Iconic, Giant Shell Faces Risk of Extinction

The state shell of Florida, the horse conch is an enormous invertebrate with a shell that can grow as big as two feet long. The largest living snail in North America, the horse conch doesn’t live as long as scientists previously thought.

Researchers have found the population of the gastropods (Triplofusus giganteus) is shrinking and the iconic shell is at greater risk of extinction than believed.

Lead author Greg Herbert, associate professor at the University of South Florida School of Geosciences, tells Treehugger he is most fascinated by the conch’s size.

“It's been described as nature's traffic cone. It's about the same size and has a bright orange shell, and the animal inside has a bright red body,” Herbert says.

“I'm also an ecologist, and horse conchs are fascinating because they are apex predators when they reach full size. Florida has some of the largest shelled mollusks in the world, and the horse conch eats most of them. The dead empty shells sit on the seafloor and become homes for fish and crabs and many other species.”

Until recently, the state of Florida maintained a list called “Species of Greatest Conservation Need," which was the basis for setting funding priorities for research and management.

“The horse conch was on that list but removed because it was decided that there were more species than the state had resources to monitor; there weren't consistent criteria for determining which species were on the list or left off, and basic biological data to establish threats were missing,” Herbert says. “My lab is in a good position to help fill some of these gaps.”

Researchers found that the average life span of a horse conch is between eight and 10 years, which is much shorter than previously believed.

The findings were published in the journal PLOS One.

“There are studies estimating the horse conch's maximum life span to be ‘many decades’ just based on its size,” Herbert says. “When I spoke to other researchers, their guesstimates ranged from 50 to 80 years. We show that most horse conchs live barely one decade, and the record size shell, which is nearly two feet in length, was probably not more than 16 years old when it died.”

The record shell is on display at the Bailey-Matthews National Shell Museum on Sanibel Island, Florida. It measures about 23.86 inches (606 millimeters) and was collected off the island.

Like Rings on a Tree

For their study, researchers analyzed shell specimens that were part of museum collections. They were collected from the Dry Tortugas, Vaca Key, Sanibel Island, and Cape Romano.

Researchers didn’t want to risk harming an already threatened population in the wild.

“We wanted to avoid sacrificing any living animals for this study, especially a large old female horse conch that could still produce tens of thousands of offspring,” Herbert says. “We also needed a very large shell, and those are almost impossible to find these days. They've all been collected. But there are some left in museums from collections made in the 1960s. Those shells were large enough and old enough to help us understand the maximum lifespan of horse conchs.”

Like counting the rings on a tree, scientists dated the shells by analyzing chemical bands in the shell.

“You can see tree rings by eye and count them to get the tree's age. Some mollusks form similar visible growth bands that can be seen in cross-sections of the shell or even ornamental bands on the outer surface,” Herbert says.

However, the visible bands on mollusk shells are “notoriously unreliable” for estimating age, he explains.

“They might work for one species but not another, or the annual bands don't start until the animal is a few years old,” Herbert says. “Instead, we used a dental drill to collect samples from hundreds of spots along the growth of the shell, starting from the very tip and spiraling around to the final lip.”

They analyzed those samples with a mass spectrometer and used the information to determine each animal’s age. They found horse conches don’t live very long and also don’t seem to reproduce until relatively late in life. They start around age 6, when they give birth to as many as 28,000 offspring each year.

“It's possible that the largest horse conchs left in the wild today reproduce just once or twice in their lifetimes instead of many dozens of times,” Herbert says.

The findings are significant, researchers say, because they can help prioritize conservation efforts.

According to the Florida Fish and Wildlife Conservation Commission, the conch population fell from 14,511 in 1996, to 6,124 in 2000, 1,461 in 2015, and just 67 in 2020.

“Conserving a species means making sure there are enough offspring produced each year to keep pace with individuals killed by human harvest and natural causes. It's like balancing a spreadsheet with income and expenses,” Herbert explains.

“We still don't know how many horse conchs are 'out there' in the wild, but our research does show that the reproductive output of an individual female (the income) is 20-40 times less than what biologists previously assumed. We're hoping this research will help prioritize population surveys so the rest of the accounting can be done to protect this and other species.”