Seashell: Definition, Types, and Examples

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What are Seashells?

A seashell is a hard, protective exoskeleton created by sea-dwelling invertebrates that is frequently seen washed up on beaches across the world. Molluscs, crabs, oysters, barnacles, brachiopods, annelid worms, and sea urchins are the most frequent creatures that generate seashells.

While the majority of seashells are exterior, certain species (such as cephalopods) have internal ones. Because the seashell is a component of the animal, empty shells indicate that it died of natural causes or was eaten by another species. Calcium carbonate and a little amount of protein make up seashells.

How Seashell Form?

The extracellular secretion of proteins forms different layers of seashells, which are later coated by calcium carbonate. As a result, the shell grows from the bottom up, with the continuous secretion of new material at the animal-shell interface. The mantle is the tissue responsible for shell development.

The mantle is located at the point where the animal’s body meets the shell. To fit the greater size of the animal and offer appropriate protection, the shell expands and becomes progressively robust as the animal grows. The mantle produces three different layers of the shell:

i. Outer Proteinaceous Periosteum

The protein on the outside the non-calcified layer on the shell’s outer surface is known as aceous periosteum. It is made up of a thin, rigid coating of black protein that protects the shell’s edge as it develops. This layer also serves as a structural basis for the succeeding layers, as well as allowing calcium ions to accumulate, which promotes crystallisation.

ii. Prismatic Layer

The prismatic layer, which is composed of a hard, prismatic calcium carbonate with a chalky appearance, is the middle layer of the shell. The same specialised mantle cells create the prismatic and periosteum layers.

iii. Inner Pearly Layer (Nacre)

The inner pearly layer is likewise calcified, but it is a pearly lamellar material produced by mantle surface epithelial cells. Nacre is also known as “Mother of Pearl” because of its incredible strength. Nacre is made up of a “brick-like” arrangement of calcium carbonate sheets interspersed with biopolymers, which gives it elasticity, strength, and crack resistance.

Types of Seashell

Seashells come in a wide range of species, each of which is recognised by the species from which it is derived. Mollusc-produced seashells are by far the most frequent forms of seashell seen on beaches. Other species that generate seashells include sea urchins, corals, arthropods, and brachiopods, in addition to molluscs. The morphology of each can be used to differentiate them.

i. Mollusk Shells

While both marine and freshwater mollusc species exist, marine species are considerably more numerous. Gastropods (snails) and bivalves (clams) are the most common molluscs, and they come in a variety of colours and sizes.

Molluscs also include cephalopods (such as squid) with an interior shell, as well as chitons. Scaphopods have tiny, tusk-shaped shells, and chitins have a shell made up of eight distinct plates that overlap yet flex to allow the creature to move while still providing protection.

ii. Bivalves

Bivalves are the most common seashell seen on beaches, and they include both saltwater and freshwater species. Bivalves are made up of two shells that are linked by a flexible hinge and shelter and protect the organism inside.

Filter-feeding bivalves with eyes and an open circulatory system are commonly collected for pearls. Oysters, mussels, clams, and scallops are examples of common bivalve species. The following is an example of bivalves:

Gastropods: Gastropods are sea snails with spiral or conical shells, the majority of which are spiral or conical. Gastropod shells are typically tiny, but they can vary in overall form and size. Hermit crabs like gastropod seashells, which are the most prevalent form of shell. Gastropod shells are generally covered in a thick coating of nacre, which makes them extremely resistant. 

Arthropods: Arthropods are crustacean species (such as shrimp, crabs, and lobsters) with a hard exoskeleton made of chitin and calcium carbonate that is shed as the animal matures. The carapace is made up of many plates that make up the exoskeleton. The animal is susceptible throughout the moulting process until the new exoskeleton solidifies because the exoskeleton must be removed. The exoskeleton is frequently washed up on beaches and can be classed as a “seashell” in the broadest sense. 

Annelids: Annelids are a group of marine worms that have a hard calcium carbonate tube that resembles a seashell. In contrast to mollusc shells, annelid tubes have only two layers: a protein layer and a calcium carbonate layer. Some species burrow or attach their tubes to the substrate.

iii. Brachiopods

The lamp shell, which resembles clams in appearance, is the most common brachiopod. Brachiopods come in a variety of sizes and have two shells called “valves” that protect the organism’s dorsal and ventral surfaces and are connected by muscle or a hinge.

The valves are made up of three layers, similar to mollusc shells: an exterior layer made up of proteins, a middle layer made up of calcium carbonate, and an interior layer made up of calcium and protein. Brachiopods, like molluscs, have a mantle at the hinge that secretes the different components that make up the shell.

iv. Sea Urchins

Sea urchins have a hard shell made of calcium carbonate called a “test,” as well as a dermis and epidermis that enclose the organs. Five different grooves are scattered throughout the test, each with two rows of calcium carbonate plates. Tubercles cover the plates and serve as a connection for the sea urchin’s distinctive spines. A peritoneum, or inner lining, is also present in the test. The following is an example of a sea urchin test:

Corals: Both hard and soft corals can wash up on the beach, and both have a calcium carbonate skeleton or skeletal components. Hard coral polyps have a skeletal tube made up of vertical plates called septocostae (pictured below), which are connected by a coenosteum and a thin covering known as an epitheca. Soft corals, on the other hand, develop spiky sclerites as a result of protein chain cross-linking and/or calcium carbonate production.

Coccolithophore: Coccolithophores are phytoplankton that produce calcium carbonate exoskeletons called “coccoliths.” Coccoliths are translucent and may be shed and duplicated as the animal grows, allowing for the passage of light necessary for photosynthesis. While the exact function of this type of seashell is unknown, it is thought that coccoliths give protection from predators as well as the marine environment.

Seashell Citations
  • Molluscan shell colour. Biol Rev Camb Philos Soc . 2017 May;92(2):1039-1058.
  • Salmonella and eggs: from production to plate. Int J Environ Res Public Health . 2015 Feb 26;12(3):2543-56.
  • The evolution of mollusc shells. Wiley Interdiscip Rev Dev Biol . 2018 May;7(3):e313.
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