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Scaphopoda

By Renee Sherman Mulcrone

Diversity

There are approximately 900 species of Scaphopoda, commonly called tusk shells. (Brusca and Brusca, 2003)

Geographic Range

Scaphopods are found in marine regions around the world. (Brusca and Brusca, 2003; Jones and Baxter, 1987)

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. (Barnes, 1987; Jones and Baxter, 1987; Lamprell and Healy, 2001; Spear, 1994)

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. (Barnes, 1987; Reynolds, 1996; Shimek, 2005)

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. (Barnes, 1987; Jones and Baxter, 1987)

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. (Barnes, 1987)

There is no parental investment after release of gametes. (Brusca and Brusca, 2003)

  • Parental Investment
  • pre-fertilization
    • provisioning

Behavior

Scaphopods burrow by projecting their foot into the substrate and contracting pedal retractor muscles to pull the animal downward. Extension of the foot may help with water intake, and the scaphopods probably use foot movements to expel wastes from the posterior opening.

The two orders of this group may have slightly differing burrowing behaviors. Individuals in the Gadilida burrow up to 30 cm in captivity and burrow this deep in the ocean. Many species in the Dentaliida burrow with the concave side just below the substrate. (Brusca and Brusca, 2003; Jones and Baxter, 1987; Lamprell and Healy, 2001; Shimek, 2005)

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. (Brusca and Brusca, 2003; Jones and Baxter, 1987)

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. (Barnes, 1987)

Predation

Scaphopods are fed on by fish and crabs. Their burrowing behavior is thought to keep them from predators. (Shimek, 2005)

  • Known Predators
    • fish
    • crabs

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. (Shimek, 2005)

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. (Reynolds, 1996; Spear, 1994)

  • Positive Impacts
  • body parts are source of valuable material

Conservation Status

No scaphopods are currently listed or given special status.

  • IUCN Red List [Link]
    Not Evaluated

Other Comments

Scaphopods do not have a heart, gills or vessels for circulation. Instead, circulation is through simple hemolymph sinuses and gas exchange takes place across the mantle and body surface. (Brusca and Brusca, 2003)

Contributors

Renee Sherman Mulcrone (author).

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

References

Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

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

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

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.

Reynolds, P. 1996. "The Scaphopod Page, Class Scaphopoda, Phylum Mollusca" (On-line). Accessed February 04, 2005 at http://academics.hamilton.edu/biology/preynold/Scaphopoda/default.html.

Shimek, R. 1990. Diet and habitat utilization in a Northeastern Pacific Ocean scaphopod assemblage. American Malacological Bulletin, 7: 147-169.

Shimek, R. 2005. "Scaphopods, Some Natural History" (On-line). Accessed February 04, 2005 at http://rshimek.com/Scaph1.htm.

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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