APLACOPHORANS: Aplacophora

PHYSICAL CHARACTERISTICS

Aplacophorans (ah-plak-oh-FOR-ans) are mollusks and are related to clams, mussels, octopuses, and squids. Their worm-shaped bodies range from long and slender to almost ball-shaped and measure between 0.039 and 3.9 inches (1 to 100 millimeters) or more in length. A thick sheet of skin, or mantle, covers their bodies. The mantle makes up the shells, or valves, in most mollusks. However, aplacophorans are the only mollusks that do not have any valves. Instead, their mantles produce sharp, needlelike projections or small, scaly plates that are imbedded in the mantle over the back. These projections and scales make most aplacophorans look shiny. Like the valves of other mollusks, these projections and plates are hard and made up of a mineral called calcium carbonate. The surface of the mantles themselves are smooth, rough, bumpy, or spiny.

The head is poorly developed, and there are no eyes or tentacles. At the back of the mouth of most species is a rough structure called the radula (RAY-jeh-leh). The radula is covered with lots of rows of small teeth. The edges of the teeth are sawlike and made up of even smaller teeth called denticles (DEHN-te-kelz). The radula is used to scrape bits of food off rocks and other hard surfaces. In most mollusks, the radula is ribbonlike in shape but not in aplacophorans. Their radulas are part of their gut. There is a bundle of nerves, or ganglion, inside the head that is attached to a nerve chord that runs along their underside. The body cavity is small. The circulatory system is open, and the blood is not always contained inside blood vessels. There are no kidneylike organs, and there is either one or a pair of reproductive organs.

They do not have a well-developed muscular foot like other mollusks. They move slowly, either with the help of tiny bristles on their bodies called cilia (SIH-lee-uh) or on a track of mucus produced by a groove underneath their bodies. The mucus smoothes the way for the aplacophoran as it glides over the track. At the end of the body is a cavity that has the openings to the reproductive organs and the anus. The anus is the opening at the end of the digestive system where solid waste leaves the body. Breathing organs may or may not be present.

GEOGRAPHIC RANGE

Aplacophorans are found in all oceans.

HABITAT

They are found at depths ranging from 16 to 17,390 feet (5 to 5,300 meters). One group of aplocophorans lives on hydroids, corals, or on the ocean bottom. The other group burrows into the ocean bottom and lives in their tunnels upside down.

DIET

Some species prey on sea anemones, corals, hydroids, sea fans, and their relatives, as well as other organisms. Others are scavengers and swallow sand and mud that contain bits of food.

RADIAL VERSUS BILATERAL

Aplacophorans were first discovered by a Swedish naturalist in 1841. At first they were thought to be related sea cucumbers (Echinodermata). Not until 1875 were they properly recognized as mollusks. Echinoderms have radial symmetry. This means that their bodies are arranged around an imaginary line, or axis, through the center of their bodies; there are no distinct left or right sides. Mollusks, however, have bilateral symmetry. Their bodies always have a left side and a right side.

BEHAVIOR AND REPRODUCTION

Very little is known about the behavior of aplacophorans.

Aplocophorans are either male, female, or hermaphrodites. Hermaphrodites are individual animals that have both male and female reproductive organs at the same time. Depending on the species, the eggs are fertilized in the water or inside the female's body.

APLACOPHORANS AND PEOPLE

Aplacophorans are used by scientists to study the origins of all mollusks.

CONSERVATION STATUS

Aplacophorans are not considered threatened or endangered.

NO COMMON NAME (Spiomenia spiculata): SPECIES ACCOUNT

Physical characteristics: The curved body is widest at the middle and becomes slightly narrower toward the rear. They are covered with long spines. The longest spines are at the rear of the body. The radula has twenty-two to twenty-five teeth. Each tooth has twenty-two or twenty-three denticles.

Geographic range: They are found in the West European Basin of the Atlantic Ocean.

Habitat: They live on the sea bottom at depths of 6,560 to 13,120 feet (2,000 to 4,000 meters).

Diet: Spiomenia spiculata eat diatoms and possibly sponges.

Behavior and reproduction: Very little is known about their behavior or reproduction, other than that they are hermaphrodites.

Spiomenia spiculata and people: Spiomenia spiculata is of interest to scientists studying mollusks.

Conservation status: Spiomenia spiculata is not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.

Periodicals:

Arnofsky, Pamela. "Spiomenia spiculata, Gen. et sp. nov (Aplacophora: Neomeniomorpha) Collected from the Deep Water of the West European Basin." The Veliger 43, no. 2 (2000): 110-117.

Web sites:

"Class Aplacophora." http://animaldiversity.ummz.umich.edu/site/accounts/information/Aplacophora.html (accessed on March 30, 2005).

"Shelled Marine Mollusks of Temperate Australia. Interactive Information and Identification." http://www.danceweb.com.au/marine/data/majgrps.htm#top (accessed on March 30, 2005).

Welcome to the Aplacophora Homepage.http://www.whoi.edu/science/B/aplacophora/ (accessed on March 30, 2005).