The largest of these snails is Achatina achatina, the giant Ghana snail. They are also known as the Giant Tiger Snail. It has excellent tiger-like stripes, so I like that name best. In Ghana, this snail is harvested and eaten. That is the main reason people attempt to smuggle them into the U.S.; as food items. The other reason is to keep them as pets. This species is now found in the Caribbean and Florida.
The species with the greatest pest potential (see below) is Achatina fulica, which I will call the Giant East African Snail. This snail is native to the region around Tanzania. This species has established populations in many Pacific tropical islands, including Hawaii. They are also found in the Caribbean and Florida. The Pacific island infestations may have started with the American military importing them as a potential food source during World War II.
Archachatina marginata or the Western African Snail (AKA Banana Rasp Snail) is native to Cameroon, and the Congo regions. Its range overlaps that of the Giant Tiger Snail. It has yet to be discovered in the U.S., but a population is on the Caribbean island of Martinique.
The best analysis I have seen on rating the danger/pest potential of snails in the U.S. was done in 2009 (Cowie et al. 2009). They list these three species of large snails on their list, ranked as highest pest potential, 1, to least, 46. These species, Achatina fulica, Achatina achatina, and Archachatina marginata. are ranked 10, 33, and 41, respectively. The other related species did not make their list.
The snails are hermaphrodites, containing both male and female organs. The more rambunctious species grow to more than 20 cm, lay about 1000 eggs a year, and live up to 10 years. That length of life is from a captive population; they are unlikely to survive that long in the wild. I should point out that several “news” articles state that these snails lay 25,000 eggs a year, a bit of fear-mongering, I think. All these species are simultaneous hermaphrodites, and mating is typically done with each transferring and receiving during the same coupling. They can hold viable sperm from a coupling partner for some time. They also can self-fertilize, but observations suggest this is rare.
What do they eat:
These snails reportedly eat everything. That, of course, is not true, but they have a wide and variable diet. They are considered agricultural pests because they eat lots of plants, lots and lots. Mead (listed in sources) gives us a long list of what they do and do not eat: A short list of things on which they have been observed feeding: Arrowroot, bananas, beans, breadfruit, cabbage, cocoa, cotton, melons (of many types), papaya, plantains, pumpkin, rubber, squash, and yam.
Another part of the abject terror (a little tongue in cheek here) that these snails spread is that they are disease vectors for plant pathogens, potentially causing damage by spreading the disease in agricultural areas. They also may aid the spreading of disease that can infect humans: Via rat lungworm, leading to meningitis. Don’t eat raw snails or unwashed plant material.
Out of Africa:
These snails have widespread distribution outside of Africa. The snail that has spread most widely is A. fulica, originally from east Africa. Let me go in order, approximately, of their spread. The spread, as far as known, begins from early 1800 to 1840, generally moving east into the India ocean. It was present in mainland India by 1847 and in Ceylon by 1900. From there, the spread appears to increase speed into Indonesia and then to some Pacific Islands, making it to Hawaii (at least Maui and Oahu) by 1938 and the big island by 1951. World War II magnified the spread (snails were moved on purpose to some Pacific Islands). Before the war’s end, the snail had made it to China, Australia, and the U.S. mainland, though whether these far-slung arrivals established populations is unknown.
The spread continues, but the establishment of populations requires the right conditions.
The principal agent of dispersal is humans, of course. Some of the spread is on purpose, mainly as a food source. The snail is eaten by humans but is also used for duck feed, among other uses. In some countries (England, for example), these snails are common pets. Unintended spread also occurs, much more than in the 1800s, since the snails can now come from so many other areas.
Survival:
These snails thrive in wet tropical conditions with slightly alkaline soils. They can survive for long periods in a state of estivation. During estivation, organs become atrophied and attenuated. During this time, they form an epiphragm covering their aperture. They survive dry periods like this but also might be able to survive cold weather similarly. These snails can survive a wide temperature range: They may estivate when the temperature climbs too high, 46C, or too low, 10C. They have infested Florida in the U.S. But even though they have been introduced to England and Germany, populations still have not become established. Cold winters are the snail’s bane.
Snail eradication:
In Hawaii, someone had a brilliant idea (sarcasm) of bringing in another snail that would keep the giant snail population at bay. The predatory snail of choice was the Rosy Wolf Snail, Euglandina rosea. I suspect you can guess the disaster that was caused. In Hawaii, the predator preferred to eat native snail species. We (collectively humans) should know by now that introducing species to control other introduced species is fraught with danger. Due diligence is rarely done: Check the Civeyrel and Simberloff paper below for more details.
In Florida, the attempts to eradicate the snail (the first attempt took 10 years and over $1 million, the second attempt took more money, and the third took even more money as it is ongoing) appeared to be successful, but….do I even have to state it? Of course, they failed. Or in the case of the ongoing one, will fail. Wasted money? Well, it is Florida, so they might as well spend it on something besides attacking education.
Typically non-native species do not thrive in their new environment. Of the small percentage that does, only a few of those truly become a problem. Then typically, the ones that become a problem are the ones that make the news. Even those problematic species, the ones we term invasive, go through a boom and bust cycle (see any biology 101 textbook for more on this concept) where the problem appears to be more pervasive (those damn things are everywhere; boom) than it is; because then bust occurs (look at all these empty shells).
Closing Words:
We, again, I collectively mean humans, have already fucked up the planet to such a great degree that species invasions are not the greatest of our problems, and indeed they could be the saviors of biodiversity in some cases. From another angle, humans are the most damaging invasive species. The practice of immediately vilifying non-native species, whether they become invasive or not, needs to stop. The world is more complex than that. Check out the Davis et al. and Burlakoba et al. papers below for a dive into another way to consider new species and the possible benefits of their presence.
Take-home message: Don't kill non-native species just because they are non-native.
Sources and Further Readings:
Burlakova LE, Karatayev AY, Boltovskoy, D. and Correa NM. Ecosystem services provided by the exotic bivalves Dreissena polymorpha, D. rostriformis bugensis, and Limnoperna fortunei. Hydrobiologia. https://doi.org/10.1007/s10750-022-04935-4
Civeyrel L, Simberloff D. 1996. A tale of two snails: Is the cure worse than the disease? Biodiversity of Conservation 5: 1231–1252.
Cowie RH, Dillon RT, Robison DG, and Smith JW. 2009 Aline non-marine snails and slugs of priority quarantine importance in the United States: A preliminary risk assessment. American Malacological Bulletin 27: 113–132.
Davis MA, Chew MK, Hobbs RJ, Lugo AE, Ewel JJ, Vermeij GJ, Brown JH, Rosenzweig ML, Gardener MR, Carroll SP, Thompson K, Pickett ST, Stromberg JC, Del Tredici P, Suding KN, Ehrenfeld JG, Grime JP, Mascaro J, Briggs JC. Don't judge species on their origins. Nature. 2011 Jun 8;474(7350):153-4. doi: 10.1038/474153a. PMID: 21654782.
Mead AR. 1961. The giant African snails: A problem in economic malacology. University of Chicago Press. Chicago.
O’Loughlin LS, and Green PT. 2017. The secondary invasion of giant African land snail has little impact on liter or seedling dynamics in rainforests. Austral Ecology 42: 819–830.
Sharma S and Dickens K. 2018. Effect of temperature and egg laying depths on giant African land snail (Gastropoda: Achatinidae) viability. Florida Entomologist 101: 150–151.