Overview

Comprehensive Description

Diversity

There are approximately 900 species of Scaphopoda, commonly called tusk shells.

  • Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
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Distribution

Geographic Range

Scaphopods are found in marine regions around the world.

Biogeographic Regions: indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea

  • Jones, A., J. Baxter. 1987. Molluscs: Caudofoveata, Solenogastres, Polyplacophora and Scaphopoda. London: E. J. Brill/Dr. W. Backhuys.
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Physical Description

Morphology

Sexual Dimorphism

Sexual Dimorphism in shape of the posterior apex of the mantel; Sexual Dimorphism in gonad shape and color visible in species with translucent shells (e.g. Dentalium antillarum)
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Physical Description

Scaphopod shells usually have four layers, and these are used for identification. The shell is curved, tubular, and shaped like an elephant tusk. Most average 3 to 6 cm long, but can range from 4 mm to 15 cm. Fossils show specimens 30 cm long.

The scaphopod shell is open at both ends. The wider end of the shell where the head and foot extends out is the   anterior end. The posterior is the narrow end of the shell which usually is at or below the substrate.

The shell surrounds a large mantle cavity, and wraps around the viscera to form a tube. The mantle cavity goes along ventral side to a smaller opening at the other end. No ctenidia are present, and gas exchange is through the mantle surface. Cilia an currents move water thorugh posterior aperature. Occasional muscular contractions expell water from the posterior end of the shell.

The head is a short, conical projection (probosicis) with a   mouth. Lobes on each side of the head have threadlike tentacles, called captacula, which are used to capture food.

Other Physical Features: heterothermic ; homoiothermic; bilateral symmetry

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Sense organs are much reduced. A radula is present in the buccal mass, and gas exchange occurs via a ciliated respiratory epithelial area in the mantle. The pericardium, heart and blood vessels are absent.

  • Laverack, M.S., Dando, J. "Lecture Notes on Invertebrate Zoology", 1974, Blackwell Scientific Publications.
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Ecology

Habitat

Scaphopods are all marine species whose habitat ranges from shallow sub-littoral areas up to waters that are 4570 m deep. Most scaphopods are found in waters greater than 6 m. Scaphopods burrow in sediments ranging from muds to medium-coarse gravel. The two orders of this group may have slightly differing burrowing behaviors. Individuals in the Gadilida may burrow up to 30 cm in captivity. Many species in the Dentaliida burrow with the concave side just below the substrate.

Habitat Regions: saltwater or marine

Aquatic Biomes: benthic ; coastal

Other Habitat Features: intertidal or littoral

  • Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
  • Lamprell, K., J. Healy. 2001. Scaphopoda. Pp. 85-128 in A Wells, W Houston, eds. Zoological Catalogue of Australia, Vol. 17.2. Melbourne, Australia: CSIRO Publishing.
  • Spear, B. 1994. "Introduction to the Scaphopoda, the tusk shells" (On-line). Accessed February 04, 2005 at http://www.ucmp.berkeley.edu/mollusca/scaphs/scaphopoda.html.
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Trophic Strategy

Food Habits

Scaphopods are selective deposit feeders, mainly feeding on microscopic organisms, particularly diatoms and foraminiferans. Each tentacle of the   capatula has an adhesive know at the tip to capture prey. Tentacular cilia brings smaller particles back to the scaphopod mouth. The tentacles retract to bring larger items to the mouth. The radula is used to break down prey. Food is digested extracellularly in the stomach, then travels from the stomach to intestine. Waste is expelled into mantle cavity through the anus.

Primary Diet: omnivore

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Associations

Ecosystem Roles

Scaphopods selectively feed on sediments, although the importance of this in the ecosystem is unknown. Hermit crabs are known to use the shells.

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Predation

Scaphopods are fed on by fish and crabs. Their burrowing behavior is thought to keep them from predators.

Known Predators:

  • fish
  • crabs

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Known predators

Scaphopoda (chitons/scaphopods) is prey of:
Chondrichthyes
Scombridae
Actinopterygii
organic stuff
Octopus
Decapoda
Stomatopoda
Anomura
Asteroidea
Gastropoda

Based on studies in:
Puerto Rico, Puerto Rico-Virgin Islands shelf (Reef)

This list may not be complete but is based on published studies.
  • Opitz S (1996) Trophic interactions in Caribbean coral reefs. ICLARM Tech Rep 43, Manila, Philippines
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Known prey organisms

Scaphopoda (chitons/scaphopods) preys on:
Ectoprocta
Cirripedia
Ascidia
Porifera
Cnidaria
Anthozoa

Based on studies in:
Puerto Rico, Puerto Rico-Virgin Islands shelf (Reef)

This list may not be complete but is based on published studies.
  • Opitz S (1996) Trophic interactions in Caribbean coral reefs. ICLARM Tech Rep 43, Manila, Philippines
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Life History and Behavior

Behavior

Communication and Perception

The scapopod   captacula may have tactile receptors, but this is unknown. Scaphopods have lost eyes, tentacles and osphridia found in other molluscs. The buccal cavity has a sub-radular (below the radula) organ which may be chemoreceptive.

Perception Channels: tactile ; chemical

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Life Cycle

Development

After fertilization, the egg develops into a free-swimming trocophore larvae, then a bilaterally symmetrical veliger. The veliger usually metamorphoses in 5-6 days. At this point it becomes benthic.

Development - Life Cycle: metamorphosis

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Reproduction

Scaphopods are gonochoristic or dioecious. Eggs are released singly through the right   nephridium. Sperm is also released through the nephridium. Eggs are planktonic and fertilization is external.

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); oviparous

There is no parental investment after release of gametes.

Parental Investment: pre-fertilization (Provisioning)

  • Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
  • Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
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The larva is of trochophore type.

  • Laverack, M.S., Dando, J., "Lecture Notes on Invertebrate Zoology", 1974, Blackwell Scientific Publications.
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Evolution and Systematics

Evolution

Classification

These two clades seem unambiguously supported by both molecular and morphological data; traditionally ranked as orders although why not subclass? Opinions welcome, decisions on this extend to other cases like the archeogastropod groups.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:154Public Records:95
Specimens with Sequences:108Public Species:17
Specimens with Barcodes:104Public BINs:32
Species:22         
Species With Barcodes:20         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Scaphopoda

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Conservation

Conservation Status

No scaphopods are currently listed or given special status.

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Relevance to Humans and Ecosystems

Benefits

Economic Importance for Humans: Positive

Shells of the genus Dentalium were culturally significant with Pacific Northwest natives (Amerinds)until the late 1800s. The shells were collected on strings and used as necklaces and money.

Positive Impacts: body parts are source of valuable material

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