With their feathery arms and long stalks, sea lilies look like their land-based namesakes. But unlike the lilies of the land, sea lilies are able to pick up and move. Over 200 million years of being eaten by sea urchins they have evolved escape strategies, including pulling themselves out of reach along the ocean floor.
“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,” the University of Michigan said in a news statement about the paper.
Feather star on a coral from Palau.
Photo by F. J. Gahn
Baumiller’s study builds on previous research on present-day sea lilies and urchins, tracing the evolutionary arms race between them way back in time.
“With their long stalks and feathery arms, sea lilies look a lot like their garden-variety namesakes. Perhaps because of that resemblance, scientists long had thought that sea lilies stayed rooted instead of moving around like their stalkless relatives, the feather stars,” U-M said.
“But in the 1980s, Baumiller and collaborator Charles Messing of Nova Southeastern University’s Oceanographic Center in Dania Beach, Florida, observed sea lilies shedding the ends of their stalks to release themselves from their anchor points and using their feathery arms to crawl away, dragging their stalks behind them.
“Then, while going through hundreds of hours of video shot during submersible dives, the two researchers came across footage that offered an explanation for why sea lilies might get up and go. The videos showed sea urchins lurking in gardens of sea lilies, some of which appeared to be creeping away from the predators.”
In some photos, the sea floor around the urchins was littered with sea lily arms, “like table scraps left from a feast,” U-M said.
“Further studies by Baumiller, Messing and Rich Mooi of the California Academy of Sciences suggested that sea urchins don’t simply scavenge bits of dead sea lilies that they find on the ocean floor; they bite pieces right off their prey, giving sea lilies plenty of reason to shed their stalk ends like lizards’ tails and scoot away.”
“First, the researchers put sea urchins into a tank with detached crinoid arms, pieces of crinoid stalks and arms, and live crinoids. Every urchin that was given the opportunity at least nibbled on crinoids, and one even ate a whole feather star. This experiment not only confirmed that urchins prey on crinoids, but it also revealed that crinoid parts that pass undigested through urchins bear characteristic scratches and pits that match the size and shape of the teeth in the urchin’s ‘mouth,’” the University of Michigan said.
The researchers looked for the same kinds of bite marks on more than 2,500 crinoid stalk fossils from Poland, dating back to the middle of the Triassic period, some 225 million years ago. More than 500 of the fossils had the same bite marks, they found.
A sea urchin with a piece of crinoid stalk (the white) in its jaws.
Photo by F. J. Gahn
The findings suggest that the development of motility in crinoids, that in some groups included swimming and floating, were stimulated by their interactions with predators, the researchers said.
The time frame is significant, too, said Baumiller, professor of geological sciences and a curator at the U-M Museum of Paleontology.
“Some of the best examples of the effects of escalating interactions between predators and prey come from something called the Mesozoic Marine Revolution (MMR), a dramatic increase in the diversity of predators and their prey that started during the late Mesozoic Era, about 150 million years ago. But the new study suggests that, at least for crinoids and their predators, the arms race began even earlier,” U-M said.
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