Escalating arms race: Predatory sea urchins drive evolution

Apr 17, 2012 by Jim Erickson
A sea urchin (the whitish globular object on the right, covered with purple-tipped spines) feeding on a crinoid (red-and-black arms with white bands) in an aquarium. The urchin uses its spines and the teeth of its jaw to manipulate and ingest the crinoid. The sea urchin is about the size of a baseball. Image credit: F.J. Gahn and M.A. Strong.

(Phys.org) -- Nature teems with examples of evolutionary arms races between predators and prey, with the predator species gradually evolving a new mode of attack for each defensive adaptation that arises in the prey species.

These adaptations are often portrayed as reciprocal, with prey and predator acting as two sides in an endless evolutionary tug of war known as co-evolution.

Now a University of Michigan paleontologist and his colleagues report an example of a much more one-sided, asymmetrical predator-prey relationship. The predators in this case are sea urchins, and over a nearly 200-million-year span they appear to have been in the driver's seat, forcing upon marine animals called crinoids, which include the sea lily.

Hungry sea urchins even drove the formerly stationary, or sessile, sea lilies to develop the ability to escape by creeping along the ocean floor.

"In this case, the predators have a greater impact on their prey than the other way around," said Tomasz Baumiller, professor of earth and environmental sciences and a curator at the U-M Museum of Paleontology. "What we're showing is evidence for the evolutionary concept called escalation."

This video is not supported by your browser at this time.
The above video shows a sea urchin approaching a feather star crinoid from the left, attacking it, then consuming it. The video was shot at the Coral Reef Research Foundation in Palau in January 2010. Both specimens were collected by scuba divers and placed in a temperature-controlled water tank. This time-lapse video clip covers a roughly six-hour period. Video by Tomasz K. Baumiller.

The escalation hypothesis was proposed by and paleontologist Geerat Vermeij in 1987. It predicts that long-term ecological shifts are caused by the evolutionary response of prey to predation pressure. The idea has been contentious in part because hard evidence to support it has been difficult to come by.

Baumiller and his colleagues present their findings in an article titled "Predator-induced macroevolutionary trends in Mesozoic crinoids," published online April 16 in the . The other authors are Przemyslaw Gorzelak of the Institute of Paleobiology at the Polish Academy of Sciences and Mariusz A. Salamon of the University of Silesia in Poland.

Sea lilies have long stalks and feathery arms that they extend to catch bits of plankton or detritus passing in the current.

In previous work, Baumiller and his colleagues showed that fragments of modern-day sea-lily stalks that pass undigested through sea urchins often bear bite marks: distinctive scratches and pits that match the size and shape of the teeth in the urchin's "mouth."

Nearly identical bite marks have been preserved in the fossil record across Central Europe in places like Poland. In a 2010 PNAS paper, Baumiller and others used more than 2,500 crinoid-stalk fossils to show that sea urchins preyed on crinoids 225 million years ago, in the early Mesozoic Era.

The 2010 paper provided a snapshot in time. The new report is more like a feature-length film. It shows how sea urchins and crinoids interacted over a 160-million-year span, beginning 225 million years ago and continuing through the end of the Mesozoic Era 65 million years ago.

Over that span many crinoids became motile, developing the ability to escape sea urchins, some by shedding their stalk ends like lizards' tails and crawling away. Other crinoids remained sessile.

In their latest PNAS paper, the researchers show that bite-mark frequencies on crinoids generally increased throughout the Mesozoic. Moreover, the frequencies of bite marks on motile crinoids were lower than those on sessile crinoids, a pattern consistent with the idea that motility constitutes an effective escape strategy.

The researchers examined more than 5,000 fossil crinoid specimens collected at 32 sites in Central Europe. In addition, they analyzed genus-level diversity histories of sea urchins and crinoids obtained from the Paleobiology Database and other literature sources.

