The tale of a spider and a pitcher plant: Study explains how two predators can benefit from collaboration

November 12, 2018, National University of Singapore
A study conducted by ecologists from the NUS Department of Biological Sciences showed that the crab spider Thomisus nepenthiphilus lives exclusively in the slender pitcher plant Nepenthes gracilis, and provides supplementary nutrients for its host. NUS doctoral student Mr Lam Weng Ngai is a key member of the research team. Credit: National University of Singapore

Two recent studies by ecologists from the National University of Singapore (NUS) have shed light on the relationship between the slender pitcher plant and its 'tenant', the crab spider Thomisus nepenthiphilus, providing insights into the little known foraging behaviours of the spider.

Thomisus nepenthiphilus is found only in the slender plant Nepenthes gracilis, which is native to Singapore and can also be found in Indonesia, Borneo, and Malaysia. Although the pitcher plant is a carnivore that traps and devours insects to supplement its nutrient requirements, the crab spider Thomisus nepenthiphilus is able to exploit the pitcher plant's sweet-smelling nectar to catch its while at the same time, provide supplementary nutrients for its host.

"Our two studies provide important insights into the circumstances that favour cooperation over parasitism, and the results are pivotal in attaining a better understanding of these interactions," said research supervisor Associate Professor Hugh Tan, who is from the Department of Biological Sciences at the NUS Faculty of Science.

The benefit of being 'robbed'

In the first study, Assoc Prof Tan, together with doctoral student Mr Lam Weng Ngai and former undergraduate student Miss Robyn Lim, found that pitchers that are without any crab spider get more nutrients out of each prey, while those inhabited by 'tenants' trap more prey but get less nutrients from each prey.

Their laboratory experiments found that the crab spider ambushes flies that feed at the pitcher plant and sucks the body fluids of the insect prey. The crab spider subsequently drops the carcasses of the prey, which still contain some nutrients, into the fluids in the pitcher for it to digest. As such, although the crab spider 'steals' from the pitcher plant and gets the first taste of the prey, the net effect of this 'burglary' can still be beneficial to the pitcher plant as it gets the residual nutrients from the prey.

The findings suggest that when resources are scarce, this partnership between the crab spider and the pitcher plant is beneficial. However, when resources are abundant, this partnership is not favourable. The results of the study were published in the journal Oecologia on 14 August 2018.

"A trend that has been observed in recent mutualism research is that under more stressful conditions, the frequency and intensity of mutualism between the different organisms increases. Our findings support this observation. In other words, the age-old adage 'a friend in need is a friend indeed' is true not just for humans, but also for and animals," said Assoc Prof Tan.

Although the crab spider Thomisus nepenthiphilus ‘steals’ prey from its host, the slender pitcher plant Nepenthes gracilis, a study by ecologists from the National University of Singapore found that the net effect of this ‘burglary’ can still be beneficial to the pitcher plant as it gets the residual nutrients from the prey discarded by the crab spider. Credit: Lam Weng Ngai, NUS Department of Biological Sciences
Big prey, big gains

Assoc Prof Tan and Mr Lam also conducted additional experiments in the natural habitat of the plants. Through field surveys, the researchers identified the species of prey that were found to be in greater numbers in pitchers that were inhabited by spiders, and those that were not.

Laboratory experiments were conducted to measure the contents of these prey species to estimate how much nutrients the pitchers would obtain if the prey had been trapped with, and without, the help of the crab spiders.

"Our results confirm the findings of our earlier study – the T. nepenthiphilus crab spider does indeed help the N. gracilis pitcher plant catch many different species of prey. More importantly, the net contribution of T. nepenthiphilus to N. gracilis' nutrition appears to be proportional to the size of prey that T. nepenthiphilus catches," explained Mr Lam.

He elaborated, "If the crab spider only catches small prey, such as mosquitoes or scuttle flies, the net benefit to the pitcher plant will be negative—it will be 'stealing' nutrients from the pitcher plant. However, when the crab spider catches large insects like cockroaches or large bugs, the pitcher plant will benefit, as the 'service charge' paid to the crab spider becomes small compared to the total amount of nutrients gained through the interaction. As such, the residual nutrients that the receives from the carcasses discarded by the crab is a good trade-off."

The results of this study were published in the Journal of Animal Ecology on 10 October 2018.

Theoretical model to be constructed

Based on the insights gained from these two studies, the research team is now constructing a theoretical model on mutualisms that involve the provision of nutrients by one species to another. Such a model will allow scientists to examine the factors that make mutualisms stable, and monitor how changes in the environment, such as global warming or habitat modification, will alter the ecological outcomes.

Explore further: Ants and carnivorous plants conspire for mutualistic feeding

More information: Robyn Jing Ying Lim et al. Novel pitcher plant–spider mutualism is dependent upon environmental resource abundance, Oecologia (2018). DOI: 10.1007/s00442-018-4246-8

Weng Ngai Lam et al. The crab spider-pitcher plant relationship is a nutritional mutualism that is dependent on prey-resource quality, Journal of Animal Ecology (2018). DOI: 10.1111/1365-2656.12915

Related Stories

When enemies come to help

April 11, 2018

"The enemy of my enemy is my friend." Now, researchers at the University of Zurich show that this principle also holds for crab spiders and flowering plants. While it's true that the spiders eat or drive away useful pollinators ...

Pitcher plant uses power of the rain to trap prey (w/ Video)

June 13, 2012

During heavy rain, the lid of Nepenthes gracilis pitchers acts like a springboard, catapulting insects that seek shelter on its underside directly into the fluid-filled pitcher, new research has found. The findings were published ...

Pitcher plants 'switch off' traps to capture more ants

January 13, 2015

Insect-eating pitcher plants temporarily 'switch off' their traps in order to lure more prey into danger, new research from the University of Bristol, UK, and the University of Cambridge, UK, has found.

A pitcher perfect relationship

July 5, 2011

(PhysOrg.com) -- It seems counterintuitive, but in rare cases carnivorous plants and herbivorous animals nourish each other in a mutually beneficial relationship.

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

3 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

rrwillsj
not rated yet Nov 12, 2018
As I understand the conclusion of the researchers in this article? They measured the nutrients left in prey corpses after the spider fed? But they did not measure the nutrients contributed to plants by the spiders excrement?

Manure is a useful source of nutrients after being processed. It all adds up considering the resource-poor environment the pitcher plants inhabit.
Iochroma
not rated yet Nov 12, 2018
It is now understood that other species of Nepenthes catch relatively few prey, but rely mostly on the excrement, of birds, or an bat that roosts inside the pitcher.
rrwillsj
not rated yet Nov 13, 2018
lochroma, source please?

And where in the hell does any pitcher plant exist, big enough to let a bird or bat roost? So I can stay away from them!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.