A new study from the University of Michigan has challenged a longstanding ecological principle by uncovering surprising interactions between mantis shrimp and clams. The findings reveal that mantis shrimp kill their cohabitant clams, prompting a reevaluation of the competitive exclusion principle.
A new study from the University of Michigan has revealed startling discoveries about the life-or-death interactions among marine clams and their predatory mantis shrimp hosts. The research, led by ecology and evolutionary biology graduate student Teal Harrison, upends the competitive exclusion principle, a cornerstone of ecological theory.
Under the traditional competitive exclusion principle, only one species should occupy a given niche within a biological community. Yet, countless examples in nature show various species seemingly occupying the same space simultaneously. Harrison and her adviser Diarmaid Ó Foighil, a professor of ecology and evolutionary biology, took a closer look at one such community of marine clams living with a mantis shrimp host.
Seven species of so-called yoyo clams reside within the burrows of their mantis shrimp hosts. Six attach to the burrow walls, leveraging a foot to spring away from danger, thus earning the name “yoyo.” In contrast, a seventh species uniquely attaches to the mantis shrimp’s body. The researchers were intrigued by how these species coexist, especially given their shared dependence on a predatory host.
“We’ve got this remarkable situation where all these clam species not only share the same host but most of them have also evolved, or speciated, on that host. How is this possible?” Ó Foighil said in a news release.
Unexpectedly, Harrison’s field samples showed that burrows containing multiple species of clams were exclusively populated by burrow wall yoyo clams. When the body-attaching species was introduced in a lab setting, the mantis shrimp killed all burrow wall clams, a finding contrary to theoretical expectations.
“Teal had two sets of unexpected results. One of them was that the species that should co-occur with the yoyo clams doesn’t. And the second unexpected result was that the host can go rogue,” Ó Foighil added. “The interesting twist is the only survivor was a clam attached to the mantis shrimp’s body. Anything on the burrow wall, it killed. It even went outside the burrow and killed one that had wandered out.”
This finding is a twist on the competitive exclusion principle, which would predict more frequent cohabitation between niche-differentiated clams and yoyo clams. Instead, the evolution of a new niche — attaching directly to the mantis shrimp — led to the exclusion of other clams.
“It was very surreal,” said Harrison of her lab findings in the news release. “It honestly didn’t even dawn on me that they were eaten right away because it was so far from what I was expecting to find. They are commensal organisms, they cohabitate with these mantis shrimp in the wild, and there was no possible way we would know whether this behavior was already happening this way in the wild or not. I just wasn’t expecting it.”
Harrison’s initial devastation over the unexpected outcomes shifted to excitement for Ó Foighil.
“Teal was understandably distraught when the experiment ‘failed’ after all her hard work, but I was excited. When you get a completely unexpected result in science, it’s potentially telling you something brand new and important,” he said.
The researchers are now focused on uncovering why the mantis shrimp’s behavior differed. One hypothesis suggests that during the larval stage, yoyo clams and host-attached clams may occupy different burrows. Another possibility is that the mantis shrimp’s lethal reaction is triggered in mixed clam populations.
Their findings indicate the potential for ecological associations to “break down catastrophically,” shedding new light on commensal relationships in marine environments. Next steps include further studies to determine larval recruitment to host burrows and to assess how the presence of host-attached clams influences mantis shrimp behavior.
The study, “Within-host adaptive speciation of commensal yoyo clams leads to ecological exclusion, not co-existence,” was published in PeerJ.