Jewels From the Seashore

BY AWAKE! WRITER IN NICARAGUA

HAVE you ever experienced the thrill of coming across a fascinating seashell glistening on the sand? If so, you are not alone. People of all ages treasure seashells for their great variety and special beauty.

Shells can be found on almost every seashore on earth. A seashell, however, is more than just something lovely to hold and admire. Each shell was formerly the home of the soft-bodied creature that built it​—a mollusk. By conservative estimates, more than 50,000 species of mollusks exist today!

The shells discussed in this article can all be found on Nicaragua’s Pacific Coast. I collected many of them along a two-mile [3 km] stretch of beach called Poneloya and Las Peñitas. Others were given to me by local fishermen. Please allow me to introduce you to some of these shells, along with the sea creatures that made them.

The Large Gastropods

Most shells are made by mollusks belonging to two main classes: Gastropoda (gastropods) and Bivalvia (bivalves). Gastropods, including all types of snails, have a head that usually bears tentacles and eyes. These mollusks creep and glide along on a single fleshy foot. That is why they are called gastropods​—a term that means “belly-footed.”

How does a gastropod breathe and eat? Many marine varieties breathe through a siphon, that is, a tubelike organ protruding from the head. The siphon enables them to suck in water through their gills. Some gastropods have a proboscis, an additional tubular organ used for feeding. Many gastropods also have a radula​—a horny ribbon that bears small hard teeth. This apparatus works like a rasp to tear up the mollusk’s food. All gastropods possess a nervous system, a circulatory  system, a digestive system, and reproductive organs.

How can you tell if a shell was made by a gastropod? If it was, it will be a univalve​—a shell consisting of one piece, usually coiled. Murex, tun, cowrie, cone, and turritella shells are all made by gastropods. Let us take a closer look at some of these jewels.

Growing in Spurts​—Drilling for Food

Murex shells from all over the world tend to be intricately shaped. I found two species, the beautiful pink and brown royal murex and the equally attractive root murex. Where do their spiny ridges come from? Mollusks create ridges, called varices, during periods of slow shell growth. Between the ridges you can see shell material that was produced during growth spurts. The book Shells​—Treasures of the Sea explains: “Adding new shell consumes so much energy that many gastropods slow down their metabolism and go into a period of inactivity so more energy can be devoted to rapid shell growth. Usually the animal will bury itself or hide in some other manner to avoid disturbance from predators. . . . The growth spurt may last from a few days to several weeks, depending on the species of mollusk.”

The murex itself is a formidable predator. At times while collecting shells, I wondered why some shells had a perfect little hole bored through them. I later learned that the murex is one of several mollusks that can drill a small hole into the shell of its prey with the aid of its radular teeth. The murex then inserts its proboscis through the hole and enjoys its meal!

I was intrigued to learn that the murex is referred to indirectly in the Bible. Two species common in the Mediterranean Sea, Murex trunculus and Murex brandaris, were the source of a costly purple dye used to color cloth in Bible times. (Esther 8:15; Luke 16:19) A gland of the murex produces a yellowish fluid that turns deep violet or reddish purple when exposed to air and sunlight. Since each murex yields only a tiny amount of fluid, it has been estimated that it would take nearly 12,000 murices to make one and a half grams of dye. No wonder Tyrian purple, as the dye was known, was accessible only to the wealthy! *

Grinning Shells

I have also collected a species of tun shell, the grinning tun. Tuns can be found around the world, since their larvae may float freely for weeks or even months before settling to the sea bottom to develop. You cannot help but notice the handsome, broad ribs going around the shell as well as the special feature  of the grinning tun​—its thickened lip buttressed with teeth. It is believed that this lip helps to protect the delectable snail inside from being eaten by hungry crabs.

My collection includes fine specimens​—both juvenile and mature—​of the crown conch, so called because of its “crown” of spikes. I also have a juvenile triton, a species that can grow to six inches [15 cm] in length, as well as a Pleuroploca princeps, especially prized for its unusual orange color. Such large deep-water shells rarely wash up on the beach. These beautiful specimens are among those given to me by fisherman friends, who caught them in their nets or lobster traps. They kept the meat for food and graciously gave me the beautiful shells.

