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City officials first noticed the eye-catching pink clusters around the Nueva Street bridge last October. San Antonio River Authority staff were alerted and came to investigate.
One could easily mistake the 2-inch, caterpillar-shaped clusters for a wad of chewed-up bubble gum or some runaway cotton candy. But the bright pink masses are actually clutches of invasive apple snail eggs. Each clutch contains an average of about 2,000 eggs.
Upon inspection, San Antonio River Authority biologists scraped and crushed the invasive interlopers and have been removing the snails ever since. So far, more than 1,000 egg clusters have been removed from the banks of the urban reaches of the San Antonio River, and 58 adult snails have been eradicated.
The prolific gastropods, Pomacea maculata, grow as large as baseballs and eat all vegetation. Native to the Amazon, the snails can tolerate a wide range of temperatures but dislike extreme heat and will die if frozen, said Romi Burks, a biology professor at Southwestern University who has studied the snails since 2004.
When river temperatures get into the 80s and 90s, the snails “aestivate,” which means they bury into the sediment and retreat into their shell – “a summer version of hibernation,” said Burks. “They can stay in a reduced metabolic state for several weeks to a couple months.”
From early summer to late fall, typically a female climbs up a plant or the concrete channel of the San Antonio River early in the morning or late at night to lay about one clutch per week just above the waterline. Clutch size varies from a few hundred eggs to several thousand, with an average of 2,000.
It takes seven to 14 days for eggs to hatch depending on temperatures. Just after laying, the matrix holding the eggs together appears gelatinous and bright pink. “Like bubblegum,” said Burks. “If you sink those, they will not hatch.”
For three to six days, they are noticeably pink, and they fade as they mature, turning pale pink, then white, then grey. Knocking the mature clutches into the water can add viable hatchlings to the water body. Burks recommends crushing egg clutches whenever possible.
Burks provided the following numbers for putting the snails’ reproductive life into perspective. Average clutch size: 2,000 eggs. Average hatching efficiency: 70%. Number of hatchlings added to water per clutch: 1,400. Number of clutches per female in Texas in summer: 20 (conservative). Number of possible hatchlings: 28,000.
Assuming a mortality rate of 99.99 percent, about three adult snails per female would be added to the population annually, she said.
“This might be an overestimation in survival but it’s easy to see how snail abundance easily gets out of control,” said Burks.
Females contain reproductive glands that are toxic to predators and the eggs contain similar neurotoxins, and the snail’s native predators are few in Texas.
Given this natural advantage, the snail population is growing fast. Their increased presence poses a threat, especially to the channeled, low-flow urban stretches of the river where greenery and shelter for native species are at a premium.
“If the snails are eating aquatic vegetation, it’s no longer available for refuge or for larger fish to use as spawning habitat,” said Chris Vaughn, an aquatic biologist for the San Antonio River Authority who oversees SARA’s snail removal effort. “A small largemouth bass needs things to hide behind and nine times out of ten, that’s an aquatic plant.” With no shelter or places to spawn, that hurts the fish population, he said.
The snails found their way into the San Antonio River most likely through an aquarium release, said Burks.
Pet stores sell a variety of freshwater snails assumed to be good aquarium species and Pomacea maculata have slipped into home aquariums. Aquarium owners often buy snails as pets and to help keep algae out of their aquariums.
Calls to local pet stores asking for apple snails were met with “We’ve never sold them.” Burks said that is not the case.
When they’re babies, many snail species look alike, said Burks. It’s not uncommon for suppliers and aquarium store staff to inadvertently confuse Pomacea maculata with other species. Customers seeking snails unknowingly take home this species as babies. When the tiny snail then gets as big as a baseball, they release it to nature, thinking they are doing the right thing.
Vaughn said SARA is less concerned about the snails damaging the Mission Reach section of San Antonio’s award-winning $375 million riparian restoration south of town than it is about the urban sections of the river.
“They’re more of a threat to aquatic vegetation in the Museum Reach,” he said, adding that currently SARA assesses the risk of damage to the Mission Reach as “very low…we don’t feel their (the snails’) fitness is suitable to those reaches.”
Burks disagreed. “The snails are absolutely a cause for concern on the Mission Reach,” she said. Since they’re general herbivores, they’ll eat what is there, she said. “For the most part, they are voracious, not picky eaters. … They can survive in the whole river stretch. As long as there is food and places for them to lay eggs, the snails can survive.”
The snails have wreaked havoc in streams and rivers around Houston, said Burks. Their spread increased following Hurricane Harvey in 2017, when floodwaters carried them to new areas where they normally didn’t thrive.
For anyone contemplating apple snails as a foraged escargot dinner, Burks urged caution. The public “really shouldn’t handle the snails.”
The “foot” of the snail – the meaty part seen when the creatures creep along the bank – is edible. But the bright pink reproductive organ of the female snail contains neurotoxins that can cause problems when eaten.
“Be careful to clean all the organs off, just like when cleaning a fish,” said Burks. And never eat raw snails, because uncooked they can carry a roundworm parasite called Angiostrongylus cantonensis that can cause a type of meningitis, she said.
She added that apple snail is “not bad grilled.” Her favorite snail recipe? Apple snail en papillote. “It’s like the Greek spanakopita – finely diced with garlic and spinach. You don’t even know you’re eating snail,” she said.
This story has been updated to clarify information about aquarium snail species, the apple snail’s spread in Houston, and the prevalence of its Texas predators.