If anyone is curious as to the outcome of my email, I did get a response from Carol Kelloff from the Smithsonian Institute, who forwarded my question to Cavan Allen of the USDA Agricultural Research Service, who said:
"That looks to me like L. laevigatum. L. spongia is the one with the obviously cordate leaf base and longer petioles. I'd still be curious what Dr. Haynes has to say."
So, that's the first answer, and it sounds like I may get a response from someone named Dr. Haynes, which I assume to be the Dr. Haynes of the University of Alabama Herbarium, who focuses his research on the systematics of aquatic vascular plants.
I also received a response from Ann Perry of the USDA Agricultural Research Service who inquired for me with the scientists at the ARS National Plant Germplasm Laboratory who had this to say:
"From a nomenclatural standpoint, there are seven names in Limnobium Rich. that have been validly published. The genus name has also been used for a group of mosses, but there it is an illegitimate later homonym. Two species of Limnobium are currently accepted. Limnobium spongia is native to North America. Limnobium laevigatum is native to Central and South America. It is sometimes treated as a subspecies of L. spongia, which is probably why the vendor said there was only one species of Limnobium."
She also forwarded my questions to Dr. Harlan Svoboda, of the U.S. National Arboretum, so I could still receive more clarification, but it looks so far as if what I have is the South American species, Limnobium laevigatum.
I'm going to make one more attempt to educate the seller (because it seems like I should at least try before filing a complaint against them with the BBB) and I'm also going to reach out to the California Invasive Plant Council and let them know that their inaccurate web page is being used by an aquatic plant seller who is trying not to take responsibility for selling the wrong, non-native species, Limnobium laevigatum, as Limnobium spongia to people using their website as his defense, and try to educate them as well and convince them to update their website so pondmegastore can't use it as an excuse to sell the wrong plant to people anymore. Here's my draft email to them so far - I'm holding off on sending it until I get more responses from all the organizations I emailed in case they have further clarification or useful information I should include:
Hello. I hope you will take the time to discuss with me the misinformation on your webpage on Limnobium Rich. (Frogbit).
You see, I have recently purchased some Limnobium spongia from a business (not in California), and they sent me Limnobium laevigatum (Amazon Frogbit/South American Frogbit/West Indian Spongeplant) instead of Limnobium spongia (American Frogbit/American Spongeplant). When I pointed out that they had made a mistake and sent me the wrong plant, they told me that Limnobium laevigatum (Amazon Frogbit/South American Frogbit/West Indian Spongeplant) doesn't exist and that Limnobium laevigatum is just a synonym for Limnobium spongia. They referenced your webpage at https://www.cal-ipc.org/plants/profile/limnobium-laevigatum-profile/ as proof that they sent me the correct plant. They swear that there is only one species of Limnobium because your website says so.
I have since reached out to several international organizations on the matter and have been assured that there are in fact two species of Limnobium: Limnobium laevigatum and Limnobium spongia, the former being a South American native species with the latter being a North American native species. Experts in the field of taxonomy are quite positive that both species exist.
It is quite easy to distinguish between the two species as the Limnobium laevigatum has a green underside with larger, honey-comb like cells, a shallower basal notch, and a more rounded shape, while Limnobium spongia has a reddish-brown tinted underside with smaller cells, a more lobed/heart-like shape, and is also larger. See https://idtools.org/uploads/idtools/350/239/Limnobium.07164.JPG for a side by side comparison. I'm sure if you look at samples of your invasive Frogbit in California, you will find it to be Limnobium laevigatum, as all the example photos I have found of the California invasive depict Limnobium laevigatum (no red-brown on the undersides of leaves and round/oval in shape).
What this comes down to is that there is a widely-known dishonest commercial aquatic plant seller online in the USA that is selling the non-native Limnobium laevigatum as Limnobium spongia on their website, and using your website as their defense in doing so. I'm reaching out to you because I feel that with the number of customers who have had a problem with getting the wrong species from this seller (the business has been repeatedly reported to the Better Business Bureau for their bad business practices), and due to the invasive nature of Limnobium laevigatum, it would be prudent of you to update your website so that it reflects the scientifically recognized two separate species of Limnobium Rich., both Limnobium laevigatum and Limnobium spongia (which are both likely to be invasive there even if only one is currently present), and note that the two are not synonymous as your website indicates.
It may be that California has not yet seen an invasion of the North American native Limnobium spongia, and therefore hasn't been made aware that there is a native species of Limnobium in North America, since it is not native to California. The invasive plant you have in California is Limnobium laevigatum, which is what is pictured on your webpage. For clarification, please refer to https://ucjeps.berkeley.edu/cgi-bin/get_cpn.pl?30986&expand=1 section titled "Editorial Comments 2", which states: "Correspondence 2 indicates that, according to Dean Kelch (pers. comm. to Baldwin), Fred Hrusa has determined that relevant California material belongs to Limnobium laevigatum (Humb. & Bonpl. ex Willd.) Heine and not Limnobium spongia (Bosc) Rich. ex Steud., and that the latter name has therefore been misapplied in California [there is one record in CCH labeled Limnobium spongia, but it is from material grown in a greenhouse in CA], a situation opposite that presented in The Jepson Manual [Ed. 2]."
I hope you will look into this matter and update your website to reflect the scientifically accepted taxonomy regarding Limnobium Richard and remove the references to it as Limnobium spongia, which it is not. Your webmaster could simply change the title line to "Limnobium laevigatum" and the synonymous line to say, "Sometimes confused with Limnobium spongia" or list some accepted synonyms such as "Hydromystria laevigata" and "Hydromystria stolonifera". The rest of the webpage appears to be correct.
I'm sure the California Invasive Plant Council does not want to be associated with a known dishonest aquatic plant seller who is trying not to take responsibility for selling the wrong, non-native species to people across the country, including to customers with good intentions and trying to not introduce non-natives to their gardens – people living where Limnobium spongia is native and Limnobium laevigatum is not.
Here are some informative links on which you can learn about the two different species of Limnobium Rich. and learn how to distinguish one from the other:
I hope you will consider this important issue and make the necessary changes on your website as quickly as possible, before anyone else is shipped a non-native species!
