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)
Biogeographic Regions:
arctic ocean 
; indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea.
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)
These animals are found in the following types of habitat:
saltwater or marine .
Aquatic Biomes:
benthic .
Systematic and Taxonomic History
Once classified with holothurian echinoderms, aplacophorans were recognized as mollusks in 1875. Although chitons and aplacophorans have been put into separate groups, they may form a monophyletic clade, referred to as Aculifera (or Amphineura).
Aplacophorans are separated into two sub-classes: 1) Chaetodermomorpha (=Caudofoveata), which contains about one third of the known aplacophoan species and 2) Neomeniomorpha (=Solenogastres). Both these sub-classes are sometimes considered as classes.
Chaetoderms lack pedal grooves, have ctenidia paired in a small posterior mantle cavity, have a cuticular shield around the mouth and the stomach and digestive glands are separate. Neomenioids can move with a narrow foot in a ventral groove, ctenidia are lacking, they have no oral shield and the stomach cavity and digestive glands are combined. (Brusca and Brusca, 2003; Ponder et al., 2000; Schander, 2000; Scheltema, 2001)
- Chaetodermomorpha = Caudofoveata
- Neomeniomorpha = Soleogastres
- vermiform body
- reduced foot
- reduced posterior mantle cavity
- gonads empty to pericardial cavity with U-shaped gametoducts which exit to mantle cavity
- no nephridia
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)
Some key physical features:
ectothermic ; heterothermic ; bilateral symmetry .
Development
Development involves metamorphosis from a trocophore larva. (Ponder et al., 2000)
Special features of growth:
metamorphosis .
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)
Key reproductive features:
gonochoric/gonochoristic/dioecious (sexes separate); simultaneous hermaphrodite; sexual ; fertilization (external , internal ); ovoviviparous ; oviparous .
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)
Key behaviors:
motile .
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)
Primary Diet:
carnivore ; omnivore ; detritivore .
Ecosystem Roles
Neomenioids live with and feed on cnidarians but the effects on the ecosystem are unknown.
Contributors
Renee Sherman Mulcrone (author), Animal Diversity Web Staff.
