In 1862 Augustus Gould described a species of bivalve from shells. He called the genus Julia. Since he only had shells to work with, determination of a species should be premature (one would think but that hasn’t stopped many “species” from being named with less). And, of course, mistakes are made, perhaps especially when the evidence is shells only, or fossils — how do you stop from describing a young T. rex as a separate species from an adult T. rex? It’s not only a problem with fossils and shell-only evidence. The extant snail, Littorina saxatilis, has been described as a separate species or subspecies over 100 times. But I digress, let me get back to Julia.
Let us now leap forward in time not quite 100 years. In 1959, a species of tiny two-valved animal, Tamanovalva umax, was described: Though it had two valves it was a snail, not a clam or scallop. Let me restate this, I known you can just reread that last sentence, but here goes. Kawaguti and Baba described a snail with two shells, pulled from Tamano Bay in Japan, in 1959.
Then in 1961 two researchers found a living two–shelled animal from the genus, Julia. It turns out these tiny mollusks were not clams, or scallops, or mussels; they were also snails.
The big news, in case you didn’t catch it: Julia is a tiny sea snail with two shells.
In case you can’t imagine this:
Here are some images of species in the other genera.
Pacific species currently known:
Berthelinia pseudochloris
Julia exquisita
Julia zebra (with stripes on shell)
Tamanovalva Umax (known from Japanese water)
Caribbean species currently known:
Bertelinia caribbea
Bertelinia limax
Let me end this by just throwing out there that the larvae of these snails have a single shell.
What? Really? WTF.
Notes:
These snails present phenotypic plasticity in their radula. The radula is the toothed tongue that snails use to scrap algae off rocks, or in the case of these snails, punch holes in plant cells, dependent on what they are eating. Rapid radula change has been presented as a quick pathway to evolution and rapid divergence of species. This fact puts into question the use of radular teeth array and counts as a way to determine species. If the change is too quick then population members will vary and the lines between some divergence and true speciation is blurred. This occurs in the mitochondria genome of snails, which make the DNA bar coding method (which use the mitochondrial gene CO1) less worthwhile on gastropods than it is on, say, mammals.
Older journals are sometimes hard to find, for example, an English speaking audience might not even try to for the original articles from the Proceedings of the Japan Academy. But, they contains a wealth of information, and often have been made freely available, and this journal is in English (for those, like me, who are encumbered by the one language affliction).
A quote from the Proceedings of the Japan Academy website:
“The Proceedings of the Japan Academy, Series B was founded in 1912 as the Proceedings of the Imperial Academy by the then Imperial Academy of Japan (now the Japan Academy). …The entire content is now freely available online through J-STAGE.” https://www.japan-acad.go.jp/en/publishing/pja_b.html
Images:
Rudman, W.B., 2002 (March 11) Tamanovalva limax Kawaguti & Baba, 1959. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/tamalima
And
Sea Slugs of Hawaii website: by Cory Pittman and Pauline Fiene
http://seaslugsofhawaii.com/species/Berthelinia-pseudochloris-a.html
And
The image on the snail on Caluper: © Daniel Stoupin / Pete West / BioQuest Studios 2018
Readings:
Baba K. 1961. On the identification and the affinity of Tamanovalva lima, a bivalved Sacoglossan Mollusc in Japan. Publication of the Seta Marine Biology Laboratory 9: 37-62.
Gould AA. 1862. Descriptions of new genera and species of shells. Proceedings of the Boston Society of Natural History 8: 280–285.
Jensen KR. 1993. Morphological adaptations and plasticity of radular teeth of the Sacoglossa (=Ascoglossa) (Mollusca: Opisthobranchia) in relation to their food plants. Biological Journal of the Linnean Society 48: 135–155.
Hilgers L, Hartmann S, Pfaender J, Lentge-Maaß N, Rintelen T, and Hofreiter M. 2020. Radula diversification promotes ecomorph divergence in an adaptive radiation of freshwater snails. bioRxiv; http://dx.doi.org/10.1101/2020.01.17.910034.
Kawaguti S, and Baba K. 1959. A Preliminary Note on a Two-valved Sacoglossan Gastropod, Tamanovalva Umax, n. gen., n. sp., from Tamano, Japan. Biological Journal of Okayama University 5: 177-184.
Kawaguti S, and Yamasu T. 1962. Julia japonica found living as a bivalved gastropod. Proceedings of the Japan Academy 38: 284-287.
Kay AE. 1962. Julia exquisita Gould, a Bivalved Gastropod. Pacific Science 16: 434–435.
Wong NLWS, and Sigwart J. 2019. Natural history clues to the evolution of bivalved gastropods (Mollusca: Gastropoda: Sacoglossa: Juliidae). Marine Biodiversity. 1-13.
Yonow N, and Jensen KR. 2018. Results of the Rumphius Biohistorical Expedition to Ambon. 1990. Part 17. The Cephalaspidea, Anaspidea, Pleurobranchida, and Sacoglossa (Mollusca: Gastropoda: Heterobranchia).
Trowbridge CD, Hirano YM, and Hirano YJ. 2011. Inventory of Japanese Sacoglossan Opisthobranchs: Historical Review, Current Records, and Unresolved Issues. American Malacological Bulletin 29: 1-22.