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Aplacophora

By Renee Sherman Mulcrone

Diversity

There are approximately 320 described species in Aplacophora. However, it is likely there are many other species that have not been described. (Brusca and Brusca, 2003; Scheltema, 2001)

Geographic Range

Aplacophorans are found throughout the oceans over the world. (Barnes, 1987)

Habitat

Exclusively marine, aplacophorans mainly burrow into the substrate in water more than 20 m deep, and may reach densities up to 4-5 per square meter. Aplacophorans in the sub-class Chaetodermomorpha are limited by a minimum salinity of 28-30%. (Jones and Baxter, 1987)

Physical Description

Aplacophorans are small, cylindrical, worm-like, and usually less than 5 cm long, but can range from 1 mm to 30 cm. Like other mollusks, it has no outer shell, but the epidermis secretes calcareous spicules or scales which are embedded in dorsal mantle. These spicules give the aplacophorans a sheen. Chaetoderms have a scaly appearance. All aplacophorans have a simple mantle cavity.

The radula is not ribbon-like as in other mollusks, but is an expansion of the foregut epithelium. The teeth of the radula may be in simple plates in transverse rows, up to 50 rows with 24 teeth per row. (Barnes, 1987; Brusca and Brusca, 2003; Ponder, et al., 2000)

Development

Development involves metamorphosis from a trocophore larva. (Ponder, et al., 2000)

Reproduction

Aplacophorans may be monoecious or dioecious with single or paired gonads. All discharge gametes through gonopericardial ducts into the pericardial (heart) chamber. Gametes then pass through gametoducts to the mantle cavity where they are then released outside the body. Animals in the Chaetodermomorpha have external fertilization while those in Neomeniomorpha are internally fertilized, and sometimes even brooded. (Brusca and Brusca, 2003; Ponder, et al., 2000)

Members of the Neomeniomorpha sometimes brood the eggs. However, most species release the gametes without further parental care. (Ponder, et al., 2000)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • provisioning
    • protecting

Lifespan/Longevity

Little is known about aplacophoran lifespans.

Behavior

Aplacophorans move via cilia through or on substrate. (Brusca and Brusca, 2003)

Communication and Perception

Little is known about the perception of the Aplacophora. The animals have a simple nerve ring around the esophogus and poorly developed ganglia. (Brusca and Brusca, 2003)

Food Habits

Chaetoderms burrow and feed on microorganisms and detritus while neomenioids live with and feed on cnidarians. (Ponder, et al., 2000)

Ecosystem Roles

Neomenioids live with and feed on cnidarians but the effects on the ecosystem are unknown.

Other Comments

Because of their deep water benthic existence, little is known about this group of animals.

Contributors

Renee Sherman Mulcrone (author).

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

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.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

detritivore

an animal that mainly eats decomposed plants and/or animals

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.

internal fertilization

fertilization takes place within the female's body

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.

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

saltwater or marine

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

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 and Dr. W. Backhuys.

Ponder, W., M. Shea, D. Beechey, R. McBain. 2000. "Aplacophora (the spicule worms)" (On-line). Shelled Marine Mollusks of Temperate Australia. Accessed February 07, 2005 at http://www.danceweb.com.au/marine/data/majgrps.htm#apla.

Schander, C. 2000. "The Taxonomy of the Aplacophora (Chaetodermomorpha or Caudofoveata & Neomeniomorpha or Solenogastres), Sclerite-Bearing Deep-Sea Mollusks" (On-line). Accessed February 07, 2005 at http://www.whoi.edu/science/B/aplacophora/.

Scheltema, A. 2001. Aplacophora. Pp. 1-18 in A Wells, W Houston, eds. Zoological Catalogue of Australia, Vol. 17.2. Melbourne, Australia: CSIRO Publishing.

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