Scaphopoda (plural scaphopods , from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as tusk shells or tooth shells, are a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

of shelled marine

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s belonging to the

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

Mollusca Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

with worldwide distribution and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range in length (with ''Fissidentalium metivieri'' as the longest). Members of the order Dentaliida tend to be larger than those of the order Gadilida. These molluscs live in 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 not as common or as easily visible in the beach drift as the shells 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

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

s. Molecular data suggest that the scaphopods are a sister group to the

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s, although higher-level molluscan phylogeny remains unresolved.

Classification

The group is composed of two subtaxa, the Dentaliida (which may be paraphyletic) and the monophyletic Gadilida. The differences between the two orders is subtle and hinges on size and on details of the radula, shell, and foot. Specifically, the Dentaliids are the physically larger of the two families, and possess a shell that tapers uniformly from anterior (widest) to posterior (narrowest); they also have a foot which consists of one central and two lateral lobes and which bends into the shell when retracted. The Gadilids, on the other hand, are much smaller, have a shell whose widest portion is slightly posterior to its aperture, and have a foot which is disk-like and fringed with tentacles which inverts into itself when retracted (in this state resembling a pucker rather than a disk). According to the

World Register of Marine Species The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive catalogue and list of names of marine organisms. Content The content of the registry is edited and maintained by scien ...

: * Dentaliida da Costa, 1776 ** family Anulidentaliidae Chistikov, 1975 – 3 genera ** family Calliodentaliidae – 1 genus ** family Dentaliidae Children, 1834 – 14 genera ** family Fustiariidae Steiner, 1991 – 1 genus ** family Gadilinidae Chistikov, 1975 – 3 genera ** family Laevidentaliidae Palmer, 1974 – 1 genus ** family Omniglyptidae Chistikov, 1975 – 1 genus ** family Rhabdidae Chistikov, 1975 – 1 genus * Gadilida Starobogatov, 1974 ** sub-order Entalimorpha Steiner, 1992 *** family Entalinidae Chistikov, 1979 – 9 genera ** sub-order Gadilimorpha Steiner, 1992 *** family Gadilidae Stoliczka, 1868 – 8 genera *** family Pulsellidae Scarabino in Boss, 1982 – 3 genera *** family Wemersoniellidae Scarabino, 1986 – 2 genera

Evolution

Fossil record

There is a good fossil record of scaphopods from the Mississippian onwards, making them the youngest molluscan class. 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.

Phylogeny

The scaphopods are largely agreed to be members of the

Conchifera Conchifera is a subphylum of the phylum Mollusca, containing five extant classes: Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda. Conchiferans can bear a single shell as in snails and ammonites, a single pair of shell ...

, however their phylogenetic relationship with the other members of this subphylum remains contentious. The Diasoma concept proposes a clade of scaphopods and bivalves based on their shared infaunal lifestyle, burrowing foot, and possession of a mantle and shell. Pojeta and Runnegar proposed the extinct

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 ...

as the stem group of the Diasoma. An alternative hypothesis proposes the cephalopods and gastropods as sister to the scaphopods with helcionellids as the stem group. A review of deep molluscan phylogeny in 2014 found more support for the scaphopods, gastropods, or cephalopods than for scaphopods and bivalves, thus the shared body features of scaphopods and bivalves may be convergent adaptations due to similar lifestyles. Analysis of the scaphopod nervous system demonstrated that both scaphopods and cephalopods share a similar nervous system structure, with ventrally shifted pedal nerves and lateral nerves that extend dorsally. These similarities led to the conclusion that scaphopods are sister to the cephalopods with gastropods as sister to them both. More recent research, including the sequenced genome of tusk shells, support the Diasoma model with bivalves as the sister group.

Orientation

The morphological shape of the scaphopod body makes it difficult to orient it satisfactorily. As a result, researchers have often disagreed as to which direction is anterior/ posterior and which is ventral/ dorsal. According to Shimek and Steiner, " e apex of the shell and mantle are anatomically dorsal, and the large aperture is ventral and anterior. Consequently, the concave side of the shell and viscera are anatomically dorsal. The convex side has to be divided into anteriorly ventral and dorsally posterior portions, with the anus as the demarcation. Functionally, as in

s, the large aperture with the foot is anterior, the apical area posterior, the concave side dorsal and the convex side ventral."

Anatomy

Shells

The shells of the members of the Gadilida are usually glassy-smooth and narrow, with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators. The Dentalids, by contrast, tend to have strongly ribbed and rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze. This makes them harder to detect by animals such as ratfish, which can sense the electrical signals given off by the most minute muscle movement.

