Recent studies reveal that a microscopic freshwater parasite, which is known to pose health risks to humans, protects its colonies with a specialized group of non-reproductive soldiers. This finding elevates this species of parasitic flatworm to the same level as intricate animal communities like ants, bees, and termites, which also have diverse classes of workers and soldiers that relinquish reproduction to benefit their colonies.
Recent investigations by scientists at UC San Diego’s Scripps Institution of Oceanography have discovered that a minute freshwater parasite linked to human health issues employs a group of soldiers that do not reproduce to safeguard its colonies.
This revelation, published today in the Proceedings of the National Academy of Sciences and supported by the National Institutes of Health, positions this species of parasitic flatworm alongside complex animal societies such as ants, bees, and termites, which also exhibit distinct classes of workers and soldiers that forfeit reproduction for the benefit of their community.
When this particular flatworm, Haplorchis pumilio, enters the human body—typically through raw or undercooked fish—it can lead to gastrointestinal troubles and, in severe instances, stroke or heart attack. The Food and Drug Administration recommends thoroughly cooking fish or freezing raw fish for at least one week to eliminate these trematodes. Although specific statistics on Haplorchis pumilio are not available, foodborne trematode infections are responsible for approximately 2 million life years lost due to disability and death each year globally.
In contrast to bees and termites, the colonies of this flatworm species do not reside underground or in tree hollows; instead, they exist within the body of a living snail. The parasites do not kill the snail; rather, they absorb nutrients for years while continuously generating free-swimming clones that seek out fish, the next host in their intricate life cycle.
“In comparison to bees and termites, we can host significantly more flatworm colonies in laboratory settings,” said Ryan Hechinger, an ecologist at Scripps and the study’s lead author. “These flatworms could prove to be invaluable tools for exploring fundamental sociobiological questions—like the evolution of such social structures.”
While other species within the trematode group also develop soldier castes, those have been shown to retain some inactive reproductive tissues, suggesting a potential for future reproduction. This study provides the first evidence of trematode soldiers that are so specialized in their function that they entirely lack reproductive tissues and appear permanently incapable of reproduction, according to Hechinger.
Haplorchis pumilio constructs nest-like colonies within the freshwater snail Melanoides tuberculata before progressing through two additional hosts during its life cycle—commonly fish and, ultimately, a warm-blooded animal. Both the trematode and snail are native to regions of Africa and southern Asia, but they have also become co-invasive in parts of the Americas, including California, Texas, and Florida.
Researchers identified the soldier caste during the phase of the trematode’s life cycle when it forms colonies inside the snail. These soldiers protect the colony from invaders, such as competing parasites, using a large mouth adapted to create a strong suction capable of tearing open adversaries and extracting their insides.
“These worms are determined to defend their home,” stated Dan Metz, the lead author of the paper and a postdoctoral researcher at the University of Nebraska-Lincoln, who conducted this research during his doctoral studies at Scripps. “If another parasite enters their snail, it’s akin to someone intruding into your home and endangering your family. You would do everything possible to protect your family, and so will these worms.”
Metz’s encounter with these worms was somewhat accidental. While strolling by Lake Murray in San Diego, California, he spotted an unfamiliar snail at the water’s edge and decided to take it back to the lab. It turned out to be Melanoides tuberculata, and it was infected by the trematode Haplorchis pumilio.
Upon a detailed examination of the parasite colony, Metz and Hechinger observed that many of the colony members were approximately half the length and exhibited different proportions than the rest. These smaller worms were identified as the soldiers, which, despite being less than half a millimeter long (0.02 inches), possessed mouths five times larger than their much larger, reproductive counterparts.
“These soldiers essentially function as mobile jaws,” explained Metz. “Their primary role is to bite.”
Through rigorous experiments and close scrutiny, Metz and Hechinger uncovered numerous pieces of evidence indicating that these soldiers are distinct from their reproductive relatives within the colony.
Firstly, all soldiers were devoid of any reproductive organs, while even the smallest immature reproductive worms— including unborn ones still residing within their parent— displayed detectable reproductive structures. Secondly, the researchers found no worms displaying body proportions that bridged the physical differences between soldiers and reproductive worms. Soldiers of various sizes maintained disproportionately large mouths compared to their body size, while no reproductive worms exhibited similar mouth-centric proportions. Lastly, experimental trials confirmed that soldiers consistently attacked other parasites, whereas reproductive worms displayed little to no aggressive behavior.
The study further revealed that having a dedicated soldier caste appears to enhance ecological dominance for this particular trematode species, especially within its invasive range in Southern California. Among other trematodes that have also invaded Southern California and infect the same snail, researchers determined that the soldier-producing species accounted for approximately 94% of all deaths among the different trematode species. This additional firepower in trematode conflicts led to significantly lower populations of competing trematode species than would typically occur.
“The soldiers pose no threat to humans—they only exist within the snails,” remarked Metz. “A different life stage of the worm is responsible for infecting warm-blooded animals, including humans. However, having soldiers enhances the parasite’s ability to infect snails, increasing its chances of transmission to humans.”
This parasite is expanding its reach globally, and understanding its biology is critical not only for public health but also for gaining insights into the evolutionary origins of complex social structures. There may also be numerous other species of trematodes exhibiting permanent soldier castes.
“We’ve only scratched the surface of this area,” Hechinger noted. “There could be as many as 200,000 trematode species worldwide, and we have only studied a fraction of them in the context of social organization. There is a wealth of knowledge waiting to be uncovered regarding how trematodes form complex societies with distinct divisions of labor.”
