Being a parasite isn’t all free drinks and fabulous parties in the most exclusive host clubs. Although the resources parasites acquire are ill-gotten (more or less), there may not be loads of them. So … budgeting. Parasites have to economize, just like the rest of us.

One area in which many parasitic plants seem to budget is seed. Many make tiny, dust-like seeds adapted for wind dispersal. The only trouble is, many of these same plants have forest floor residences, since they no longer need light to make food and face few photosynthetic competitors there. But such muffled environments are about as unfavorable for wind dispersal as it gets.

The flowers (top) and fruits of three parasitic plants studied by Kenji Suetsugu in Japan. Credit: Suetsugu 2017

So some parasitic plants have turned to an alternative seed mobilizer: animals. Although an animal is not an unusual choice, one animal, recently discovered to be doing this work in the forests of Japan, is.

Japanese scientist Kenji Suetsugu suspected animals were dispersing the seeds of three small parasitic plants in Shizuoka Prefecture on the island of Honshu. Parasitic plants are different from carnivorous plants like the Venus flytrap, which catch insects for protein but still make their own sugars using sunlight and are thus still green.

Parasitic plants like mistletoes steal all or nearly all their food directly from other plants. Other types of parasitic plants steal from the underground fungi called mycorrhizae that partner with trees, effectively stealing from both the fungi and the tree that hosts it. Both types of parasites are often pink or pallid rather than green. They frequently lack leaves or possess shrunken ones, as leaves are now effectively beside the point.

Some parasitic plants -- particularly the ones in shady forests and including the three in Japan -- also make fleshy fruits that don’t split open when ripe. That seemed intriguing to Suetsugu. Fleshy fruit is usually calculated to appeal to animal who will eat it and deposit the seeds elsewhere. So he staked out the flowers to see what, if anything, might be nibbling upon them.

After a combined 190 hours of nocturnal observation, the surprising answer was: camel crickets. It was the first time camel crickets have ever been documented to disperse plant seeds.

A Japanese camel cricket eats the fruit of the parasitic plant Monotropastrum humile. Credit: Suetsugu 2017

Camel crickets only superficially resemble true crickets and are not closely related to them. The omnivorous insects frequent caves and other damp, cool environments like basements where they may make a nuisance of themselves (one species on the east coast likes to chew on just about anything it can get its mandibles on, possibly including you. Their default – and highly upsetting -- reaction to being surprised is to jump towards the thing that surprised them).

Not only were Japanese camel crickets seen visiting the fruits often, mammal exclusion experiments demonstrated they were eating lots of it. They pooped hundreds of intact seeds. The seeds did not survive the same trip through ground beetles also seen eating the fruit.

Studies of certain plant families show that many plants, not just parasites, have evolved fleshy fruits in response to moving into a forest. The three plants in this study are from three different families and parasitize different things. Two parasitize the mycorrhizal fungi that partner with plant roots. The third directly parasitizes trees.

Yet in a striking example of convergent evolution, all three have evolved the same seed dispersal mechanism in response to the same shady habitat. It might have helped that their seeds were already tiny, which makes it easy for insects to swallow them. And compared to vertebrate seed dispersers like mice or birds, insects require far fewer fruit calories to perform the same service – an excellent choice for a plant on a budget.

Reference

Suetsugu, Kenji. "Independent recruitment of a novel seed dispersal system by camel crickets in achlorophyllous plants." New Phytologist 217, no. 2 (2018): 828-835.