Why?

[Lagoon Island Pearls]

...out of all of those species of shellfish happily not being particularly nacreous, why do those few go to all that bother? ... What is it that triggers those few species to make colourful shell?

Steve might be happy now that this question has been rephrased

The answer is simple. Shell strength.

The benthic zone is the thinnest region of all zones in our oceans, mostly because benthic animals live either on the surface of the sea-bed sediments or within the top few centimetres of bottom mud. Even this zone is muddied (no pun intended) because some seabed creatures, particularly the shrimps, fish and some cephalopods, move away from the bottom from time to time, sometimes swimming tens or even hundreds of metres above it.

Venus clams live within the substrate. Although they can survive above the sediments, they will have markedly deformed shells. Epifaunals, such as Pinctada, or mussels etc, generally live above the substrate, attaching themselves to rocks, other creatures, pilings etc. This kind of exposure increases the risk of predation and physical damage, thus needing a stronger shell. This can be acheived, by increasing one element of the shell building process. In the case of oysters, they have thick prismatic layers. In mussels, thick periostracum and in the case of abalone and pinctada.... thick nacre.

There is some discussion between the terms epifaunal and epibenthic. Some say it's the same, but I've always felt they should be seperate, because it represents two very different environments, despite the proximity to one another.

Scallops and cockles break the rules of sedentary habitation. Most pectins can bury themselves temporarily then emerge and swim to new location. Cockles can dredge themselves out of the bottom and kick their way to a new location, then rebury themselves. Gastropods walk around most of the time, but can bury themselves or hide in nooks and crannies. These are evolutionary behaviors, because to become mobile and exposed means a necessity to have a harder shell. I still doubt color itself has anything to do with it. It's coincidental.

 

[Caitlin]

Lots of things are iridescent, like faschia on muscles, which are rarely seen during the life.

Maybe, if it is because of the molecular and or chemical structure, the question should be why do our eyes pick out these structurally caused anomalies and notice them in the first place? Why do we have an eye for beauty?

 

[SteveM]

The long and short of this thread is this:

Nacre color is not adaptive. It is merely a byproduct of a unique microstructure evolved during the lower Ordovician era as a desperate measure to survive Earth's most intensive epoch of inter-species warfare.

Ever since, nacreous species have been on the steady decline, in favor of bulkier, less costly shell microstructures.

On the other hand, non-nacreous calcium carbonate, as evidenced in molluscs such as melo-melo and conch, certainly indicates adaptiveness in its colors and patterns, expressed as they are on the shell exteriors.

 

[pearlescence]

I've heard from my biologist friend - this moves things along a smidge:
This is both a very silly question and a very deep one - several very deep ones!

First the 'silly' q: as Ian says (and as Wendy well knows), because it's on the inside it makes no difference to the animal's life chances. That is, the colour makes no difference! The chemistry that generates the colour might be dreadfully important, of course! The structure of the mother-of-pearl is such that the chemistry shines through, as it were; there's chemistry going on in the tissues under the m-o-p. It's not important that our blood is red; what's important is that it's full of haemoglobin - which is red; OK?

Now it may be (nearly Ian's other point...) that the mantle, that secretes the inner and outer aspects of the shell, does something - anything - on the inside as a result of what it does to the outside. I'm sure this is well known to mollusc experts, because the mantle is an amazing organ - but I don't know the stories about inside, only some of the ones about outside, the shell patterns on eg conches and cowries. These outside patterns seem to be desperately important, of course; but it's the same mantle that lays down the inside; not quite the same part of the mantle, but it might well have some 'resonance' - and, after all, it doesn't 'matter'!!!

Hope that helps...

So the colour and iridescence is about as much use as a nice plaid pattern of blood vessels on the exterior of my liver, in evolutionary terms. ...I'll be following up on this more

 

[SteveM]

On issues of molluscan phylogeny and genomics I've read several papers by a superb worker at the University of Queensland, Daniel J. Jackson. This one specifically addresses the complex modularity of the gastropod mantle, and the expression of the patterns and colors of gastropod shells.

