Examining a Sea Shell

Here is a photograph of a sea shell. My family picked it up at a beach in Galveston, Texas three years ago. It may look unspectacular. After all, it is broken. It appears to be heavily eroded, with parts of the shell missing and numerous holes pocketing its surface. However, there are actually many interesting details about this shell. Let us take a closer look.

Those who are familiar with sea shells, and with molluscs in particular, may recognize this shell as a gastropod. Gastropods typically have coiled shells. Snails, conches, and limpets are familiar gastropods. There are more interesting details, as well. Perhaps you have noticed the unusual, tan patches on the shell. What are these patches? Let us take a closer look.

Here is a front view of our shell. A tan patch can be seen on the right side, partially surrounding a hole. Another patch can be found inside the broken tip. Notice the tiny holes on these patches? Those tiny holes are chambers that used to hold colonial animals. A colonial animal is a type of animal that lives in colonies. Often, the individual animals in a colony are connected to each other somehow. In this case, each individual animal creates a shell around itself, and this shell is then connected with its neighbors. All of these connected shells creates an encrusting patch that grows on a solid surface, such as on our shell. The particular type of colonial animal seen on this shell is a bryozoans. In life, each tiny animal would extend a coil covered with tiny hairs called cilia. It would use this cilia to sweep small food particles into its mouth. In fact, there are two types of bryozoans found on this shell. Take a look at the next picture.

 Now we are looking at the inside of the broken shell. Notice the tan patch, showing the same type of bryozoan seen in the previous picture. Right above it is a darker patch with larger holes. This is a second type of bryozoans. Notice that the tan bryozoan actually overlaps the darker bryozoans. This means that the darker bryozoan grew on the shell first, likely after it had been broken open, exposing the inside of the shell to the outside environment. Later on, the tan bryozoan began to grow and partially covered the other bryozoan. However, something else came along and began growing on top of the tan bryozoan. Can you see what it is? Look inside the shell, on the left, at the top. Do you see a small group of cup shapes?

Here is another view of the same cup-like structures. Now we are looking at the underside of the coral. Look in the hole, up inside the top arch. The cups are visible here. Each cup had numerous lines radiating from the center out to the rim of the cup. These are another type of colonial animal: they are coral. Each cup would have housed a single coral animal in life. After the tan bryozoan began to grow and grew over the darker bryozoan, coral began to colonize the shell and grew on top of the tan bryozoan. This entire shell once housed three different colonies of animals. Such amazing details can be found on such an innocuous looking, broken sea shell.

~Written by Steven King