Ann Arbor Times

A recent study from the University of Michigan reveals that a unique association between mantis shrimp and clams defies a well-established ecological principle. The competitive exclusion principle posits that only one species can occupy a particular niche in a biological community at any one time. However, in nature, multiple species often appear to share the same niches simultaneously.

Graduate student Teal Harrison and her adviser Diarmaid Ó Foighil examined seven marine clam species living within the burrows of predatory mantis shrimp. Six of these species, known as yoyo clams, attach to the burrow walls using a long foot to spring away from danger. The seventh species attaches directly to the mantis shrimp’s body and does not exhibit this yoyo behavior.

Ó Foighil commented on this unusual arrangement: “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?”

Harrison's field samples revealed unexpected results: burrows containing multiple clam species were exclusively inhabited by burrow wall yoyo clams. In laboratory experiments, when the host-attached clam was introduced, the mantis shrimp killed all burrow-wall clams.

Ó Foighil explained: “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.” He added, “The interesting twist is the only survivor was a clam attached to the mantis shrimp’s body.”

The competitive exclusion principle suggests that different niches should lead to cohabitation rather than exclusion. However, Harrison’s data indicate otherwise; evolution of a new niche led to ecological exclusion among these commensal clams.

“It was very surreal,” Harrison noted about her lab findings. “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.”

Despite initial disappointment over what seemed like experimental failure, Ó Foighil saw potential for groundbreaking discoveries: “When you get a completely unexpected result in science, it’s potentially telling you something brand new and important.”

The researchers speculate various reasons for this exclusion mechanism but remain uncertain. They plan further studies to investigate whether both types of commensals can recruit as larvae to the same host burrows and if changes in mantis shrimp predatory behavior are responsible.

Co-authors Ryutaro Goto of Kyoto University and Jingchun Li of the University of Colorado contributed significantly to this research.