exactly what the worm is and which species of spiders are affected.
“New Zealand has a large number of endemic spiders . . . over 90% of them are endemic. If something were to happen to them it would not be good for our biodiversity.
“Spiders hold a special place in the food chain. When the worms take over, it flips that idea on its head. The parasite completely changes it, the spiders aren’t filling their normal role.”
It is a slow, horrible process for the spider, and always results in its death.
There are few ways to tell whether a spider is parasitised, but those infected often grow larger limbs.
“They can be very abnormal. Their legs get shorter but thicker, the spider is physically changing. The abdomen gets bigger and swollen too. They look like zombies,” said Mendis.
But it is not always the case. The only real way to tell is to wait until the spider dies and the nematode emerges, ready for the next stage of its life cycle.
While researchers know how the nematodes come out, they don’t know how they get into the spiders. It is part of what Mendis is trying to discover.
So what do the nematodes want from the spiders?
Mendis said it is just one stage of their life cycle.
“They can’t live without a host. They consume the energy and nutrients of the host, and grow inside it.”
The nematodes need water to progress to the next stage of their life cycle, which is why they drive the spiders towards moisture.
“In that moist environment they can grow, mate and lay eggs. Then those eggs hatch and the cycle begins again.”
Spiders are often found drowned. Even if the spider survives the nematode emerging from its body, it will soon die.
Because spiders carrying parasites will go out of their way to find water, Mendis is using water traps to collect the nematodes.
It has only been 35 years since mermithids were first reported in New Zealand, and there is still more to do before we can fully understand their relationship with spiders, Mendis said.
In the meantime, she asks people to reconsider how they think of spiders, given their vital role in our ecosystem.
“Spiders may look disruptive and creepy, but they’re not the enemy.
“They’ve got their own problems to deal with.”
-Allied Media
