Everything about our next invasive species could probably be summed up with the word "gross." From its huge size (as large as a softball) to the thousands of slimy pink egg cases it leaves behind, to the parasites it carries to its impact on native wildlife, there's very little to like about these inching invaders from South America.

In this installment of AL.com's series on invasive species, we look at the Amazonian apple snail, a non-native gastropod perilously perched on the edge of the ecological jewel that is Mobile-Tensaw Delta.

Dennis Pillion | dpillion@al.com

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In this photo by Ben Raines, a baseball-sized Amazonian apple snail slides over egg casings on a concrete wall at Mobile's Langan Park.

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'Nasty little critter'

At dusk in Mobile's Langan Park, a snail the size of a grown man's fist begins its long, slow journey out of the water and up the concrete piling of a bridge.

The snail slides its way up the vertical surface, slowly oozing out a pink smear in its wake. That smear is a clutch of eggs, thousands of them, and they're fueling an invasion that is all but impossible to completely stop. It's revolting to normal people and wildlife biologists alike.

"They're kind of a nasty little critter," said Dave Armstrong, District 5 Supervisor at the Alabama Division of Wildlife and Freshwater Fisheries.

Armstrong is leading the state of Alabama's efforts to control this invasive mollusk in state waters, an effort that's seen limited success. The snails appear to have been successfully extirpated from one residential lake in Baldwin County -- where they were discovered before they had firmly taken hold -- but Mobile's Three Mile Creek is another story.

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Introduced through aquariums

Somewhat ironically, these 'nasty critters' were first spread to the U.S. through the freshwater aquarium trade. They are sold small as algae eaters for freshwater tanks, but are occasionally released back into the wild if they outgrow their tank.

"The main idea is that they often get a little too large or maybe too abundant, and for whatever reason the aquarium owner decides they don't want them anymore so they decide to just release them in the waterways," said Bill Walton, a specialist with the Alabama Cooperative Extension System and assistant professor of aquaculture and recreational pond management at Auburn University.

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They smell bad

Armstrong said he was surprised the snails were popular in the pet trade due to their large size, messy egg cases and other negative aspects of keeping them in captivity.

"They kind of smell for one," he said. "They're worse than a turtle. They foul their own water very quickly, so I don't understand the appeal of keeping them in an aquarium."

And yet some aquarium suppliers did sell them, so the species made its way from South America and the Amazon River basin to freshwater aquariums in Mobile, to Three Mile Creek.

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Still legally sold

Armstrong said there is no legal prohibition in Alabama against selling the snails in the pet trade. He's explored the idea of trying to limit their availability or encouraging distributors to choose other species that are less destructive if they escape captivity. So far, he said, he's had little success with either.

Casi Callaway, who runs the Mobile Baykeeper organization and has organized several apple snail roundups in Mobile, said she's seen the tell-tale blue aquarium gravel during round-ups where someone has emptied their tank into the water. The snails in the photo above were removed from Mobile's Langan Park during a Baykeeper snail roundup.

"They're so cute but they get too big for your fish tank and you don't want to kill them, so you set them free... to destroy the entire Mobile-Tensaw Delta," she said.

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Langan Park is ground zero in Alabama

Though invasive apple snail populations have taken hold in several coastal states in the Southeast, in Alabama the population (as far as we know) is limited to Three Mile Creek and its tributaries in Mobile, and especially the lake at Mobile's Langan Park.

Since the snails were first discovered in Langan Park in 2008, numerous roundups have been held to try and remove as many of the snails and their egg cases as possible. The Division of Wildlife and Freshwater Fisheries also received a grant in 2014 to treat the area with a copper-based medication harmful to snails but safe for fish.

During the most recent roundup this summer, volunteers organized by Mobile Baykeeper collected 800 apple snails and thousands of snail eggs. Many more remain.

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Lake a lost cause

Armstrong said the population in Three Mile Creek proper may be on the decline, thanks to tidal influence, but in the lake in Langan Park, their numbers are "exploding again."

