Lanxes: Oddballs of the Lymnaeidae

This is the post excerpt.

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Banbury Springs Lanxes, Idaholanx fresti Clark, Campbell & Lydeard, 2017 (photo from U.S. Fish and Wildlife Service)

Although I don’t have a favorite animal, if forced to pick a group I would most likely opt for the fascinating and insanely diverse mollusks. Comprising over 85,000 extant species, this ancient lineage has representatives in nearly all imaginable ecosystems. 80% of those 85,000 species belong to the class Gastropoda, meaning “stomach foot” and including all the animals often referred to as snails and slugs. The term “limpet” is used for those gastropods that possess a flat external shell into which they are unable to withdraw. Taxonomically speaking, true limpets belong to the Patellogastropoda, a clade comprising five marine families, but the term is applied to many superficially similar yet unrelated snails in multiple families, both marine and freshwater.

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Shell of Scutellastra barbara (Linnaeus, 1758), a species of true limpet in the Patellidae (photo copyright G. & Ph. Poppe – Conchology, Inc)

Freshwater limpets are usually quite small, rendering them overlooked and obscure. Among this already obscure category of mollusks, one of the most poorly known lineages is the Lancinae, a subfamily of the Lymnaeidae. Lymnaeids are a worldwide group of freshwater snails, many of which tend to favor stagnant or slow-moving water. The majority of lymnaeids have scalariform shells with a prominent, sometimes highly inflated, body whorl. Differing markedly from most lymnaeids in shell morphology, lancines were often grouped with the ancylins, a diverse lineage of freshwater limpets with a worldwide distribution. It was not until 1925 that two studies, Pilsbry (1925) and Baker (1925), decided to look past the shape of the snails’ shells and examine their internal anatomy. Later studies would validate these findings and confirm that the lancines were indeed firmly nested in the lymnaeidae.

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Lateral (E) and ventral (F) views of the kneecap lanx, Lanx patelloides (Lea, 1856) (photo from Campbell et al. (2017))

The Lancinae is represented by only four extant species, all found in northwestern North America. They seem to prefer cool, well oxygenated water associated with fast flowing springs. Relative to other limpets, their lungs are very poorly developed, negating the possibility of survival out of water for extended periods of time. As far as freshwater limpets go, lanxes are rather large with some species reaching approximately 20 mm (0.7 in) in length. Considering that just four species are found in the entirety of the Pacific Northwest, these snails generally have respectably sized distributions, with one notable exception. The Banbury Springs lanx, Idaholanx fresti Clark, Campbell & Lydeard, 2017, is currently recorded from only four isolated springs in Gooding County, Idaho, the Thousand, Banbury, Briggs, and Box Canyon Springs, which all empty into a roughly 5 mile (8 km) section of Snake River. Due to this extremely limited range, I. fresti has been federally recognized as endangered by the United States since 1973 when it was included in the U.S. Endangered Species Act, 44 years before it was recognized as a distinct species.

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Shortface lanx, Fisherola nuttalli (Haldeman, 1841) (photo copyright Ian Gardiner)

Although the lancines are the only modern patelliform lymnaeids, the fossil record shows that this body plan evolved multiple times among the Lymnaeidae during the Miocene epoch, which lasted from 23 to 5 mya. This lengthy time period saw a gradual draining of the European Paratethys Sea, creating a massive system of lakes. One of these lakes, the Pannonian lake, or Lake Pannon, harbored the evolution of a unique deep-water, limpet-like lymnaeid lineage known as the Valencienniinae. A series of fossils document the transition of shallow water snails with highly inflated shells to patelliform limpets that favored the deepest levels of the lake. The genus Delminiella was another Miocene deep-water patelliform lymnaeid from the Paratethys, but evolved separately from the valencienniines in a completely different lake, the Dinarid Lake System.

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Fossil of the Late Miocene – Early Pliocene deep-water patelliform lymnaeid Valenciennius reussi (Neumayr in Neumayr & Paul, 1875) (photo from Croatian Natural History Museum)

REFERENCES

Baker HB (1925) Anatomy of Lanx, a limpet-like lymnaeid mollusk. Proceedings of the California Academy of Sciences 14: 143–169.

Campbell, D. C., Clark, S. A., & Lydeard, C. (2017). Phylogenetic analysis of the Lancinae (Gastropoda, Lymnaeidae) with a description of the US federally endangered Banbury Springs lanx. ZooKeys663, 107.

Harzhauser, M., & Mandic, O. (2008). Neogene lake systems of Central and South-Eastern Europe: Faunal diversity, gradients and interrelations. Palaeogeography, Palaeoclimatology, Palaeoecology260(3), 417-434.

Pilsbry, H. A. (1925). The family Lancinae distinguished from the Ancylidae. Nautilus38(3), 73-75.

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