An operculum (; ) is a
corneous
Corneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals.
The word is generally used to describe natural or pathological anatomical structures made o ...
or
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines.
In zoology
''Calcare ...
anatomical structure like a
trapdoor
A trapdoor or hatch is a sliding or hinged door that is flush with the surface of a floor, ceiling, or roof. It is traditionally small in size. It was invented to facilitate the hoisting of grain up through mills, however, its list of uses has ...
that exists in many (but not all) groups of
sea snail
Sea snails are slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the Taxonomic classification, taxonomic class Gastropoda with slugs, which are distinguishe ...
s and
freshwater snail
Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs t ...
s, and also in a few groups of
land snail
A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. ''Land snail'' is the common name for terrestrial molluscs, terrestrial gastropod mollusks that have gastropod shell, shel ...
s, including the
Helicinidae,
Cyclophoridae,
Aciculidae
The Aciculidae are a family (biology), family of small land snails that have operculum (gastropod), opercula (an operculum is a little door that closes the shell when the animal retracts into it). In other words, Aciculidae are Terrestrial mollus ...
,
Maizaniidae,
Pomatiidae
The family Pomatiidae is a taxonomic family of small operculate land snails, terrestrial gastropod mollusks that can be found over the warmer parts of the Old World. In the older literature, this family is designated as Pomatiasidae.
This fam ...
, etc.
The operculum is attached to the upper surface of the foot and in its most complete state, it serves as a sort of "trapdoor" to close the
aperture of the shell when the soft parts of the animal are retracted. The shape of the operculum varies greatly from one family of gastropods to another. It is fairly often circular, or more or less oval in shape. In species where the operculum fits snugly, its outline corresponds exactly to the shape of the
aperture of the shell and it serves to seal the entrance of the shell.
Many families have opercula that are reduced in size, and which are not capable of closing the shell aperture. Opercula have sometimes been modified: in the
Strombidae
Strombidae, common name, commonly known as the true conchs, is a Taxonomy (biology), taxonomic family (biology), family of medium-sized to very large sea snails in the superfamily (zoology), superfamily Stromboidea, and the Epifamily Neostromboid ...
the operculum is claw-shaped and is used to push into the substrate in a leaping form of locomotion.
Virtually all
pulmonate
Pulmonata or pulmonates is an informal group (previously an order, and before that, a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group inclu ...
snails are inoperculate, i.e. they do not have an operculum, with the exception of the
Amphiboloidea. However, some
terrestrial pulmonate species are capable of secreting an
epiphragm, a temporary structure that can in some cases serve some of the same functions as an operculum. The epiphragm may be distinguished from the true operculum by its homogeneity and want of growth marks.
In
ammonite
Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
s (extinct shelled cephalopods), a calcareous structure known as the
aptychus (plural aptychi) existed. When these were first described they were thought to be
valve
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...
s of a
bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
species, then for many years after that they were considered to be a form of paired or single operculum-like structures belonging to ammonites. More recently the aptychus or paired aptychi have been hypothesized to be a
jaw apparatus of ammonites.
Functions
Perhaps the most essential function of the operculum in gastropods is to allow snails to resist drying out, or
desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...
. This is very important in intertidal marine snails during low tide, and this also enables operculate freshwater and land snails to survive periods of drought and dry weather.
In those marine species where the operculum completely seals the shell, it can also serve as a protection against
predator
Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
s when the snail body is retracted.
Anatomy

In life, the operculum is attached at the ending of the
columellar muscle with an opercular disc dorsally to the upper surface of the posterior part of the foot. However, in ''
Buccinum'' a layer of long cylindrical
epithelial cell
Epithelium or epithelial tissue is a thin, continuous, protective layer of Cell (biology), cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial (Mesothelium, mesothelial) tissues line ...
s, with mostly distinct nuclei, and long divided processes enter between the muscular fibres. The operculum, a cuticular development of these cells, is composed, as may be seen in the corneous opercula of ''
Murex
''Murex'' is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly called "murexes" or "rock snails".Houart, R.; Gofas, S. (2010). Murex Linnaeus, 1 ...
'',
''
Purpura'', ''
Triton'', of very thin superimposed layers. The cylindrical cells are attached with their head to the lowermost layer.
The operculum grows in size as the shell grows, such that the operculum remains in proportion to the
apertural size. In many species, when the animal is active and crawling, part of the underside of the shell rests on the outer surface of the operculum.
In many species of marine shelled snails which live subtidally, the operculum is greatly reduced in size, and no longer serves to seal the shell entrance. In a large number of families it has been eliminated completely.
