Have you ever noticed mysterious clusters of smooth pea-shaped spheroids—often white- or pink-colored—stuck to the stems of aquatic plants or debris? More than likely, these are the eggs of an aquatic snail. If you ever find yourself paddling through the Florida everglades, the egg clumps you see may be laid by the beautiful Florida apple snail, Pomacea paludosa. Although you might be more interested in the avian, reptilian, and mammalian fauna native to this biodiverse ecosystem, it is good to remember that much of the “charismatic” diversity we see in nature is maintained by humble keystone species, such as insects and snails.
As taxonomic bias goes, snails are often largely ignored by both the scientific community and public. In fact, a recent meta-analysis published in 2017 looked at this inherent species bias in publication rate and highlighted that it is often societal preferences, rather than targeted research efforts, that drives this “taxonomic chauvinism”1. In this analysis, gastropods were the third least-represented taxon in biodiversity studies—followed closely by insects and arachnids—sampled proportionally to the number of known species in each group. Birds, on the other hand, were over-represented. This means that taxonomic groups that have more species are often the least-featured in scientific literature. Interestingly, there was a significant correlation between public interest—measured as “the number of web pages referenced by a search engine”—and research efforts. Whether this correlation is causal is hard to ascertain, but the skew exists and we must ask the question: does this bias matter to conservation efforts?
In the case of the Florida apple snail, it certainly does! Although classified as a species of Least Concern by the IUCN Red List of Threatened Species2, it is a critical component of the diet of the endangered3 Everglade snail kite, Rostrhamus sociabilis plumbeus. Snail kites are raptors that are hyper-specialist molluscivores, i.e. the birds almost exclusively consume apple snails as a part of their natural prey base. Their long, curved bill is evidently adapted to extract snails from the shell.
Left: Note the long, curved bill of the Everglade snail kite. [Source] Right: Snail kites often leave behind shell middens at their perch sites. [Source]
Apple snails have a hard, calcareous operculum or “door” that protects them from predators and from drying out when they aestivate. Nonetheless, the kites are able to locate and process enough snails and can thrive when the density of snails is high. In fact, the apple snail is an important prey item to several other wetland predators, including other bird species, fish, and reptiles4. This has important implications for wetland restoration projects that aim to create a varied mosaic of habitats that maintains faunal and floral diversity.
Snails have long been used as indicators of environmental perturbation or pollution, especially in aquatic environments5. In this particular case however, the Florida apple snail has been touted a good indicator species for Everglades restoration success4. An ideal habitat is complicated to achieve since these snails require two conflicting habitat features to thrive: (1) flooded conditions that are robust to drying events and (2) emergent aquatic plant species on which they lay their eggs4. Habitat restoration is further complicated by the fact that the Everglade snail kite benefits from flooded environments whereas the sympatric wood stork, Mycteria americana benefits from lower water levels6.
Florida apple snails are hard to see when submerged and kites survey large expanses of wetland habitat in search of individual snails that surface to breathe. Apple snails are aquatic snails in Ampullariidae family and are unusual in that they are amphibious: they have both gills and lungs. This means they must surface from watery depths from time to time to breathe. Some species even lay their eggs above the water-level to protect against underwater predation from fish, aquatic insects, and reptiles. Thus, these eggs can withstand long periods of desiccation. Ampullarids are thought to have a Gondwanan origin and currently have a pantropical distribution, found across aquatic habitats in South and Central America, Asia, and Africa7. Beyond their role as prey items, apple snails are also known to be intermediate hosts for a variety of parasite species that can infect humans, wildlife, and domesticated animals alike7.
This species is also the largest native aquatic snail in North America2 although it is now threatened by closely-related invasive apple snails that can often be quite large8. These non-native snails both feed and reproduce at faster rates and thus may outcompete the smaller, slower-growing Florida apple snail8. Although these bigger snails might make a tasty morsel for the Everglade snail kite, the invasive species are often too large for the birds to consume9. Other perturbations to native apple snail populations include temperature changes, which influence not only snail behavioral responses but also kite movement and foraging behavior10. Thus snail population dynamics have important implications for conservation efforts of other native species. The next time you see a cluster of small eggs or a snail in a pond or stream, think about all the other animals that might be directly impacted by this small, unobtrusive gastropod. If we start to value animals besides those that are obviously charismatic, our appreciation for all creatures great and small could reveal even more interesting ecological and evolutionary mechanisms that maintain biodiversity.
1Troudet, J., Grandcolas, P., Blin, A., Vignes-Lebbe, R., & Legendre, F. (2017). Taxonomic bias in biodiversity data and societal preferences. Scientific reports, 7(1), 9132. Retrieved from https://www.nature.com/articles/s41598-017-09084-6
2Cordeiro, J. & Perez, K. (2011). Pomacea paludosa. The IUCN Red List of Threatened Species 2011: e.T189339A8718219. http://dx.doi.org/10.2305/IUCN.UK.2011-2.RLTS.T189339A8718219.en. Downloaded on 06 February 2018.
3US Fish and Wildlife Service. (2008). Everglade Snail Kite, Rostrhamus sociabilis plumbeus, 5-Year Review: Summary and Evaluation. Everglade Snail Kite, Rostrhamus sociabilis plumbeus, 5-Year Review: Summary and Evaluation.
4Darby, P. C., Bennetts, R. E., Croop, J. D., Valentine-Darby, P. L., & Kitchens, W. M. (1999). A comparison of sampling techniques for quantifying abundance of the Florida apple snail (Pomacea paludosa Say). Journal of Molluscan Studies, 65(2), 195-208. Retrieved from https://doi.org/10.1672/0277-5212(2002)022[0489:MOFASI]2.0.CO;2
5Goodnight, C. J. (1973). The use of aquatic macroinvertebrates as indicators of stream pollution. Transactions of the American Microscopical Society, 1-13. Retrieved from http://www.jstor.org/stable/3225166
6Simberloff, D. (1998). Flagships, umbrellas, and keystones: is single-species management passé in the landscape era?. Biological conservation, 83(3), 247-257. Retrieved from https://doi.org/10.1016/S0006-3207(97)00081-5
7Hayes, K. A., Cowie, R. H., & Thiengo, S. C. (2009). A global phylogeny of apple snails: Gondwanan origin, generic relationships, and the influence of outgroup choice (Caenogastropoda: Ampullariidae). Biological Journal of the Linnean Society, 98(1), 61-76. Retrieved from https://doi.org/10.1111/j.1095-8312.2009.01246.x
8Morrison, W. E., & Hay, M. E. (2011). Feeding and growth of native, invasive and non-invasive alien apple snails (Ampullariidae) in the United States: invasives eat more and grow more. Biological Invasions, 13(4), 945-955.
9Mac Stone & Alisa Opar (2017). See One of the World’s Coolest, Most Specialized Raptors in Action. Audubon: Science. Spring 2017. Retrieved from http://www.audubon.org/magazine/spring-2017/see-one-worlds-coolest-most-specialized-raptors
10Stevens, A., Welch, Z., Darby, P., & Percival, H. (2002). Temperature Effects on Florida Applesnail Activity: Implications for Snail Kite Foraging Success and Distribution. Wildlife Society Bulletin (1973-2006), 30(1), 75-81. Retrieved from http://www.jstor.org/stable/3784638
Main featured image [Source]