Tusk shell
Encyclopedia
The tusk shells or scaphopods are a class
of shelled marine
mollusks. The scientific name of this class is Scaphopoda, meaning "shovel-footed". Shells of species within this class range from about 0.5 to 15 cm in length. Members of the Order Dentaliida
are generally significantly larger than those of the Order Gadilida
.
These molluscs live on soft substrates
offshore (usually not intertidally). Because of this subtidal habitat
and the small size of most species, many beachcombers
are unfamiliar with them; their shells
are usually not nearly as common or as easily visible in the beach drift as the shells of sea snail
s and clam
s.
Molecular data suggests that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains somewhat unresolved.
The aragonitic shells of scaphopods are conical and curved in a planispiral way, and they are usually whitish in color. Because of these characteristics, the shell somewhat resembles a miniature elephant
's tusk, hence the common name tusk shell. However, unlike an elephant's tusk, the shells of these molluscs are hollow and open at both ends; the opening at the larger end is the main or anterior aperture
of the shell. The smaller opening is known as the apical aperture. Some tusk shells are minute, and most are 4 to 6 cm (1.6 to 2.4 in) long; however, a few species reach 15 centimetres (5.9 in) in length.
The scaphopod shell bears a prominent cross-lamellar layer, and has a bioeroded tip.
(which may be paraphyletic) and the monophyletic Gadilida
.
The differences between the two orders is subtle and hinges on details of the radula and shell.
of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow through the substrate. The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting up into the water.
Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by cilia. There are no gill
s; the entire surface of the mantle cavity absorbs oxygen from the water. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.
A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food, which they then convey to the mouth. The mouth has a grinding radula
that breaks the bit into smaller pieces for digestion. The remainder of the digestive system consists of an oesophagus, stomach
, and intestine
. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The anus
opens on the underside of the animal, roughly in the middle of the mantle cavity.
The scaphopod vascular system is rudimentary, lacking both heart and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. Metabolic waste is excreted through a pair of nephridia
close to the anus.
The nervous system is generally similar to that of gastropod
s. One pair each of cerebral and pleural ganglia
lie close to the oesophagus, and effectively form the animal's brain
. A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body. Scaphopods have no eye
s, or other distinct sensory organs.
, and external fertilisation. They have a single gonad
occupying much of the posterior part of the body, and shed their gamete
s into the water through the nephridium.
Once fertilised, the eggs hatch into a free-living trochophore
larva, which develops into a veliger
larva that more closely resembles the adult, but lacks the extreme elongation of the adult body.
The Ordovician Rhytiodentalium kentuckyensis has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from ribeirioid rostroconch molluscs such as Pinnocaris
. However, a competing hypothesis suggests a Devonian/Carboniferous origin from a non-mineralized ancestor, or from a more derived, Devonian, conocardioid rostroconch.
As such they were the most recent of all molluscan classes to evolve. They are most closely related to the extinct molluscan class Rostroconchia
.
hexagonum, a scaphopod mollusc, were strung on thread and used by the natives of the Pacific Northwest
as shell money. Dentalium shells were also used to make belts and headdresses by the Natufian culture of the Middle East
, and are a possible indicator of early social stratification.
Shells of the species Dentalium pretiosum were used as money.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of shelled marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
mollusks. The scientific name of this class is Scaphopoda, meaning "shovel-footed". Shells of species within this class range from about 0.5 to 15 cm in length. Members of the Order Dentaliida
Dentaliida
Dentaliida is one of the two orders of scaphopod mollusks, commonly known as elephant's tusk shells. The order Dentaliida contains most of the larger scaphopods.-Families:* Anulidentaliidae* Calliodentaliidae* Dentaliidae* Fustiariidae* Gadilidae...
are generally significantly larger than those of the Order Gadilida
Gadilida
Gadilida is one of the two orders of scaphopod molluscs, which are commonly known as tusk shells.The families within the order Gadilida are usually much smaller organisms than the families within the other order of scaphopods, the Dentaliida....
.
These molluscs live on soft substrates
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
offshore (usually not intertidally). Because of this subtidal habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
and the small size of most species, many beachcombers
Beachcombing
Beachcombing and beachcomber are words with multiple, but related, meanings that have evolved over time.A beachcomber is someone who "combs" the beach, and the intertidal zone in general, looking for things of value, interest or utility....
are unfamiliar with them; their shells
Seashell
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...
are usually not nearly as common or as easily visible in the beach drift as the shells of sea snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s and clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...
s.
Molecular data suggests that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains somewhat unresolved.