Smaller but Eye-Catching

On days when the receding high tide leaves behind a long ribbon of shells on the sand, there are plenty of interesting smaller gastropod shells to be found right on the beach. These include many eye-catching varieties​—cowries, cone shells, olive shells, auger shells, sundial shells, moon shells, and turritellas. Since there are so many species around the world, some collectors specialize in collecting shells from only one family. There are, for instance, more than 500 species of cone shells alone!

One of the most attractive features of gastropod shells is their spiral design. Sundial and auger shells are particularly pretty examples of such a design. As they grow, gastropod  shells follow a mathematically regular pattern. Thus, as they increase in size, they retain their basic form. The mollusk produces this spiral shape by continually adding shell to the edge, coiling around an imaginary axis running straight through the shell. The resulting shell becomes a strong, compact home for the mollusk inside​—functional and beautiful!

Clams, Scallops, and Other Bivalves

What about the other class of mollusks mentioned earlier, the Bivalvia? Their shells are also plentiful on Nicaraguan beaches. A bivalve shell is composed of two hinged halves, called valves, that fit together precisely, enclosing and protecting the animal inside. The clam is a well-known bivalve. Bivalve mollusks have no head, but they do have sensory organs called chemoreceptors that enable them to detect taste or scent in the water. Generally, bivalves feed by filtering out organic material from the seawater. Some have a narrow, fleshy foot with which they propel themselves. The scallop, also a bivalve, is able to swim by forcefully snapping its valves together, producing a jet stream that shoots the scallop backward. To go forward, it squirts out water behind itself. But how does a scallop know when danger lurks nearby? It boasts dozens of small but sensitive blue eyes situated in two rows around its body. These can detect the passing shadow of a predator.

Probably the most prized product of bivalves is nacre, or mother-of-pearl. Shells consist largely of crystallized calcium carbonate​—a component of the mineral salts in seawater. However, the lustrous beauty of nacre comes from a particular type of calcium carbonate called aragonite. On the inside of their shells, certain mollusks lay down microscopically thin aragonite crystals in a pattern resembling overlapping roof tiles. These crystals refract and reflect light, producing the shells’ much-admired iridescent appearance. In fact, some species of bivalves use nacre to coat irritants inside their shells, such as a grain of sand. In time, layer after layer of nacre is applied and an irritating grain of sand may be transformed into a beautiful and precious pearl.

The Mollusk’s Unique Feature

I have saved for last what many consider the most amazing feature of the mollusk​—the mantle, an organ that all mollusks have in common. This is the highly specialized region on the animal’s upper surface that is responsible for the creation of new shell. Says Shells​—Treasures of the Sea: “The mollusk carries [calcium carbonate] in its dissolved form through its blood and secretes it through tube-like pores in the mantle, . . . along the growing edge of the shell.” The mollusk also secretes a protein that causes the calcium to crystallize in water.

At the same time, pigment cells in the mantle implant the wide variety of delightful colors and patterns in these jewels as they grow. Malacologists​—scientists who study mollusks—​puzzle over the function of colors and shell patterns. Color and pattern do not seem to play an important role in the mollusks’ ability to recognize one another. And such features do not always camouflage the animal in nature. But the enormous variety of color, pattern, and shape in the mollusks’ shells certainly captivates us!

So the next time you are walking along a sandy beach and pick up a glistening shell, keep something in mind. Whether it be a common bivalve or a beautifully coiled gastropod, you will be holding in your hand a jewel​—the former home of a soft-bodied little creature known as a mollusk.

[Footnote]

[Blurb on page 26]

Gastropods consist of just one piece, while bivalves produce hinged two-piece shells

[Picture on page 25]

ROOT MUREX

[Picture on page 25]

ROYAL MUREX

[Picture on page 25]

GRINNING TUN

[Picture on page 26]

COWRIE

[Picture on page 26]

CONE SHELL

[Picture on page 26]

OLIVE SHELL

[Picture on page 26]

AUGER SHELL

[Picture on page 26]

CROWN CONCH

[Picture on page 26]

TRITON

[Picture on page 26]

PEARL OYSTER

[Picture on page 26]

PLEUROPLOCA PRINCEPS

[Picture on page 26]

MOON SHELL

[Picture on page 26]

TURRITELLA

[Picture on page 26]

SUNDIAL SHELL

[Picture on page 27]

VENUS CLAM

[Picture on page 27]

SCALLOP