Land hermit crabs were caught on camera snacking on the fruits of a parasitic plant. Since the fruits attach to the animals' bodies and some seeds even survive passage through the digestive tract, the Kobe University study is the first to document land hermit crabs as both external and internal seed dispersers.
In the coastal forests of a tropical Japanese island near Okinawa, there grows an inconspicuous plant that is easy to mistake for a mushroom. It is a non-green parasite feeding on the roots of other plants and produces up to a million tiny seeds, each covered in a minuscule, dry fruit. The seeds, in particular, present a mystery to scientists: How do they get dispersed? "Many people assumed that they simply fall by gravity and are carried away by the wind, but the humid and windless forest floor makes this seem rather implausible," says Kobe University botanist SUETSUGU Kenji.
His previous studies showed that the fruits of plants in the family Balanophoraceae, with their dull coloration and yeasty scent, are in fact dispersed by invertebrates such as ants, crickets, and cockroaches, with plants whose fruits are firmly attached being visited by insects with stronger mandibles, and those with easily detachable fruits being visited by smaller invertebrates. Balanophora fungosa, too, has dull colors, a yeasty scent and easily detachable fruits, so Suetsugu suspected the dispersers to be ants. But when he covered the plants with nets that let through ants but not larger animals, he found that the plant's fruits were no longer picked up. So, the Kobe University researcher set up time-lapse cameras to observe who the real disperser was.
The main disperser turned out to be the land hermit crab Coenobita brevimanus, as Suetsugu now reports in the journal Ecology. He found that the crabs scraped off the fruits with their pincers, sometimes removing a plant's entire fruit ensemble overnight. As a result, some of the fruits ended up attached to the crabs' bodies, and Suetsugu spotted animals with their cargo even several meters away from the plants. He even found that some of the seeds survived passage through the crabs' digestive tract, and considering that land hermit crabs can travel up to 100 meters, they are likely effective dispersers of the plant. "I was excited to find this because these animals have usually been regarded as omnivores, scavengers or seed predators rather than potential plant partners," says Suetsugu. In his newest paper, he writes that "this study provides the first empirical evidence that hermit crabs can function as both external and internal seed dispersers."
What is more, Suetsugu found that the animals seemed to actively seek out the mature plants, as he didn't see them around immature plants. This means that the animals were probably attracted by the scent the mature plants give off, and that this scent is a direct advertisement to invertebrates who otherwise feed on almost anything they can find on the ground and get their pincers on.
"When it comes to seed dispersal, large vertebrates have received most of the attention, but small-bodied invertebrates are often overlooked. Nevertheless, they may also help maintain seed movement, especially on islands, in fragmented habitats, and on shaded forest floors," explains Suetsugu. Every new finding brings him closer to creating a predictive framework for when and why such animals become important dispersers. For land hermit crabs, this is the first observation, but, as he writes in his study, "their abundance in coastal forests and omnivorous feeding habits suggest that this may represent a more general pattern."
This research was funded by the Japan Science and Technology Agency (grant JPMJPR21D6).
Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan's leading comprehensive research universities with over 16,000 students and over 1,700 faculty in 11 faculties and schools and 14 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society's challenges.