Back to the early Ordovician and World War I: We must give credit where credit is due?it was when Cephalopods (Nautiloids) discovered flight (buoyancy) that the stakes were raised.

 

[Caitlin]

I am glad you can read that and translate it for us! this kind of info is so interesting, but I would never see it if it weren't for the clever, thinking folks on this forum!

 

[pearlescence]

I never did get an answer as to why some molluscs have colourful nacre.

 

[CortezPearls]

I never did get an answer as to why some molluscs have colourful nacre.

Well, it all has to do with: Genetics, Health, and the Environment.
So, mollusks that can secrete nacre crystals will do so based on their genetics (nacre thickness), but their health will be very important (better crystals are produced in healthy animals) and the environment has quite a say in health!

 

[Lagoon Island Pearls]

Aragonite is pseudo-hexagonal, likewise are bee hives. Mineralizing epithelial cells and bees share adjacent spaces equally to maximize efficiency.

Gaming boards and computer maps very often use hexagonal grids to maximize contiguity with adjacent cells. Six pathways as opposed to four using squares.

The termination habits of most crystals allow for the passage, reflection or refraction of light. Imperfect, non-precise or varied angles of massive crystal structures may produce colour shifting.

In all likelihood, iridescence is co-incidental to structural strength and cellular adjacency.

 

[SteveM]

Surmising here, the scientific stuff is great and explains the 'how', but the title of this thread remains unaddressed. Wendy?

 

[pearlescence]

Lots on why nacre and discussion on biology but indeed, nothing on why different colours firmly embedded in that nacre. If you look at a Tahitian shell it is a dazzling rainbow of colours. Purple dye aside the colours in various freshwater hybrids are being developed rapidly.
But all that colour should be there for a reason. Since molluscs cast their futures to the wind - or rather into the water - there is no potential mate to impress or any other biological reason. Neither does it matter whether or not the molluscs have eyes or any sort of visual sense because mostly it's dark where they live (although this might go some way to explaining eg the Cortez UV fluorescence)

 

[Lagoon Island Pearls]

Lots on why nacre and discussion on biology but indeed, nothing on why different colours firmly embedded in that nacre.

Colour originates from conchiolin which is a bio fluid. Molluscs have growth cycles, roughly nine per year while the other two or three months are hibernation periods. At the end of each lunar cycle there is a period of quiescence. During rest periods these fluids accumulate as opposed to being precipitated hence become darker as they're infiltrated with other bodily fluids. Toward the end of the cycle these flush out and run clear. Molluscs hibernate because of lower temperatures and less available food, thus conchiolin mainly runs thin and clear. This is why pearls are harvested during winter months when these thin, clear micro layers present with maximum lustre.

Purple represents the thinnest nacre possible between growth cycles. This is readily apparent at the outer edges of most bivalves where the epithelium is at it's thinnest. Being micro thin, the orientation of aragonite crystals (and likely prismatic calcite lathes) are on an equal plane hence appearing uniform at that end of the spectrum. As nacre become thicker, the orientation becomes less uniform, thus presenting as iridescence. Aragonite in conchiolin does not "orient" greater than a few degrees and this is why we almost never see red pearls. We see some cherry colors in oceania pearls which undoubtably have thick nacre.

UV fluorescence is from calcite. I've never measured the thickness of a pearl sac from SC oysters, but I'd hazard to guess they're among the thinnest of pearl molluscs and have a greater ratio of prismatic calcite to aragonite thus overall more purple(ish) than other colours.

Colour (as perceived by others) is coincidental to growth rates. Mollusks have chromataphores, thus color is a thing. This is apparent in giant clams. Likewise octopus, which is clearly a survival mechanism aka camouflage. At times an expression of emotion, (ie) bright red when defensive. Epithelial cells don't have eyes as we know them, but are capable of detecting adjacent cells, not so much by physical contact or colour itself, but likely by the passage of light. In mollusks with eyes, they've clearly evolved from the mantle itself.

The sky and water are not blue. They merely appear blue.

