Although the next morning was quite cool, we walked the beach, looking for seashells but we didn’t find many. However, the next day we walked the beach after the tide had gone back out, which was an ideal time for finding seas shells. The two of us agreed that looking for seashells could become addictive.
Seashells are the external skeletons of a class of marine animals called mollusks. While our skeletons are inside our bodies, mollusks’ skeletons are outside their bodies. These shells camouflage and protect the marine animals from predators, storms and strong ocean currents. Just as our bones are made of calcium, these seashells are primarily made of calcium.
After a mollusk dies, its shell is left behind. At times, nature recycles this shell, with other animals, such as the hermit crab, moving
into the shells. Of course, many of these shells , which are swept up on the beach by the tides, are picked up by beachcombers to be used for decorations.
Mollusks are divided into many types; however, there are two main types: bivalves and univalves. These names are derived from the Latin words: bi meaning two and uni meaning one. An easy way to remember this is to think of bicycle (bi), which has two wheels, and unicycle (uni), which has one wheel.
A bivalve mollusk has two shell halves, which form a whole shell. Clams and oysters are examples of bivalve mollusks. Most mollusks are of the bivalve type.
A univalve mollusk has a one-piece shell that is usually a spiral-type shell. The big conch shells that we would have been so excited to find on the beach are among the univalves.
After the animal that created the seashell dies, the shell is either washed up onto the beach or remains in a tide pool where the creature lived.
The exoskeleton of a mollusk has three distinct layers, which are mostly composed of calcium carbonate, with only a small amount (approximately two percent) of protein. The mantle tissue, which is located under and touching the shell, secretes proteins and minerals to form the shell.
`Thus, seashells grow by adding materials at the margins. Since this shell is not shed, the mollusk shell continues to enlarge to accommodate body growth.
In comparison, a turtle’s shell is part of its skeleton. The outer surfaces are made of a tough keratin. Underneath the keratin are the dermal tissues and calcified shells. The carapace (the turtle shell) is formed by fusion of vertebrae and ribs during development.
Turtle shells are mostly made of calcium phosphate, while the exoskeletons of mollusks are made up of mostly calcium carbonate. Both types of shells are water proof and strong, allowing for protection. We know that Mother Nature does not always give up her secrets since scientists do not know why there are two distinct types of exoskeletons
(hard outer coverings).
Unlike seashells, a turtle’s shell has living cells, blood vessels and nerves, including a large number of cells on the shell’s surface. A
turtle can feel a fingernail being scraped across its carapace. If a
turtle’s shell is damaged, the turtle can repair itself. Unlike seashells, turtles grow their shells from the inside out just as we humans grow our bones.
Those who have walked a beach looking for seashells know that not all shells are the same. Seashells are collected from all over the world because of their elegance and bright colors. Throughout history, people have used shells to make jewelry and other items, and in earlier times, certain shells were used as currency.
On our fourth morning of walking the beach, I finally convinced Mary Alice that we had collected enough shells. Although she really wanted to find a sand dollar, Mary Alice agreed that it was time to return to Troy.