Marine Predators Have Grown Larger as Their Prey Remain the Same Size

New findings support the idea that increasingly powerful animals at the top of the food chain can drive evolutionary trends in their prey.

Shells can survive a remarkably long time in the fossil record, such that if a person slurped up several oysters and carefully saved the shells, scientists millions of years into the future might be able to document the ancient meal.

Adiël Klompmaker was struck by a similar idea a few years ago, when he was a postdoctoral researcher at the Florida Museum of Natural History. He was preparing a database of drill holes created by marine predators in shells, and it occurred to him that the shells had untapped potential as “smoking gun” evidence for over 500 million years of marine predator-prey interactions.

He and his colleagues have since conducted a detailed analysis of thousands of such shells, revealing that numerous marine predators have grown steadily larger and more powerful over time, while their preferred prey has remained relatively small.

The findings, reported in the journal Science, support what is known as the “escalation hypothesis.” This theory, in part, holds that increasingly powerful and metabolically active animals at the top of the food chain can drive evolutionary trends in their prey, such as by affecting their defensive arsenals and ability to move.

“There is a long-term increase throughout the last 500 million years in the size of drilling predators and predator-prey size ratios of drilling predators and their prey,” said Klompmaker, who is now a postdoctoral researcher at the University of California, Berkeley.

“Although prey size did not increase or decrease over the last 500 million years, they may have responded to their predators by becoming more mobile,” he added. “Some of them started to burrow into the bottom of the oceans, and some developed a more fortified shell.”

Klompmaker and his colleagues first found a strong correlation between predator size and the diameter of drill holes that they produced in the shells of prey. The predators in this case were primarily members of two marine gastropod families known as moon snails (Naticidae) and murex snails (Muricidae), but they also included various species of octopus and other groups of snails. Their prey commonly consisted of clams, other snails, and brachiopods, otherwise known as lamp shells.

The researchers next compiled data from 6943 drilled shells to examine trends in the size of drill holes, prey size, and predator-prey size ratios from the Cambrian Period (541–485.5 million years ago) to the present. The difference in size ratios increased 67-fold, providing evidence for the ever-widening gap in size between certain big marine predators and their tiny prey.

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Klompmaker and senior author Seth Finnegan of UC Berkeley explained that a combination of three factors likely led to the increase in predator size and predator-prey size ratios. First, the density of prey items increased over time, such that predators encountered prey more often and with less search effort.

“Secondly, the amount of meat per shell rose as the relatively less nutritious brachiopods declined in abundance and the meatier mollusks increased in abundance over time,” Finnegan said.

Thirdly, the scientists believe that as predation ramped up, the hunters themselves were increasingly vulnerable to their own enemies, such as crabs and fish. Pursuing small, easy prey might have lessened the threats posed by these other predators.

Humans are part of the equation now, but the researchers are not yet certain what affect we are having on the escalation hypothesis.

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“Marine predators, especially big ones, have been particularly vulnerable to human exploitation, including many large predatory fish and multiple groups of crustaceans, however, how these threats impact drilling predators such as moon snails is more difficult to evaluate,” co-author Michal Kowalewski of the Florida Museum of Natural History said. “In fact, it is possible that the decline of higher level predators, some of which prey on predatory mollusks, may benefit drilling predators.”

“On the other hand,” he added, “many recent environmental changes negatively affect all benthic organisms, so the long-term impact of humans is difficult to predict.”

Co-author John Warren Huntley of the University of Missouri said that in future, he and his colleagues hope to expand their database of information gathered on drill holes to refine their estimates of long-term trends in marine predation intensity. The researchers are also analyzing parasite-host interactions dating back to the Cambrian Period.

“Initial results suggest contrasting temporal trends between parasite prevalence and predation frequency,” Huntley said.

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