Debunking another widely held myth. Nacreous v Non-Nacreous

[Lagoon Island Pearls]

As a side note, this is very interesting and something I've alluded to for a long while.

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Zhu et al. (2016) reported that no distinguishable myostracum layer could be found in the clam R. philippinarum, at odds with our study (Figures 1O, P). Such an inconsistency might be due to the different sampling methods. Because in Zhu’s work, they examined the fracture surface perpendicular to the shell growth direction, which might lead to their missing the myostracum. Indeed, the myostracum in R. philippinarum was only 5 µm in height (Figures 1O, P), which was quite difficult to figure out. It would be much easier to image this structure in the fracture surface along the shell growth direction, as we did in the present study.

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The author describes the Manila Clam, although in modern times it's been reclassified as Venerupis philippinarum because it falls into the category of "venus" clams as opposed to genus Ruditapes or just Tapes.

This clam is considered an invasive species here, despite having little or no adverse effect on wild stocks of clams or other marine fauna. It hitch-hiked it's way to North America when Japanese oysters were relayed for aquaculture in the 1920s. It has a greater resilience to frost, which means it can inhabit the shoreline higher along the tide than indigenous species. They are greatly significant to the commercial clam fishery and an ideal species for aquaculture. At the same I operated Lagoon Island, I also cultured Manilas at Indian Island, near Tofino, BC.

They are exceedingly delectable, mainly because of their size. As such it's not uncommon to eat a few dozen in each sitting. Over the years, I have eaten countless numbers. Tens, possibly hundreds of thousands, yet have not encountered a single pearl. Only once, did a friend find one. It was cooked and out of the shell, so it was somewhat destroyed from a scientific standpoint.

This paper outlines the reasoning.

 

[SteveM]

Again agreed mainly because of the conflation between the two terms. The burning question being, which came first, the chicken or the egg.

We also agree on the majority of cases where once mythical non-nacreous objects are indeed nacreous in terms of structure. Clams pearls for certain. Even most gastropod pearls are nacreous. I'm not closing the book on conch though, because I'm not certain they are absent of the tabular platelet model in a minor way... especially at the myostracum as the OP suggests. I will revisit this after this post.

However I strongly disagree the nuances of cross-lamellar structures being an exception to nacre. A brick wall is a vertically offset structure bound together by mortar. If I were to build a structure using the cross-lamellar model 00-90-00, it would be exponentially stronger, but it's still a brick wall.

The burden to change something which is common knowledge in all molluscs (a barrier) for a detail that may or may not be nuanced (a structure) in the alternative is misplaced, especially knowing cross-lamellar construction occurs in 90% of otherwise nacreous molluscs, it is infinitely simpler to agree it's nacre.

We have discussed "burdens" previously, above. Here I argue that the "common knowledge" being changed (burdened?) is the predominant concept of nacre as the miraculously iridescent 5% organic microstructure with which we are all familiar. Without a more generic overarching term for the tissue/shell interface, the communications challenge being demonstrated in this discussion will not be resolved.

 

[SteveM]

Ok, revisiting the OP, this:
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Similar to Bivalvia, shell structures underlying the myostracum layer in the Polyplacophora, Gastropoda, Scaphopoda, and Cephalopoda exhibited a diverse display. Columnar nacreous were present in both Gastropoda and Cephalopoda, while composite prismatic, crossed lamellar, and simple lamellar were also found as the bulk microstructure of tested shells. Moreover, both calcite and aragonite were found to be the composition of the main part of the shell layers.
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They're all nacreous to some degree.

The nacreous and porcelainous microstructures co-exist within the same shell but preserve their distinct morphologies.

 

[SteveM]

This paper outlines the reasoning.

Should this contain a link?

 

[Lagoon Island Pearls]

The nacreous and porcelainous microstructures co-exist within the same shell but preserve their distinct morphologies.

Non-nacreous and porcelainous are subjective terms as to appearance. Nacreous, calcitic and calcareous are objective terms as to structure.

It is true in the context of comparing one to another, one is non-nacreous if you ignore the barrier part and solely rely on a (flawed?) structural description.

However, we agree in the context of the object on the whole, both "co-exist", thus nacreous to some degree. So even in complete absence of the barrier thing, it's a fallacy to suggest a subjective point overrides an objective one.

Nothing in the OP suggests structures which stand alone in the absence of elegant aragonitic structuring, in fact does exactly the opposite by alluding to nacreous structures across the board, even though the latter paper I've submitted to the thread on natural arrangement alludes cross-lamellar (which is still elegant aragonite as opposed to massive) being the only microstructure in Class scaphopoda (Carter, 1990: Almagro et al., 2016) aka Tusk shells or Dentaliida, but is not a pearl bearing class as we know it.

