Why Do Shells Have Their Colors?

by Gary Rosenberg, ANSP
As a curator of a large shell collection, I am often asked why shells have their colors. The most frequent form of the question is, "Are those the natural colors?" I resist saying "No, we painted all 12 million specimens in our spare time," and explain that the colors are indeed natural and are generally thought to serve as camouflage. This answer satisfies most people, but usually does not satisfy the experienced shell collector.

What about color in deep-sea shells never exposed to sunlight or in species active only at night, or where the color is hidden by thick periostracum or inside the valves and can be seen only when the animal is dead? Clearly there is more than one explanation for color in mollusks.

In broadest terms, explanations can be categorized as visual or non-visual. In visual explanations, color serves either for camouflage or communication. If the species involved have only black and white vision, then degree of contrast, not color itself, is what is important. Non-visual explanations say that color is incidental -- it is an epiphenomenon. The color doesn't have a function in itself, but is associated with some other function. Asking why some shells have their color is like asking why mammalian blood is red -- that happens to be the color of the pigment hemoglobin when it is oxygenated.

Visual Explanations

In some cases, bright colors might serve to warn predators that a species is poisonous or distasteful, examples among mollusks being some nudibranchs and ovulids such as Cyphoma. Color is probably not used for communication within species in mollusks, except perhaps some cephalopods which have well-developed image-forming eyes. With mollusks, visual explanations of color usually invoke camouflage, which can be achieved in several ways:

Make pigments that match the background. Many excellent examples of camouflage are found among land snails, which often blend in with leaf litter. But camouflage occurs even in the deep sea. There's no sunlight, but some predators produce their own light. Thus pelagic deep sea fish are black, merging with the void, but bottom dwelling organisms in the deep sea tend to blend in with the bottom. (This might also involve not making pigment, if the substratum is white.)

Take pigments from something eaten. For example, simnias on sea whips take purple or yellow pigments from their hosts. A yellow simnia transplanted to a purple host will thereafter deposit purple shell.

Look different from conspecific neighbors. This would make it hard for predators to form a search image and might explain the enormous variation in species like Donax variabilis and Umbonium vestiarium. Both species burrow in sand at the edge of the intertidal zone where waves break, scattering them like pebbles, and in both, almost the full range of variation in the species can be collected in a few handfuls.

Use chromatophores to blend into the background. For example, the cuttlefish Sepia, which does not have color vision, tries to match the contrast pattern of the background. It can be misled in an aquarium by gravel of unusual colors.

Non-visual Explanations

Result of crystal structure. Nacre in abalones is a good example. The nacre is not seen when the animal is alive, and its color is incidental to its function.

Metabolic waste products stored in the shell. This explanation is often advanced, but there are no proven examples in mollusks. Why spend energy to get waste products into the mantle for deposition in the shell instead of just excreting them? And if it were essential to get rid of waste products in this way, then albino shells should die young of metabolic poisoning. Also, shell pigments that have been studied are the same types of pigments made by other groups of organism.

Temperature regulation. In some intertidal species, a light colored shell might aid in preventing desiccation by keeping temperature lower. In subtidal species, ambient water temperature would minimize any temperature effect of shell color.

No predators so anything goes. First, few mollusk species lack predators. Analysis of stomach contents of batfish and lobsters has shown that they'll eat almost mollusk they encounter. Naticids and octopus are also omnivorous -- think how many species you've collected with drill holes in them. Look at the percentage of shells that have repaired breaks, which are often the result of failed attempts at predation. Second, if pigment has no function, it should just be lost, because it takes energy to produce: witness cave animals. Although molluscivory is widespread, many mollusk species have few predators that use vision to find their prey. So why do so many shells have pigmented color patterns, especially inside the shell or hidden by periostracum where it can't be seen?

Pigment strengthens the shell--it serves a structural function. Color patterns often align with spiral or axial sculpture. Sculpture, like corrugation in cardboard, strengthens shells against predators such as crabs, and pigment might further strengthen it. Instead of producing and transporting a thicker shell, it might be more energy efficient for mollusks to make pigments. Pigments might also impede propagation of a crack in the shell. The structural explanation also works for color inside of shells. A good example is Mercenaria mercenaria (the quahog or cherrystone clam). The purple inside the shell, hidden when the animal is alive, lies along the edges of the shell, just where the big Busycon whelks are likely to attack. The pigment in Mercenaria presumably makes it harder for Busycon to chip the shell. Under the structural explanation, albinos would be at a disadvantage, but it wouldn't be fatal to all individuals. And cave animals, which generally don't face predation (or big waves), wouldn't need pigment for structural reasons.

Mixing and matching

More than one explanation can apply to a species. Because each species has different biological characteristics, it has to be evaluated on its own merits. The explanation for one species might not apply to another. For example, Busycon contrarium (the Lightning Whelk) makes a color pattern when it is young, but stops making it as it gets older. Maybe it no longer needs camouflage against predators once it reaches a certain size (or maybe it changes habitat?). Conus leopardus (the Leopard Cone) has a thick periostracum which hides its color pattern. Large adults stop making the pattern -- maybe the shell is thick enough after a certain size that it doesn't need to be reinforced. And Mercenaria doesn't make the purple color as a juvenile -- maybe the juvenile shell is too thin to stop Busycon even if pigment is built in, so the animal concentrates its energy on growing till it reaches a size where the pigment make a difference. So far, all these explanations are merely just-so-stories. It's easy to concoct scenarios, but hard to test them. To start with, I'll need a vise and a set of matched albino and normal shells....

Thanks to Stanley Francis, Harry G. Lee, Russell Minton, G. T. Watters, and other subscribers to Conch-L whose ideas during online debate on Conch-L prompted me to write this column.