Hermit crab. Image via Wiki Commons.
In the humid, coastal forests of Japan’s Ishigaki Island, near Okinawa, a parasitic plant relies on some unusual allies to spread its seeds: land hermit crabs and cockroaches, according to a recent study.
Balanophora fungosa is already an unusual plant. It doesn’t photosynthesize, instead stealing nutrients from the roots of other plants. Most of it remains underground, with only its fleshy reproductive structures emerging above the forest floor. These can look more like mushrooms than plants.
Previously, scientists have suggested that gravity might be helping disperse those seeds. But in the latest study, researchers found that on Ishigaki, the seeds are primarily dispersed by the land hermit crab (Coenobita brevimanus) and the Australian cockroach (Periplaneta australasiae).
Crabs, cockroaches, and fungi
Initially, study author Kenji Suetsugu, a professor of biodiversity and ecology at Kobe University, Japan, told Mongabay by email that he expected ants to be the primary dispersers, due to the plant’s detachable fruit layer. To test this, he conducted “exclusion experiments” by covering some plants with mesh nets that allowed tiny ants inside but kept larger animals out. However, the fruits inside these ant-only nets went completely untouched. So Suetsugu set up a waterproof digital camera about 20 centimeters (8 inches) from the plants.
Programmed to snap photos every 50 seconds, the camera compiled 6,427 images over nearly 89 hours of monitoring. This time-lapse photography captured land hermit crabs and cockroaches aggregating under the cover of night to devour the fruits. “The biggest surprise was the identity of the main seed disperser,” Suetsugu said.
While feeding, hermit crabs carried an average of 86 fruits externally on their claws. They also ingested and defecated numerous intact seeds, some of which remained viable after passing through their digestive tracts.
“To our knowledge, this is the first empirical evidence that land hermit crabs can function as both external and internal seed dispersers,” Suetsugu said. “This expands the range of ecological roles known for these animals and, more broadly, adds to growing evidence that small-bodied invertebrates can make meaningful contributions to seed dispersal.”
The Australian cockroach also defecated seeds of fungus root, but they excreted fewer intact seeds than the hermit crabs. Both creatures are likely drawn to the fungus root by its smell, as mature fruits release a fermented odor that attracts the foraging animals.
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“Seed-dispersal research has traditionally focused heavily on birds and mammals,” Suetsugu said. “Our findings suggest that this can leave important interactions unnoticed, particularly on shaded forest floors and on islands, where abundant ground-dwelling invertebrates may provide dispersal services that are easy to miss.”
The relationship may be even more effective than it first appears. Both hermit crabs and cockroaches swallowed the fruits and later passed intact, still-viable seeds in their feces. And because land hermit crabs can travel much farther than the distances directly observed in the experiment, they could potentially carry these microscopic seeds tens of metres through the forest. For a plant that spends most of its life hidden underground, parasitizing the roots of other plants, these unlikely “gardeners” may be crucial to helping the next generation find somewhere new to grow.
He added that for him, “the broader lesson is that familiar animals can have ecological roles that remain hidden simply because nobody has watched them closely enough.”
The discovery is a reminder that seed dispersal is often far messier than the textbook image of berries carried by birds or burrs caught in fur. In tropical forests especially, tiny and easily overlooked animals may be doing a substantial share of the work. For Balanophora fungosa, survival depends not on one elegant partnership, but on a whole cast of unlikely helpers — ants, cockroaches and hermit crabs — each contributing a different piece of the plant’s reproductive cycle.
This article was originally published on Mongabay. Minor edits have been made to it.