"It's just the perfect habitat for them," Armstrong said.

Armstrong, Walton, and Callaway all said eradication does not seem like a realistic outcome at this point in Langan Park, but management is important to help keep the snails from spreading even further.

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Hard to see

One reason these roundups are limited in their effect is that it's hard to see all the snails, even the large ones.

"There's a lot of muck in the water, so they're pretty well hidden," Walton said. "You'll see the ones that are out of the water laying eggs, but you don't see a lot of the population."

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They go deep

When the apple snails first showed up in Mobile, Armstrong and others hoped that a hard freeze would mean a hard reset on the snail population. It didn't, as the snails were seen alive and active just a week after a record cold snap in early 2014.

Armstrong said the snails can burrow deep beneath the mud during the winter, or other times of intense stress, and emerge unscathed weeks or months later. Some studies, he said, have indicated the snails can burrow as much as 10 feet deep to escape the cold.

This photo by Ben Raines shows one of the invasive snails underwater in Mobile's Three Mile Creek.

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Eggs like bubble gum

The snails spend most of their time out of sight and under the water, but it's hard to miss the pink "bubble-gum" looking egg cases they lay at water's edge.

The eggs need to stay dry, at least in the early stages of development, to be viable, but the newly hatched snails will need access to the water quickly. For that reason, the eggs are prolific on plants, concrete surfaces and wooden docks protruding from the water.

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...or pink slime

Removing the eggs can be a sticky mess, as shown in this Mobile Baykeeper photo. Callaway said that during the roundups, the volunteers use scrapers and buckets to try and collect the slimy masses as completely as possible.

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Thousands of eggs

There are male and female apple snails, and once they reach sexual maturity, they can lay eggs almost continually throughout the summer. Armstrong said the snails can lay eggs from March through November in Alabama, and they can sometimes be seen even into December.

"They'll lay eggs multiple times during the warm season, about 8-9 months out of the year, they can lay eggs," he said. "They can lay approximately once a month during that time."

The snails can lay eggs when they get to be about the size of a quarter, and continue as they grow up to softball- or fist-sized. Each clutch can contain "hundreds to several thousand eggs," he said.

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Eating and breeding

The largest concern with the apple snails -- apart from how quickly they reproduce -- is that the large snails feed mostly on aquatic vegetation, which they can wipe out if they reach large enough concentrations.

"They eat a lot of wetland plants, and as they eat those, we assume they're affecting how well that marsh, that wetland works," Walton said. "Like a lot of invasive species, we find that they're sort of spectacular at a couple things. They're pretty good at eating and pretty good at reproducing."

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The snails can float

While the snails don't move very rapidly on the ground, they can actually trap enough air in their shells to float in water, which is a scary proposition for people worried about their spread to other water bodies.

"They have this ability to float," Walton said. "And when we get flooding, you can get dispersal of them that way, they can be carried over longer distances."

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Harvey spreads slow-moving Texas invasion

The flooding that impacted the Houston area after Hurricane Harvey didn't just destroy homes and displace people. Local media reports tell of the "alien-like pink bubble masses" that are spread all over the city after the storm.

Those are of course apple snail egg cases, and the sheer volume of them in the Houston area shows just how effectively flooding can transport the snails to new areas.

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Adds to fears for the Delta

While Amazonian apple snails are mostly limited to Three Mile Creek in Alabama, the events in Texas show just how far they might spread in a major flooding event.

If the invaders make their way to the Mobile-Tensaw Delta, they have the potential to devastate one of the most ecologically diverse river systems in the country.

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They could give you rat lungworm

The snails aren't just gross to look at, they can also carry some nasty parasites that could impact humans. Apple snails also serve as hosts of multiple disease-causing critters, including the rat ringworm, intestinal fluke, and the human endoparasite rat lungworm.

Mostly these parasites infect humans through consumption of uncooked snails, but it's enough reason to be cautious when handling the snails.

"Does cooking get rid of them? Probably, but you don't want people handling them in the meantime," Walton said.