Mantle

The mantle 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 upward. This end seldom appears above the level of the substrate, however, as doing so exposes the animal to numerous predators. Most adult scaphopods live their lives entirely buried within the substrate. Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

. There are no

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s; the entire surface of the mantle cavity absorbs

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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.

Feeding and digestion

A number of minute

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s 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 The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

that breaks the bit into smaller pieces for digestion. The radulae and cartilaginous oral bolsters of the Gadilidae are structured like zippers where the teeth actively crush the prey by opening and closing on it repeatedly, while the radulae and bolsters of the Dentaliidae work rachet-like to pull the prey into the esophagus, sometimes whole. The massive radula of the scaphopods is the largest such organ relative to body size of any mollusc (among whom, except for the

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

, the presence of which is a defining characteristic). The remainder of the digestive system consists of a digestive

diverticulum In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, t ...

esophagus The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

stomach The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

, and

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...

. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

opens on the ventral/ underside of the animal, roughly in the middle of the mantle cavity.

Vascular system

The scaphopod vascular system is rudimentary lacking heart auricles as well as corresponding ctenidia (gills) 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. The

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

, a characteristic feature of all other groups of mollusca, has been considered totally lost or reduced to a thin fold of the

pericardium The pericardium (: pericardia), also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong inelastic connective tissue (fibrous pericardium), ...

; however, according to more recent studies, the muscular, regularly beating perianal blood sinus is homologous to the ventricle and is therefore considered the scaphopod heart. Metabolic waste is excreted through a pair of

nephridia The nephridium (: nephridia) is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Nephridia co ...

close to the anus. The tusk shells appear to be the only extant molluscs which completely lack the otherwise standard molluscan reno-pericardial apertures. Furthermore, they also appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water (through two "water pores" located near the nephridial openings). These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.

Nervous system

The nervous system is generally similar to that of

s. One pair each of cerebral and pleural

ganglia A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

lie close to the oesophagus, and effectively form the animal's

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

. A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body, and connect to pavilion ganglia via long connectives. Radular and sub-radular ganglia are also present, as are statocysts with staticonia. Scaphopods have no

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s, no osphradia, or other distinct sensory organs. However, scaphopods do possess genes involved in photoreceptor formation and function implying scaphopods may have had eyes that degenerated over evolutionary time.

Reproduction and development

Scaphopods have separate sexes, and external fertilisation. They have a single

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

occupying much of the posterior part of the body, and shed their

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

s into the water through the nephridium. Once fertilized, the eggs hatch into a free-living

trochophore A trochophore () is a type of free-swimming planktonic marine larva with several bands of cilia. By moving their cilia rapidly, they make a water eddy to control their movement, and to bring their food closer in order to capture it more easily. ...

larva, which develops into a

veliger A veliger is the planktonic larva of many kinds of sea snails and freshwater snails, as well as most bivalve molluscs (clams) and tusk shells. Description The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxono ...

larva that more closely resembles the adult, but lacks the extreme elongation of the adult body. The three-lobed foot originates prior to metamorphosis while the cephalic tentacles develop post metamorphosis. Scaphopods remain univalved throughout their morphogenesis contrary to bivalves.

Ecology

Tusk shells live in

seafloor sediment Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles either have their origins in soil and rocks and have been transported from the land to the ...

, feeding primarily on

foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

ns; some supplement this with vegetable matter.

Human use

The shells of '' Dentalium hexagonum'' and '' Dentalium pretiosum'' were strung on thread and used by the natives of the

Pacific Northwest The Pacific Northwest (PNW; ) is a geographic region in Western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common ...

as shell money. ''Dentalium'' shells were also used to make belts and headdresses by the

Natufian The Natufian culture ( ) is an archaeological culture of the late Epipalaeolithic Near East in West Asia from 15–11,500 Before Present. The culture was unusual in that it supported a sedentism, sedentary or semi-sedentary population even befor ...

culture of 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 ...

, and are a possible indicator of early social stratification. File:Necklace PRE.2009.0.238.1-a.jpg, Tusk shell necklace from Bronze Age ( MHNT) File:Necklace PRE.2009.0.238.1-IMG 1851.jpg, Tusk shell necklace from Bronze Age (MHNT) File:Necklace PRE.2009.0.238.1-IMG 1992.jpg, Tusk shell necklace from Bronze Age (MHNT) File:Necklace PRE.2009.0.238.1-IMG 2001.jpg, Tusk shell necklace from Bronze Age (MHNT)