Hawaiian Shells and Cowrie Gloss
Cowries were used as decoration on clothing, drums, divining chains, headdresses, ritual masks, furniture, games, and in computation. Because of the demands of cowries, special systems of numeration were often used just for cowrie shell arithmetic. Along the coast of Africa they were pierced and threaded, generally in strings of forty. Another group multiplied twenty times three cowries and added ten which give (20X3) +10 =70 cowries. Other people had a unique system of cowry equivalents with a special nomenclature. First the shells were counted out by six and then ten groups of six were combined to form piles of sixty cowries. The fabulous gold wealth of one kingdom was balanced and recorded by treasurers every twenty days with the aid of cowrie shells.
Cowries were used as a sign of wealth, prestige, and power on royal thrones, adornment, crowns, and beadwork. Only a king was allowed to wear cowries in bulk. In the Pacific Islands, women dancers wore the cowrie paired or singly with a tight cord around the neck. Cowries became a favorite because of their beautiful colors, unique patterns, and high-gloss finish. This is possible because their mantle is on the outside, secreting the shell from the top down and keeping it protected, whereas most other shells are secreted from the inside out, hence the glossy interior of many shells. The live cowrie mantle is usually ornamented with spectacular papillae that provide camouflage by matching the color of the sponge it feeds upon. Cowries usually remain hidden during the day and emerge at night to feed with the mantle fully extended. Juveniles are paper-thin, coiling as they grow until maturity when the outer lip curves inward, forms teeth, and the shell thickens with a new adult color pattern. The height of an adult cowry does not change once this takes place but rather the shell thickens and the interior is dissolved to create more space inside. Curiously, young cowries stop coiling at random regardless of height, resulting in a broad size range in adults.
Hawaiian Shells and Shape
The mantle helps in the formation of the shell’s external features, such as ribs, spines, and grooves. For some species, these features provide protection from predators, added strength, or assist in burrowing. Shells of creatures like the nautilus have complex internal structures, such as multiple chambers containing water and gas that allow the creature to adjust its buoyancy. Most gastropods have coiled shells in the form of a logarithmic spiral, the only form of a coil that can both increase in size and retain its shape. Such an architecture is an efficient way to maintain strength while retaining the compactness of the shell. Some mollusks cement old discarded shells and other sea floor debris to their own shells. They use this technique as a means to camouflage themselves from predators, and to prevent sinking into soft sand or mud. While many mollusks have shells for protection from predators and environmental stresses, shells also have their disadvantages. Shells are permanent structures that mollusks must carry around for the rest of their lives, and its weight could slow the creature down. Some mollusks, such as the squid and octopus, have evolved by eliminating shells altogether.
Shell shape is a product of evolution which is greatly influenced by local environment and type of sea floor. A shell that is low and wide might indicate strong waves or many predators. A thinner, more spherical shell probably comes from deep water, or areas around the north and south pole that are poor in calcium (unlike rich tropical waters). On hard sea floors, crawling gastropods have coiled shells or flat, saucer-like shell cases that allow them to retreat into the shell when in danger. On a sandy or rubble strewn surface, shells have expanded shell edges that help stabilize the mollusk with its opening facing down. In calmer waters, sculpted features such as spines increase the volume of the shell, making the mollusk look more formidable against predators. This also increases the surface area of the shell, allowing other marine organisms to settle on the mollusk’s shell surface, serving as a physical and chemical camouflage. For burrowing in soft muddy or sandy surfaces, some mollusks have evolved smooth, long, tapering shells. Many molluscs are able to withdraw far enough into their shells to be beyond the reach of predators. Others are able to block their apertures with a hardened plug called an operculum.
Environmental changes, injuries, or abnormal conditions of the mantle are often reflected in the shell they form. When the animal encounters harsh conditions which limit its food supply or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a ‘growth line’ is produced extending the entire length of the shell. Patterns and colors on the shell after these dormant periods are sometimes quite different from previous colors and patterns. Each species of mollusk will build the external shell in a genetically predetermined shape, pattern, ornamentation, and color, while at the same time, giving itself a look all its own.
Hawaiian Shells and Formation
Appreciation of a shell’s beauty can only be enhanced by understanding a shell’s formation. The blood of a mollusk is rich in a liquid form of calcium. A soft, outer organ called the ‘mantle’ concentrates the calcium in areas where it can separate out from the blood, forming calcium carbonate crystals. The mantle continues depositing sheets of the crystal in varying thickness, shape, and orientation. Adjacent layers are often deposited with their crystal planes at right angles to each other greatly increasing the strength of the shell. The shell grows as the animal inside adds its building material to the leading edge near the opening. This causes the shell to become longer, wider, and stronger to better accommodate the growing animal inside.
The mantle orchestrates the designs and colors of the shell. Production of new shell material is influenced by several factors: sexual hormones, intrinsic rhythms, diet, acidity and temperature of water. Colors in shells are derived from organic pigments found in their food. Glandular cells collect these pigments, mix them with fluid calcite, and set this substance into the outer shell before it hardens. There are four main pigments that produce the many colors seen in shells: yellow carotenoids, black melanins, green porphyrins, and blue and red indigoids. Most color cells are located along the front edge of the mantle where new shell material is added. A straight color line or ray is formed when the color cells remain in the same position as the shell grows out. If color pigment production continually starts and stops, a pattern of dots or dashes is drawn on the shell. If the color cells actually migrate to one side, a slanting trail of color is produced. Other kinds of behavior by color cells can produce circles, triangles, and other shapes. Mollusks within a particular species have basic colors and patterns that are genetically inherited, but natural variation, like different hair color among people, gives each shell a character of its own.
