According to new research, horse conchs, the flashy marine snails that inhabit Florida's colossal state seashell, live shorter lives and reproduce later than previously thought, indicating that the Gulf of Mexico population may be on the verge of extinction.
Horse conchs are one of the most visually appealing species on the beaches of the southeastern United States, with spindle-shaped shells that can grow to be more than a foot long and red-orange bodies as bright as traffic cones.
They used to be even bigger: historic photographs of tourists in Florida show them carrying horse conch shells half the length of a small child. These sizes are no longer seen, prompting researchers to wonder.
Research Findings:
Sclerochronology—the shell version of dendrochronology, or tree-ring science—was used to investigate the lifespans of the animals, whose off-white shells have been measured to be as long as two feet from pointy top to funnel tip. Because of their size, some scientists believed the predatory snails could live for a half-century or more, with females releasing hundreds of thousands of tiny conchs into the sea over decades. According to new research, this is not the case.
"The actual lifespan is significantly shorter," says Gregory S. Herbert, a marine ecologist at the University of South Florida who led the study, which was published recently in the journal PLOS ONE. According to the findings, females conceive later in life. Because today's largest horse conchs are both smaller and younger than the historic shells used in the study, "the largest females left in the wild may have few lifetime spawning events if any," the article warns, putting the Gulf population in jeopardy.
Earlier research found that the size of the conchs has decreased over time, which Herbert describes as "the universal sign that a tipping point is near." Horse conchs, like other marine animals living near heavily populated coasts, have lost a significant amount of habitat due to development and pollution, including favourite breeding grounds along mudflats and seagrass beds. Their Gulf habitat is also warming as a result of climate change, which scientists believe puts additional strain on the animals based on the negative effects extra heat has on other large mollusks. The more immediate threat to their numbers and sizes, according to scientists, is overharvesting, primarily for their highly sought-after shells.
According to data from the Florida Fish and Wildlife Conservation Commission, the reported commercial harvest in Florida fell from a high of 14,511 horse conchs in 1996 to 6,124 in 2000, 1,461 in 2015, and 67 in 2020. The number of recreational harvests is unknown.
Drilling for Data
Herbert's team examined chemical isotopes in large horse conch shells from museum collections to estimate age and reproductive maturity. A marine mollusk builds its shell with calcium carbonate from the surrounding sea, which serves as a chemical diary of its life and environment. Scientists can see the shell of bivalves like clams in half to read wispy grey bands in the cross-section that mark time, similar to tree rings. Snails build in a spiral, making it impossible to section the shell in such a way that all of the bands are visible.
Herbert's team drilled into the shells with tiny dental bits, milling a fine powder to measure the relative weights of oxygen and carbon isotopes. The researchers milled along the lifetime of spiral growth to the lip, beginning at the pointy top of each shell—where the conch embryo once fit and began to build.
The scientists were able to age the shells by analyzing oxygen isotopes, which record temperature changes over warm and cool seasons. The carbon isotopes, which are heavily influenced by the animal's own physiology, particularly reproduction, allowed them to determine when the females first spawned. The carbon isotopes also revealed that mother horse conchs put a lot of energy into their egg masses, which are honeycomb-like structures with thousands of capsules, each of which feeds an embryonic conch that grows a perfect, pea-sized shell before hatching and crawling away.
The largest-known horse conch shell, a 23.9-inch beauty displayed at the Bailey-Matthews National Shell Museum on Florida's Sanibel Island, was included in the study. The scientists were unable to drill into the record shell, so they estimated its age by plotting its whorls alongside the growth curves and isotope values of the other shells. They concluded that the animal that built it lived for 16 years, which is the species' most likely maximum age.
According to coauthor Stephen P. Geiger, a mollusk researcher with Florida's Fish and Wildlife Research Institute, which advises state regulators on species management, the age of the record shell was a big surprise. According to conventional wisdom, the record horse conch was at least a half-century old.
According to Geiger, scientists who are still learning about the basic biology of horse conchs assumed that females would have many chances to spawn over such long lives. These assumptions aided in keeping horse conchs, like the vast majority of fish and mollusks in Florida, in the "unregulated" category, with no limit for permitted commercial harvesters and a 100-pound-a-day limit for recreational fishers. That's a lot of conch shells.
Loving Icons to Death
According to malacologist Jose H. Leal, science director and curator at the Bailey-Matthews Museum and editor of The Nautilus, one of the oldest scientific journals of mollusks, the new research is as convincing as it is urgent. While it's difficult to get people excited about protecting squishy mollusks, Leal, who was not involved in the study, believes the horse conch is a worthy cause. "It's obvious. It's magnificent. It's called the state shell."
In 1969, the Florida Legislature designated the horse conch as the state seashell. On the day of the vote, members of the Palm Beach Shell Club placed a shell on the desk of each of Florida's 160 legislators. Today, it joins a number of Florida state symbols that have been pushed to the brink of extinction by the humans who revere them.
The Florida panther, the state animal, is on the verge of extinction due to hunting and habitat loss. The manatee, the state marine mammal, is dying in large numbers due to pollution-caused loss of its seagrass food source. The sabal palm, the state tree, is succumbing to a fatal disease spread by an invasive pest and is dying out in coastal forests due to soil salinity caused by sea-level rise.
According to Herbert, the study demonstrates how sclerochronology can help fill in the gaps in mollusk life histories without collecting and killing increasingly rare animals. Despite the limited population data, the findings show that horse conchs deserve to be protected, he says. "It's like a wobbly vase that hasn't yet fallen but might if no one catches it."
(Source: National Geographic)
