Most Pisidium are now considered to be in the Euglesa genus, and Musculium is now considered a part of Sphaerium again.

In the image, your Musculium lacustre figure (Fig. 4) appears to be Euglesa casertana, and your Pisidium amnicum figure (Fig. 5) appears to be Euglesa supina. All of the specimens in the other images appear to be of the Genus Euglesa, and I do not see any Pisidium s.s. or Sphaerium in the images. I am unsure about the rest of the figures.

Thank you!!!

I disagree with L.J. Lamera's conclusion on fig 4 and 5. The umbonal cap visible in fig. 4 is typical for Shaerium (Musculium) lacustre and the specimen shown in fig.5 seems to be P. amnicum for I think it has the characterisctic shape and ridges. The rest is more difficult for it is difficult to see if the shell is matt or shiny.

Figure 4 is definitely not Musculium. The beaks are too posterior, and the shape is far too rounded. Additionally, Musculium lacustre would get much larger than these specimens. Calyculate (capped) beaks also are common in Euglesa species, especially Euglesa casertana, as stated on Page 8 of Herrington 1962: https://deepblue.lib.umich.edu/bitstream/handle/2027.42/56362/MP118.pdf;sequence=1

The striae in Fig. 5 are far too fine for Pisidium amnicum. Additionally, there appears to be clear sculpture on the umbos, confirming it’s either Euglesa henslowana or Euglesa supina. The significantly tall shape suggests supina over henslowana; henslowana is always more long than tall. Pisidium amnicum has very coarse striae and beaks that are very posterior, unlike the subcentral beaks in these specimens. Compare to the figures shown here: http://rcin.org.pl/Content/57646/WA058_73457_P255-T42_Annal-Zool-Nr-12.pdf

Dear L.J. Lamera. I still disagree with your conclusion and to prevent it from becoming a yes-no discussion I can only write that 1) I think that fig 4 is a Sphaerium (musculium) lacustre because it is whitish in color, has shoulders and a cap on the umbo (beaks). Caps in other species are rare and I agree that sometimes caps are missing in S. lacustre (especially in summer) and 2) the specimen is fig 5 has a posterior placed umbo and is not subcentral as stated by you. Furthermore, Euglesa/Pisidium is less whitish and always has a hint of yellow and the only other species with shoulders is E.milium but this species never has a cap on the umbo (at least not in The Netherlands). I know the books and I even followed two courses on Sphaeriid clams, one by Piechocki himself.

Ton van Haaren,

You said that Fig. 4 looks whitish in color, but I believe it is more brown. I can see how you believe Fig. 4 is Sphaerium lacustre but I do not believe color is an accurate way to separate Sphaeriids. Valves of every Sphaeriid species can range anywhere from white to black, but are most often light brown or yellowish. The habitat, the age of the valve, staining, water contents, and weathering can change the color of a valve, making it very inconsistent when trying to use it as a way to identify to specimens. In my personal collection (https://www.inaturalist.org/observations?place_id=any&taxon_id=871416&user_id=lj_&verifiable=any), valves of the same species can be grey, red, brown, and yellowish, depending on many factors.

Sphaerium lacustre has central beaks (or beaks that are slightly anterior), whereas Fig. 4 has beaks that are clearly posterior. Sphaerium lacustre would be more quadrate, and not fully rounded. The shell outline appears too long for Sphaerium.

Additionally, S. lacustre gets much, much larger than these specimens, and because of the defined beaks, the valves in Fig. 4 are adults. An adult S. lacustre can get up to around 10mm.

The presence of calyculate beaks isn't a way to separate Sphaerium from Euglesa/Pisidium either. The source I sent in my previous reply states that calyculate beaks can often be present on Pisidium sensu lato, and also Sphaerium species that are not S. lacustre. The beaks in Fig. 4 are only very slightly calyculate, whereas a Sphaerium lacustre at this size would have very high, fine, and capped beaks. Compare to this specimen of S. lacustre: https://www.inaturalist.org/observations/226083562, and these specimens of E. casertana: https://www.inaturalist.org/observations/73375245

Figure 5 is very high in outline, whereas Pisidium amnicum is generally more elongated. The beaks are definitely posterior, but I do not believe they are posterior enough. Additionally, as I mentioned, the striae are very fine, and Pisidium amnicum has very coarse striae. See how coarse the striae are in this image of P. amnicum: https://www.inaturalist.org/photos/449145. Compare that to E. supina, which has finer striae and a tall shape: https://www.inaturalist.org/observations/232271052

The presence of umbonal ridges (beak sculpture) rules out Pisidium amnicum for Fig. 5. The only species with beak sculpture in the area are E. henslowana, E. supina, and E. lilljeborgii, and Odhneripisidium moitessierianum.

O. moitessierianum is too small and E. henslowana is longer in outline. It may be E. lilljeborgii, but I think E. supina is more likely. I do not view Pisidium amnicum as a possibility for the reasons above: Fig. 5 has beak sculpture, and fine striae.

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To summarize, Fig. 4 cannot be S. lacustre because of the size, rounded (not quadrate) outline, and lack of very high, fine, and highly calyculate beaks. I would not say the beaks in the specimen are capped, even in the broadest sense of calyculate.

Fig. 5 cannot be Pisidium amnicum due to the tall outline, very fine striae, and most notably, the beak sculpture. A true P. amnicum would not display any beak sculpture.

L.J. Lamera

Dear friends malacologists, I found very interesting discussion, thank you for answers and collaboration with me. P. amnicum is very common in Kaliningrad region and identification of it isn't problem. M. lacustre is juvenile of course with specific top of shell formed. Please find my article with good pictures of small bivalves, and make a criticism on it. I will be very grateful to you help. As a specific bonus download my 465 photos photo album with raw material and use it for identification species of Kaliningrad region in all-european works (as regional overview). Of course, identification of my material will be yours to the honor of N.K.Alekseev (maker of this collection that I processed in COVID time). Nevertheless, I am grateful to all of you!

I think this discussion would never end. Live and fresh material of S. lacustre are fare more paler, close to white, than any other Euglesa/Pisidium. This is even visible in juvenile stages. In mature ones, if you compare the fresh material of both it still remains visible. Of course, the coloration is not of use in old material. I still think that Dmitry showed a S. lacustre ans amnicum in his paper. So, how to resolve this issue? is to ask another expert, because we strongly disagree.

I do agree there is Sphaerium lacustre and Pisidium amnicum in the article, I was specifically referring to the original attached image. It looks like a new image has been attached, showing S. lacustre as Figure 4 and Pisidium amnicum as Figure 5. Perhaps there has been a misunderstanding?

L.J. Lamera Ton van Haaren I found a very good experience for myself in discussing this issue, it is a really good and instructive story for me, firstly I should have given a link to another article dedicated to the Alekseev collection, secondly it was necessary to indicate the size of the scale bar on the images, next time I will be more attentive and unambiguous. Alas, I only confused the scientists who paid attention to me and my question, in the future I will be more careful. Here is my article from where the picture to attract attention was taken: Manakov D.V. (2022) On the N.K. Alekseev collection of freshwater mollusks from the Kaliningrad Region (Russia) // The Bulletin of the Russian Far East Malacological Society. Vol. 26. N 1/2, pp. 201-230 https://www.researchgate.net/publication/366642488

L.J. Lamera Ton van Haaren I am sorry! I apologize for my inexperience. DM