This CDC photo shows a rat lungworm removed from a human patient's vitreous humor (eye).

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Edible, but not recommended

Though the snails are edible if prepared correctly, Walton said encouraging people to eat them is probably not a good idea.

For one, there are concerns about the parasites, but perhaps more importantly, creating a market for the snails increases the risk of spreading them to new areas.

"This is a challenge with invasive species, this idea of encouraging a market for these things," Walton said. "It sounds great to have people eat the problem away, but the concern is that does this then get transported into new water bodies?

"Once you start transporting this stuff, how do you do that and ensure that by-product is never escaping?"

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A 'maculata' conception

There are actually several species of non-native apple snails in the U.S., and it can lead to confusion among scientists and wildlife agencies when discussing the finer aspects of the snails' biology or behavior.

Armstrong said the species that is impacting Three Mile Creek is the Pomacea maculata. Island apple snail is a valid common name for the species, but Armstrong says he now refers to the species in Alabama as the maculata apple snail for specificity.

The map above shows the known range for Pomacea maculata, compiled by the U.S. Geological Survey.

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Trunks like elephants

The snails also have an appendage they can use to breathe air from just below the water's surface. Armstrong said the appendage allows the snails to breathe air from under the water, if they aren't getting enough oxygen through their gills. That's an especially useful adaptation in the some of their native South American waters that have very little oxygen.

"They push their little trunk up and siphon fresh air," Armstrong said. "Actually when they're stressed, you'll notice that they put up their little siphons and it looks very similar to an elephant trunk. It's pretty wild to see it."

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Cooking spray a control method?

Newly-laid eggs will typically fail if they fall into the water, Armstrong said, but eggs that have matured a few days may survive.

Another option he looked into: using common cooking sprays to coat the eggs with oil and smother them that way. Armstrong and some of his colleagues actually sprayed egg cases with coconut oil or Pam cooking spray to evaluate whether that would be a feasible control method for snail roundups.

Armstrong said the sprays seemed to work most of the time, and were more effective when applied to freshly laid eggs. The data from that study is still being analyzed for future publication.

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Salinity may help limit spread

The snails are freshwater creatures, though research conducted at the Dauphin Island Sea Lab has shown that they can tolerate brackish water up to a certain level of salinity.

Walton said the salinity and tidal influence of Mobile Bay may be the biggest factor preventing the snails from spreading downstream from Three Mile Creek.

"We've never seen the egg casings, at least I haven't, much below Conception Street Bridge in Mobile," Armstrong said. "Since that's the deeper part of the maintained ship channel, I'm pretty sure the salinity in there stays pretty high."

If the snails found their way into the water further north, however, the invasion throughout the Delta could go unchecked.

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Bury the bodies?

Once the hundreds of snails and thousands of eggs have been collected during a roundup, there's still the issue of safely disposing of them all without further fueling the invasion.

The last thing anyone wants is for a seagull or raccoon to get into a bag of dead snails or eggs and have those eggs wind up in a new body of water. Or for a mess of eggs to find new homes via someone's boots or kayak.

For that reason, Armstrong and the Division and Wildlife and Freshwater Fisheries helps Mobile Baykeeper and other citizen groups dispose of the snail material after a roundup. The snails and eggs are dumped into a cooler with dry ice to kill them quickly and the remains are buried to prevent predators from making a meal of the dead snails and spreading the infestation.

"I don't know for a fact that [spread of the animal through dead eggs] would happen, you just think of all the terrible scenarios that could happen and you have that thought," Armstrong said.

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Next roundup Oct. 14

Mobile Baykeeper will conduct another Island apple snail roundup on Oct. 14, from 9 a.m. to 11 a.m. at Langan Park.

For more details, see the Mobile Baykeeper web site.

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More on Amazonian apple snails

For more on Amazonian apple snails and their incursion into south Alabama, see the following:

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More invasive species

See the other installments of AL.com's series on invasive species in Alabama.