A new study has revealed that wild hermit crabs can help spread the seeds of a parasitic plant growing in Japanese coastal forests in two ways: by carrying the fruit attached to their bodies, or by swallowing the seeds and then excreting them later while they are still intact.
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The provides the first direct evidence that terrestrial hermit crabs can act as both external and internal seed transporters. External transport means that fruits or seeds adhere to the animal's body as it moves, while internal transport occurs when the animal eats the fruits and then some seeds are later excreted in its feces elsewhere.
The discovery came during a study by a Japanese researcher of a plant called *Palanovarus fangusa*, a non-green parasitic plant that, unlike most plants, does not produce its own food from sunlight but rather obtains it from the roots of other plants. This plant produces a very large number of microscopic seeds, sometimes up to a million, but how they are dispersed from the parent plant has remained a puzzle for scientists.
"Many assumed that they simply fall due to gravity and are then carried by the wind, but the moist, almost windless forest floor makes this possibility unconvincing," the study's lead author, Kenji Suetsugu, a professor of botany at Kobe University.
Kenji points out that his previous research has shown that other plants from the same family use small animals to disperse their seeds, such as ants, cockroaches, and crickets; therefore, it seemed logical that Balanophora vangusa would follow a similar strategy, especially since its fruits are pale in color, easy to separate, and emit a yeast-like odor, qualities that might attract small creatures looking for food on the forest floor.
Initially, the researcher suspected ants as the primary seed carriers. To test this idea, he covered some plants with netting that allowed ants to pass through but prevented larger animals from reaching them. However, the fruit no longer disappeared as it had before, suggesting that the culprit was not ants but a larger animal. The researcher then placed time-lapse cameras around the plants to see who was visiting the fruit as it ripened.
The footage revealed that the wild hermit crab approached mature plants and used its pincers to scrape the small fruits from their surface. In some cases, its impact on the plant was significant; a single crab was able to remove almost all the fruit from a plant in one night. Some of these fruits even stuck to the crabs' bodies, turning the animals into living carriers of the seeds across the forest floor. The researcher found crabs still carrying the fruits on their bodies even after they had traveled several meters away from the plants.
The study notes that the crabs also ingested some seeds, and tests showed that some survived passing through the animal's digestive system. This means the seeds could later be transported elsewhere with the crab's excrement. "These animals thus become unusual seed carriers, as they combine external transport, when seeds or fruits adhere to their bodies, with internal transport, when they eat the fruits and then excrete the seeds later while they are still viable," explains the study's lead author.
A small relationship with big implications
The researcher also noted that the crabs did not visit immature plants, but appeared only around fully mature ones. It is likely that the scent emitted by the mature plants attracts these animals at the opportune moment, when the fruit is ready for consumption and transport. This makes the scent, according to the study, akin to a signal directed at small terrestrial creatures searching for food, but which, while feeding, may also help the plant disperse its seeds.
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