 

[pearlescence]

Octopuses do a sex thing and have eyes so the analogy is irrelevant to molluscs which don't and haven't.
And why do different locations and different species have different colours, after millenia of evolution? Pearl nacre as in pearls harvested changes colour sometimes from harvest to harvest

 

[SteveM]

The sky and water are not blue. They merely appear blue.

Walking around Green Lake in Seattle this morning on a sunny day, this very thought occurred to me. As a perverted deviation of this thread, might we consider Earth as an evolutionary biological organism that has developed this color scheme to serve as a beacon of sorts for interstellar observers, or is it just happenstance? Our ongoing search for viable exoplanets is based on chromatic analyses of their atmospheres. Of course such a theory is based upon the ability to 'see'.

 

[Lagoon Island Pearls]

Octopuses do a sex thing and have eyes so the analogy is irrelevant to molluscs which don't and haven't.
And why do different locations and different species have different colours, after millenia of evolution? Pearl nacre as in pearls harvested changes colour sometimes from harvest to harvest

Irrelevant? I think not. Octopus are molluscs. All cephalopods have eyes. Pectinidae (scallops) are bivalve molluscs and have eyes. Eyes in molluscs did not spontaneously appear out of nowhere. Cephalopods do not copulate per se. Although not broadly cast into the water column, the male donates externally and the female fertilizes herself. While merely one on one, it's not unlike other molluscs.

Humans have identical anatomies to one another, but appear differently, especially regionally. Some are exposed to more sun, hence more melanin in the skin. Some have entirely different diets. Age groups have pronounced differences. Babies have fine, perfect skin where mature adults have cellulitis, wrinkles and granular scar tissues. Humans have black, brown, red or blond hair, but nearly all senescent adults have gray hair. The same reason why molluscs have silver/grey nacre radiating from the umbo. The e-cellular layup is over-mature, while juvenile cells precipitate colorful nacre.

The colour of a cultured pearl is strictly relative to the graft donor and is highly subjective in selection criteria and region of collection. It's only been going on for a hundred and fifty years and has absolutely nothing to do with evolution.

 

[SteveM]

The colour of a cultured pearl is strictly relative to the graft donor

Shouldn't we be assuming that the premise here applies to natural pearls and the actual origin of the donor DNA?

 

[CortezPearls]

Octopuses do a sex thing and have eyes so the analogy is irrelevant to molluscs which don't and haven't.
And why do different locations and different species have different colours, after millenia of evolution? Pearl nacre as in pearls harvested changes colour sometimes from harvest to harvest

Different locations have different environments (micro-habitats), with different currents, salinity, minerals, phytoplankton, depth, turbidity, etc.
This means that some areas are better suited for the mollusks than others...so they grow faster and healthier, or the opposite so they grow stunted and unhealthy...and this helps pearls have different colors, luster, size, etc.

 

[Lagoon Island Pearls]

Shouldn't we be assuming that the premise here applies to natural pearls and the actual origin of the donor DNA?

Perhaps. In which case the environmental conditions vary from year to year. Even the direction of the tide varies day to day. Some molluscs spawn seasonally while others all year round. That likely stems from genetic diversity as opposed to individually selecting a mate.

If colour in any species was inextricably linked to sexual reproduction then would be reasonable to suggest blind or infertile individuals would not get grey hair?

We know male octopuses die shortly after donating. We know female octopuses die shortly after tending offspring to a pelagic state. Unless preyed upon, infertile octopuses live an inordinately longer period of time than the otherwise fertile age group. I'd expect the same with giant squids.

 

[CortezPearls]

Shouldn't we be assuming that the premise here applies to natural pearls and the actual origin of the donor DNA?

I believe the most important factor is the recipient oyster, since it will be the one in charge of "feeding" nutrients to the donor cells. If the donor cells are amazing, but the recipient mollusk is having a really tough time, then the donor cells will "enjoy" the same fate.

 

[Lagoon Island Pearls]

Epithelial cells don't have eyes as we know them, but are capable of detecting adjacent cells, not so much by physical contact or colour itself, but likely by the passage of light.

I should append this with an additional comment. It's not exclusive to the point. There is undoubtedly a hormonal connection too.