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.
