It's for sure the lack of food.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0044374

Keeping the tradeoff in question between reproduction and growth in naiads in mind, I can say that  the species has a preference towards shell growth-maturation based on the size of initial annual rings. In poor waters, no time for growing thick! BTW, it's habitat is narrow upstream waters. With its smaller size and variable shell thickness (?) the species seems to be well adapted to low income life style.

Australian mussels are fascinating. Especially, the ones that can tolerate arid conditions. Several 106 years of experience. 

We have a species, Pseudobithynia pentheri. Type material collected more than a century ago and some (in the same habitat) modern shelss have poorly calcified areas on the shell in a limestone area. I cant remember any example of thinner shells in hard water.

http://malaco.de/Sonderdrucke/Gloeer_et_Yildirim.pdf 

Hello Ümit,

You have been very helpful, thank you. I had not seen the paper by Chavey-Campos et al. From studies like that, and your comments, there is good reason to suppose that the shells in our limestone spring are thin because the organic supply is limited.

Although the organic content of freshwater mussel shells (and molluscs generally) is often said to be 'small', I understand that more than 30% of the organic material in the pearl mussel Margaritifera margaritifera may be in the shell, even though that species lives in streams with low organic content (see p. 84 in Bauer & Wächtler (eds) 2001, Ecology and Evolution of the Freshwater Mussels Unionoida, Springer). The balance in the trade-off between reproduction and growth appears to vary widely even among closely-related species, and generalizations are difficult.

Although I was referring to mussels in karst water, Art Bogan points out that some of the anodontine mussels in rivers have thin shells compared to those in reservoirs, and the difference there does not appear to be calcium-related. With that in mind, we might think of karst waters as just one example of a habitat with a low organic supply, and it would be interesting now to look further afield for reports of thin shells in non-karst waters.

This might be an interesting topic for a literature review! It could adopt a broad perspective, including marine species and gastropods as well as bivalves, before considering the special problems of life in fresh water.

Thanks again, and thanks also to Art for his comments – we may hear more soon.

Best wishes, Keith

PS Your work on bithyniids is also interesting. The spring fauna includes a species of Gabbia, and we should look at that more closely.

I doubt is as much related to the hardness as well because the hard water should provide plenty of CaCO3.  In some of my laboratory toxicology work, I received juvenile mussels from a hatchery in which the mussels had lots of little pock marks on their shells and appeared less healthy than mussels from other propagation facilities.  When I asked about it, the hatchery manager said they often have issues with their mussels having thinner shells because the source water at the hatchery is soft (i.e., less calcium).  In the case of your karst spring, I'd have to suspect something other than hardness.

Thanks Jennifer,

I agree that hardness alone doesn’t seem to be a likely causal factor.

My impression was that the low organic supply (recall the karst spring is clear water, with little suspended material) so that the shells do not contain the organic layers typical of those in other habitats. It’s more complicated than that.

From a recent paper by Araujo et al. (2014), shells from alkaline as opposed to acidic water may lack the organic layers, hence have thin shells. This applies to some species, and to individuals within other species. They regard the organic inclusions as a defence against corrosion, and that presumably is less important in habitats where calcium is not in short supply. This might allow the karst spring mussels to invest more energy in reproduction, compared to shell maintenance and growth.

Another paper by Bartsch et al. (2010: Influence of shell morphology on distributions of unionids in the Upper Mississippi River, J. Molluscan Studies 76, 67–76), confirms that shell thickness varies within river habitats, for reasons apparently unrelated to calcium supply.

Araujo et al. (2014) Presence of organic layers in shells of fossil and recent Unionoida (Bivalvia) and their implications. J. Molluscan Studies 80, 74–83. The presence of thin organic layers embedded in the nacreous layer in the shell of several species of Unionoida has been widely considered to be largely a reaction to prevent further shell dissolution once part of the shell has been corroded. Their microstructural characteristics have also been used as taxonomic characters to separate the families Margaritiferidae and Unionidae. This paper focuses on the description and interpretation of these organic layers in samples of four extant species of European Unionidae coming from acidic and alkaline waters, six extant species of Margaritiferidae, one Cretaceous Unionidae and two Cretaceous Margaritiferidae species. Our results demonstrate that organic layers have characterized species of Unionoidea at least since the Cretaceous and occur in specimens coming from both acidic and alkaline waters. Moreover, the presence of these layers in noncorroded shells can be interpreted as a preventative or ‘prophylactic’ measure against corrosion. Their thickness is the only useful character verified to separate the two families. In the Margaritiferidae, organic layers were found to be present in Margaritifera margaritifera, M. valdensis, M. idubedae, M. laosensis, M. laevis and M. monodonta, and absent in M. auricularia and M. marocana. In Unionidae, they appear in Protopleurobema numantina, Potomida littoralis and in some individuals of Unio mancus, U. delphinus and U. tumidiformis. We argue that organic layers played a fundamental role in shell protection during the invasion of freshwater by the earliest Unionoida, while today they function to improve fitness in acidic water in some species.

Great review on the topic, Keith!

Hi Keith

There is a nice contrast situation in Magela Creek, NT, where Velesunio angasi  have well calcified shells in waters with very low Ca levels around 0.5 mg/L and pH values 5.5-6. Where the rather thick periostracum becomes worn with age around the umbo there is often apparent dissolution of the shell layers. So this species would fit well with the hypothesis that Ca is not limiting for shell development and the organic layer is eeded in acidic waters.

Thanks Ross!

This recalls other questions/answers under Freshwater Mussels and Freshwater Bivalve. What an interesting Hons project this might offer for someone (CDU?). All the more so if there are gastropods and decapods (Cherax, Macrobrachium ... ?) for comparison.