They found that Mesozoic diversity changes in predatory sea urchins show a positive correlation with diversity of motile crinoids and a negative correlation with diversity of sessile crinoids. In simple terms, that means that as the number of different types of predatory sea urchins increased, the number of various types of motile crinoids also increased, while the diversity of sessile crinoids decreased.

That pattern is what would be expected if the escalation hypothesis is correct and predatory drove the observed long-term ecological shifts, Baumiller said.

"It appears that the predators are driving one group to lower diversities and another group toward higher diversities, and that pattern makes sense in an escalation context," he said. "In this case, the causal arrow is much more consistent with predators driving prey diversities."

Today, motile crinoids predominate in shallow-sea environments while sessile forms are restricted to deep-water sites where predator pressure is low. Crinoids, which are related to starfish, consist of two groups: sea lilies and feather stars.

Explore further: Male monkey filmed caring for dying mate (w/ Video)

Related Stories

Urged on by urchins: How sea lilies got their get-up-and-go

Mar 15, 2010

Nature abounds with examples of evolutionary arms races. Certain marine snails, for example, evolved thick shells and spines to avoid be eaten, but crabs and fish foiled the snails by developing shell-crushing claws and jaws.

Sea urchins see with their whole body

Jun 30, 2011

Many animals have eyes that are incredibly complex – others manage without. Researchers at the University of Gothenburg have shown that sea urchins see with their entire body despite having no eyes at ...

Sea urchins cannot control invasive seaweeds

Jul 13, 2011

Exotic marine species, including giant seaweeds, are spreading fast, with harmful effects on native species, and are increasingly affecting the biodiversity of the Mediterranean seabed. Some native species, ...

Decline in Alaskan sea otters affects bald eagles' diet

Oct 03, 2008

Sea otters are known as a keystone species, filling such an important niche in ocean communities that without them, entire ecosystems can collapse. Scientists are finding, however, that sea otters can have even farther-reaching ...

Recommended for you

Male monkey filmed caring for dying mate (w/ Video)

4 hours ago

(Phys.org) —The incident was captured by Dr Bruna Bezerra and colleagues in the Atlantic Forest in the Northeast of Brazil.  Dr Bezerra is a Research Associate at the University of Bristol and a Professor ...

Orchid named after UC Riverside researcher

22 hours ago

One day about eight years ago, Katia Silvera, a postdoctoral scholar at the University of California, Riverside, and her father were on a field trip in a mountainous area in central Panama when they stumbled ...

In sex-reversed cave insects, females have the penises

Apr 17, 2014

Researchers reporting in the Cell Press journal Current Biology on April 17 have discovered little-known cave insects with rather novel sex lives. The Brazilian insects, which represent four distinct but re ...

Fear of the cuckoo mafia

Apr 17, 2014

If a restaurant owner fails to pay the protection money demanded of him, he can expect his premises to be trashed. Warnings like these are seldom required, however, as fear of the consequences is enough to ...

User comments : 0

More news stories

Researchers successfully clone adult human stem cells

(Phys.org) —An international team of researchers, led by Robert Lanza, of Advanced Cell Technology, has announced that they have performed the first successful cloning of adult human skin cells into stem ...

Scientists tether lionfish to Cayman reefs

Research done by U.S. scientists in the Cayman Islands suggests that native predators can be trained to gobble up invasive lionfish that colonize regional reefs and voraciously prey on juvenile marine creatures.

Researchers develop new model of cellular movement

(Phys.org) —Cell movement plays an important role in a host of biological functions from embryonic development to repairing wounded tissue. It also enables cancer cells to break free from their sites of ...

Continents may be a key feature of Super-Earths

Huge Earth-like planets that have both continents and oceans may be better at harboring extraterrestrial life than those that are water-only worlds. A new study gives hope for the possibility that many super-Earth ...

Under some LED bulbs whites aren't 'whiter than white'

For years, companies have been adding whiteners to laundry detergent, paints, plastics, paper and fabrics to make whites look "whiter than white," but now, with a switch away from incandescent and fluorescent lighting, different ...