Instead these papers clearly outline nearly all mollusks are nacreous to some degree, thus ruling those once considered non-nacreous as nacreous.

 

[Lagoon Island Pearls]

Should this contain a link?

The link is in the title. It's from the same paper in the OP. My apology, I should have been more specific.

 

[Lagoon Island Pearls]

Aside from the assertion, I found this paper greatly intriguing because it lists several species common to us in the Pacific NW and west coast of Canada.

Abalone, mussels, scallops, oysters, Manila clams and geoducks purchased at a fish market in China.

It strikes home by directly reflecting my field work and what I've been postulating for years. That a clam is a clam is a clam irrespective of it's hemisphere of origin.

 

[SteveM]

all mollusks are nacreous to some degree, thus ruling those once considered non-nacreous as nacreous.

The Arabic naqur (mollusk shell) is turning out to be a prescient etymology.

It would be of great interest to know if you would offer any guidance for Elizabeth Strack on a future update to her comments on nacre. She's one of the most open-minded gemologists I've had the pleasure of meeting!

 

[Lagoon Island Pearls]

The Arabic naqur (mollusk shell) is turning out to be a prescient etymology.

It would be of great interest to know if you would offer any guidance for Elizabeth Strack on a future update to her comments on nacre. She's one of the most open-minded gemologists I've had the pleasure of meeting!

I would gladly, though I know woefully little of Ms. Strack's eruditions.

Much of her work is behind paywalls, in expensive books or with presentations at foreign seminars beyond my austere means.

 

[SteveM]

I would gladly, though I know woefully little of Ms. Strack's eruditions.

Much of her work is behind paywalls, in expensive books or with presentations at foreign seminars beyond my austere means.

We can expand the net to include all popular pearl literature, Strack serving a symbolic function. Thus the "burden" I most recently referenced. That's a lot of re-writing to do!

But we've made considerable noise here. Someone must be hearing!

In all of the referenced papers here I can only find 'nacre' specified in relation to the 5% organically-matrixed iridescent expression. The "Inner Shell Elegant-Aragonite Scenario" suggests that the obtention of a smooth inner shell surface (and skin of a pearl), iridescent or porcelainous, should be referred to as a "nacreous" process, in 80% to 90% of all mollusks.

That leaves 10% to 20%, and it would be of great interest to discover qualifying specimens in that class, and to compare their relative shell morphology.

Don't mind me, I'm just the smartass trombonist here.

 

[Lagoon Island Pearls]

That leaves 10% to 20%, and it would be of great interest to discover qualifying specimens in that class, and to compare their relative shell morphology.

Yes, it's an interesting tangent to the discussion. I wonder if it's shelled molluscs or all molluscs? If it's the latter, nudibranchs and some cephalopods would be the exceptions for a great part. I would gather it's the former though. We did identify scaphodpods as cross-lamellar, but I'd concede they are still nacreous in the "barrier" sense. Though persistently returning to this point, I cannot emphasize it's importance enough because where pearls are concerned, it's directly proportional to the outer surface quality in 100% of cases.

While were at it, we should touch on the differences between homogeneic or xenogeneic pearls. Most are homogeneic, namely grafts from the same species as in cultured pearls or one's formed from a mollusc's own devices (ie) cracked shells, autoimmunity etc. Xenogeneity is a little more complex in the sense that we cannot rely on the shell structure of the host to explain the structure of the pearl. My octopus pearl is a perfect example of this, where the object was borne of the tissues of another species. For certain we know some are histocompatible, but most other factors are highly speculative. Although not always or even rarely, I'd suspect some histocompatibilty with parasites.

In the hypothetical, let's say a tiny snail found it's way into the tissues of a mollusc. The creature would invariably suffocate, however mantle tissue remains viable for a time post mortem. We know this because in a cultural setting, grafts are sectioned after the donor is sacrificed. If the snail's mantle were to adopt the host's nutrients and compounds that are supplied through the mollusk’s circulatory and metabolic systems, we'd have a pearl. I've seen some rather unusual pearls over the years and suspect this may be the case and inclined to say it may happen more than we'd think in the natural world.

Then of course, we cannot rule out other phyla. Arthropoda, annelids or cnidaria (corals) to name a few. Though greatly rare, it stands to reason anomolies may occur, thus producing iscolated objects. Even humans generate calcaereous concretions, albeit from mineral salts coming out of solution as opposed to immediate epithelial behavior. Cysts are epidermal though, thus epithelial while some produce hard objects. Esoteric and mundane? Perhaps, but I find it fascinating nevertheless.