In species of
conch
Conch ( , , ) is a common name of a number of different medium-to-large-sized sea snails. Conch shells typically have a high Spire (mollusc), spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point on both ...
s, the operculum is elongated and sickle-shaped, and is used to dig into the sand to enable the conch to perform a leaping type of
locomotion.

The structure of the operculum can be described as follows:
* concentric: the nucleus is central or subcentral as in ''
Lithoglyphus'' and ''
Ampullaria'', and in other the nucleus is near the parietal margin of the shell.
* imbricated, or lamellar: when it grows only on one side, and the nucleus is marginal, as in ''Purpura'',''
Xenophora'', and ''
Paludomus''.
* claw-shaped, or unguiculate: with the nucleus apical or in front, as in ''
Turbinella'' and ''
Fusus''; it is claw-shaped and serrated in ''
Strombus
''Strombus'' is a genus of medium to large sea snails, marine (ocean), marine gastropod molluscs in the family (biology), family Strombidae, which comprises the true conchs and their immediate relatives. The genus ''Strombus'' was named by Swedi ...
''
* spiral: when it grows only on one edge, and revolves as it grows; it is always sinistral in dextral shells.
* paucispiral or oligogyrous: with few spirals as in ''
Littorina
''Littorina'' is a genus of small sea snails, marine (ocean), marine gastropod molluscs in the family (biology), family Littorinidae, the winkles or periwinkles.WoRMS (2011). Littorina Férussac, 1822. Accessed through: World Register of Marine ...
''.
* subspiral or scarcely spiral, in ''
Thiara''
* multispiral or polygyrous: having many closely spaced spirals as in ''
Trochus'' where they sometimes amount to twenty; the number of turns which the operculum makes is ''not'' determined by the number of whorls in the shell, but by the curvature of the aperture, and the necessity that the operculum should revolve fast enough to fit it constantly.
* articulated, when it has a projection, as in ''
Nerita''
* radiated is a modification of the articulated operculum in which the spiral is not so evident as in ''
Navicella''
File:Filopaludina martensi operculum.png, Example of a concentric operculum ('' Filopaludina martensi'')
File:Marstonia comalensis operculum.png, Example of a polygyrous spiral with eccentric nucleus in the operculum of '' Marstonia comalensis'', scale bar: 200 μm
File:Cremnoconchus syhadrensis operculum 2.png, Example of an oligogyrous spiral in '' Cremnoconchus syhadrensis''
File:Opercula coral reef Pliocene Cyprus.jpg, Fossil spiral opercula (both sides) from the Pliocene of Cyprus
In 1998 Checa and Jimeneze proposed three types of opercula:
* type 1: flexiclaudent spiral (mostly multispiral) operculum; its shape does not coincide with the aperture but fits by flexing into the aperture; predominantly present in archaeogastropods.
* type 2: rigiclaudent spiral (usually paucispiral) operculum; its shape fitting the aperture; present in archaeogastropods, but predominating in Caenogastropoda.
* type 3: rigiclaudent concentric operculum; its shape fitting the aperture; predominant in higher neotaenioglossans and exclusive in neogastropods.
There are two basic types of opercula in terms of their material composition:
* The most common kind of operculum is composed of a thin to rather thick corneous protein material, which is yellow to brownish in color and is usually somewhat translucent. This matter is supple when in its natural state but may become brittle when it is dried out. The operculum varies in shape, depending on the family of snails and the shape of the aperture of their shells.

* The other kind of operculum is restricted to a few families of gastropods including the
Turbinidae. This operculum structure has a corneous base with a heavy
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines.
In zoology
''Calcare ...
overlay. The calcareous surface in some genera has color or ornamentation of various kinds including, for example, pustules and incised grooves.
The different shapes of opercula can include ungulate (hooflike), claw-like, or ovate. The type and shape of the operculum is used to help identify and classify related groups (genera) of land operculates, and likewise some marine operculates.
Human uses
The oldest known human depiction of an operculum together with the shell of ''
Charonia tritonis'' is a
seal made by the
Minoan civilization
The Minoan civilization was a Bronze Age culture which was centered on the island of Crete. Known for its monumental architecture and energetic art, it is often regarded as the first civilization in Europe. The ruins of the Minoan palaces at K ...
.
As incense material
Opercula of certain
gastropods
Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...
, especially varieties from the
Red Sea
The Red Sea is a sea inlet of the Indian Ocean, lying between Africa and Asia. Its connection to the ocean is in the south, through the Bab-el-Mandeb Strait and the Gulf of Aden. To its north lie the Sinai Peninsula, the Gulf of Aqaba, and th ...