Ecology
Scaphopods feed on microscopic organisms; some supplement their diet of zooplankton with vegetable matter.Shell description
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
's tusk, hence the common name tusk shell. However, unlike an elephant's tusk, the shells of these molluscs are hollow and open at both ends; the opening at the larger end is the main or anterior aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
of the shell. The smaller opening is known as the apical aperture. Some tusk shells are minute, and most are 4 to 6 cm (1.6 to 2.4 in) long; however, a few species reach 15 centimetres (5.9 in) in length.
The scaphopod shell bears a prominent cross-lamellar layer, and has a bioeroded tip.
Habitat and distribution
Tusk shells live in seafloor sediment where they feed on detritus, foraminiferans and microscopic animals.Classification
The group is composed of a two subtaxa, the DentaliidaDentaliida
Dentaliida is one of the two orders of scaphopod mollusks, commonly known as elephant's tusk shells. The order Dentaliida contains most of the larger scaphopods.-Families:* Anulidentaliidae* Calliodentaliidae* Dentaliidae* Fustiariidae* Gadilidae...
(which may be paraphyletic) and the monophyletic Gadilida
Gadilida
Gadilida is one of the two orders of scaphopod molluscs, which are commonly known as tusk shells.The families within the order Gadilida are usually much smaller organisms than the families within the other order of scaphopods, the Dentaliida....
.
The differences between the two orders is subtle and hinges on details of the radula and shell.
Anatomy of the soft parts
The mantleMantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow through the substrate. The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting up into the water.
Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by cilia. There are no gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s; the entire surface of the mantle cavity absorbs oxygen from the water. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.
A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food, which they then convey to the mouth. The mouth has a grinding radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
that breaks the bit into smaller pieces for digestion. The remainder of the digestive system consists of an oesophagus, stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, and intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
opens on the underside of the animal, roughly in the middle of the mantle cavity.
The scaphopod vascular system is rudimentary, lacking both heart and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. Metabolic waste is excreted through a pair of nephridia
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...
close to the anus.
The nervous system is generally similar to that of gastropod
Nervous system of gastropods
The nervous system of gastropods consists of a series of paired ganglia connected by major nerve cords, and a number of smaller branching peripheral nerves.The brain of a gastropod consists of three pairs of ganglia, all located close to the oesophagus...
s. One pair each of cerebral and pleural ganglia
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
lie close to the oesophagus, and effectively form the animal's brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body. Scaphopods have no eye
Mollusc eye
The molluscs have the widest variety of eye morphologies of any phylum, and a large degree of variation in their function. Cephalopods' eyes are as complex as those of vertebrates; scallops have up to 100 simple eyes; and some bivalves have compound eyes....
s, or other distinct sensory organs.
Reproduction and development
Scaphopods have separate sexesDioecious
Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...
, and external fertilisation. They have a single gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
occupying much of the posterior part of the body, and shed their gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s into the water through the nephridium.
Once fertilised, the eggs hatch into a free-living trochophore
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
larva, which develops into a veliger
Veliger
A veliger is the planktonic larva of many kinds of marine and freshwater gastropod molluscs, as well as most bivalve mollusks.- Description :...
larva that more closely resembles the adult, but lacks the extreme elongation of the adult body.
Fossil record
There is a good fossil record of scaphopods from the Mississippian onwards, making them the youngest molluscan class; but the origin of the group remains contentious.The Ordovician Rhytiodentalium kentuckyensis has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from ribeirioid rostroconch molluscs such as Pinnocaris
Pinnocaris
Pinnocaris is a genus of rostroconch mollusc known from the Ordovician. It was once considered as a possible ancestor to the scaphopods, but this no longer seems tenable. Original descriptions from material in southern Scotland had the organism as a bivalved arthropod, but its univalved...
. However, a competing hypothesis suggests a Devonian/Carboniferous origin from a non-mineralized ancestor, or from a more derived, Devonian, conocardioid rostroconch.
As such they were the most recent of all molluscan classes to evolve. They are most closely related to the extinct molluscan class Rostroconchia
Rostroconchia
The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult typically has a single, pseudo-bivalved shell...
.
Human use
The shells of DentaliumDentalium
The word dentalium or dentalia , as commonly used by Native American artists and anthropologists, refers to tooth shells or tusk shells used in indigenous jewelry, adornment, and commerce in western Canada and the United States. These tusk shells are a kind of seashell, specifically the shells of...
hexagonum, a scaphopod mollusc, were strung on thread and used by the natives of the Pacific Northwest
Pacific Northwest
The Pacific Northwest is a region in northwestern North America, bounded by the Pacific Ocean to the west and, loosely, by the Rocky Mountains on the east. Definitions of the region vary and there is no commonly agreed upon boundary, even among Pacific Northwesterners. A common concept of the...
as shell money. Dentalium shells were also used to make belts and headdresses by the Natufian culture of the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, and are a possible indicator of early social stratification.
Shells of the species Dentalium pretiosum were used as money.