Some worms are far more socially complex than thought, according to scientists who have just discovered one species that forms social castes, including armies of warrior worms.
The study, published in the latest Proceedings of the Royal Society B, is the first to determine that any worm lives in a colony with organized division of labor. In this case, trematode flatworm parasites exist in cooperative colonies consisting of big reproducers, which release hundreds to thousands of clonal offspring daily, and specialized soldiers that defend the colony.
"The soldiers use their relatively large mouth parts to bite enemies," lead author Ryan Hechinger told Discovery News. "They sometimes swallow enemies whole."
"Soldiers sometimes rip open the body wall of the enemy and then suck out the insides," added Hechinger, an assistant research biologist at the Marine Science Institute at University of California, Santa Barbara. "You can sometimes see the eyes of the enemies' progeny inside the soldiers' guts."
He and colleagues Alan Wood and Armand Kuris collected California horn snails infected with the flatworm colonies. They analyzed both the physical appearance and behavior of the parasitic worms, including how the flatworms reacted when rivals were present.
The warrior flatworms stayed together in armies and maintained small, sleek and active bodies. Despite being 60 times smaller than reproducing flatworms, their jaws were just as big.
"It's good to be small for the soldiers because they can travel to areas of the host body where new invasions enter, so what we had was selection for a division of labor," Hechinger said. "Some individuals specialized in reproduction, becoming relatively immobile, fat slugs pumping out progeny."
The scientists think the worms started out as generalists. But as onslaught from invaders increased, traits evolved in some worms that benefited defense, while the reproducers became more specialized at what they do best.
It's now thought that all 20,000 or so trematode species live in such organized colonies.
Their cooperative societies appear to be extremely successful. Each colony lives for at least a decade, and the individual worms appear to live for quite a long time too.
"We say that because we don't generally encounter dead or dying worms," Hechinger explained. "In this study we carefully looked at 91,229 worms from 51 colonies and never detected a dead or dying worm."
In the future, the soldier worm armies may benefit humans, too.
"Hundreds of millions of people are infected by blood flukes," he said, adding that flukes (a common name for trematodes) can also attack the liver and lungs.
"Trematodes with soldier castes have a biomedical application because they may be used in the biological control of these problematic parasites," he continued. "Soldiers may also help to keep human disease-causing trematodes out of snail intermediate hosts, preventing infections in humans."
Bernard Crespi, a professor of evolutionary biology at Simon Fraser University, told Discovery News that he completely agrees with the new findings and "was astonished" when he read the paper.
"Such discoveries of sociality are quite rare and special, in providing new systems for testing theory and understanding how, why and where sociality has evolved," Crespi said.
Parasitic flatworms now join snapping shrimp, sea anemones, mole-rats and a handful of others on the relatively short list of organisms that form similarly organized castes.