, have long served as an
incense
Incense is an aromatic biotic material that releases fragrant smoke when burnt. The term is used for either the material or the aroma. Incense is used for aesthetic reasons, religious worship, aromatherapy, meditation, and ceremonial reasons. It ...
material in ancient
Jew
Jews (, , ), or the Jewish people, are an ethnoreligious group and nation, originating from the Israelites of ancient Israel and Judah. They also traditionally adhere to Judaism. Jewish ethnicity, religion, and community are highly inte ...
ish tradition, as well as
Arabian
The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world.
Geographically, the ...
cultures. The operculum of conch species ''
Strombus tricornis'' and ''
Lambis truncata sebae'' are most commonly used in regions near the
Middle East
The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq.
The term came into widespread usage by the United Kingdom and western Eur ...
. Opercula from these conches may be the
onycha incense material which is described in the
Book of Exodus
The Book of Exodus (from ; ''Šəmōṯ'', 'Names'; ) is the second book of the Bible. It is the first part of the narrative of the Exodus, the origin myth of the Israelites, in which they leave slavery in Biblical Egypt through the strength of ...
.
Operculum powder is also an important ingredient in
Chinese and
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
ese incense making. Here it is called "''bèi xiāng''" (Chinese: 貝香; ''lit.'' sea shell fragrance) or "''kai kou''" (Japanese: 甲香, ''lit.'' shell/armour fragrance) respectively. Incense producers in these countries use the operculum of many conches and other marine
snail
A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
s, including those found in Southeast Asia, South America, and East Africa. The opercula are traditionally treated with
vinegar
Vinegar () is an aqueous solution of diluted acetic acid and trace compounds that may include flavorings. Vinegar typically contains from 5% to 18% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting ...
,
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
and
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
in order to remove any
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
y smell. The clean opercula are then ground to a powder and used as a scent fixative, in a technique similar to that used in
perfume
Perfume (, ) is a mixture of fragrance, fragrant essential oils or aroma compounds (fragrances), Fixative (perfumery), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agre ...
s with certain plant
resin
A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
s.
When burnt on its own, high quality operculum reportedly smells of
castoreum
Castoreum is a yellowish exudate from the castor sacs of mature beavers used in combination with urine to territorial marking, scent mark their territory.
Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two c ...
or other animal
musks, while that of lower quality is reminiscent of burnt hair.
As a gemstone or decorative object
The operculum of certain species of
Turbinidae is sometimes used as a very inexpensive organic "
gemstone
A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...
" in rings, bracelets, amulets etc. These opercula are commonly known as "cats eye" (or more recently "Shiva's eye").
The turban snail ''
Turbo petholatus
''Turbo petholatus'' (common name: tapestry turban) is a species of sea snail, marine gastropod mollusk in the family (biology), family Turbinidae.
Description
The length of the shell varies between 30 mm and 100 mm.
The imperforate ...
'' is the species whose operculum is most renowned, although other ''Turbo'' species are also used. The operculum has two sides of differing appearance. Either side or both may be displayed as jewelry or ornament. The internal side is flat and white, with a spiral suture line, while the external side is hemispherical, glossy, and variously colored and patterned. In the case of ''T. petholatus'' there is a prominent dark 'eye' and sometimes a greenish area.
Opercula are often used in
Northwest Coast art of North America, as detailing in masks, panels and paddles. These objects are used to represent the teeth in masks, and are used as a decorative element in many of the objects which play an important part in the art and culture of Northwest Coast peoples, including dishes for food, bentwood boxes, and rattles.
Friedrich Ratzel in ''The History of Mankind'' reported in 1896 that, in
Oceania
Oceania ( , ) is a region, geographical region including Australasia, Melanesia, Micronesia, and Polynesia. Outside of the English-speaking world, Oceania is generally considered a continent, while Mainland Australia is regarded as its co ...
, chains and girdles composed of the coloured opercula of certain shells were considered valuable ornaments.
As paperweights
The largest opercula of ''
Turbo marmoratus'' have been used as
paperweight
A paperweight is a small solid object heavy enough, when placed on top of papers, to keep them from blowing away in a breeze or from moving under the strokes of a painting brush (as with Chinese calligraphy). While any object, such as a stone, ...
s.
References
*
External links
Pictures of diverse gastropod opercula This site also provides fairly extensive material on the growth, development, and physiology of the gastropod operculum as well as dozens of pictures of both the operculum and its host shell.
* Photo of opercula of some ''
Busycon'' and ''
Busycotypus'' species
{{DEFAULTSORT:Operculum (Gastropod)
Gastropod anatomy
Incense material
Mollusc products
Organic gemstones