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Rhombozoans: Rhombozoa




Rhombozoans (RAHM-boh-ZOH-uhns) are parasites (PAIR-uh-sites) that live in the kidneys of bottom-dwelling octopuses, cuttlefish, and sometimes squid. Parasites are animals or plants that live on or in another animal or plant, called a host, without helping it and usually harming it. The body of rhombozoans is made up of only eight to forty cells in a simple arrangement. They have the fewest cells of any animal.

Rhombozoans have two types of organization. One type is wormlike embryos and adults. This form consists of a central cell shaped like a cylinder and a layer of eight to thirty outer cells that have hairlike fibers. At the front of the animal, four to ten of the outer cells form a cap, the hairlike fibers of which are shorter and denser than on the outer cells toward the rear of the animal. The shape of the cap varies among species of rhombozoans. The second type of organization is an embryo that consists of thirty-seven or thirty-nine cells that are more specialized than the cells of wormlike rhombozoans. Inside these embryos are four large cells, each containing another cell that may give rise to the next generation. It is these specialized embryos that invade the host animal.


Rhombozoans live in the northern, eastern, and western parts of the Pacific Ocean; in the waters around New Zealand and Australia; in the Mediterranean Sea; in the northern, eastern, and western parts of the Atlantic Ocean; in the Gulf of Mexico; and in the Antarctic Ocean.


In the wormlike form, rhombozoans live only in the kidneys of octopuses, cuttlefish, and sometimes squid.


Rhombozoans absorb nutrients from the urine of their hosts.


In both the wormlike phase and as specialized embryos, rhombozoans swim by movement of the hairlike fibers on their bodies. Scientists do not know how the specialized embryos infect a new host and develop into wormlike rhombozoans. Wormlike embryos develop asexually from a cell of a parent and grow into adults. Asexual (ay-SEK-shuh-wuhl) reproduction is that which takes place without the uniting of egg and sperm for the transfer of DNA from two parents. Crowding of rhombozoans in a host's kidney can cause a shift from asexual to sexual reproduction. When crowding occurs, rhombozoans develop a sex organ that contains both eggs and sperm. The mature sperm unite with eggs, and the fertilized (FUR-teh-lyzed) eggs develop into the specialized form of embryos.


Rhombozoans have no known importance to people.


Rhombozoans are not threatened or endangered.


Physical characteristics: The body length of an adult Dicyemodeca deca (abbreviated as D. deca) does not exceed one–thirty-second of an inch (1 millimeter). There are twenty-three or twenty-four outer cells. The cap is disk shaped. The specialized embryos consist of thirty-five cells and are approximately 0.001 inch (33 micrometers) long.

Geographic range: D. deca lives in the northeastern part of the Pacific Ocean.

Habitat: D. deca lives in the kidneys of the giant Pacific octopus.

Diet: D. deca absorbs nutrients from the urine of its host.

Behavior and reproduction: The wormlike and specialized embryo forms of D. deca swim by movement of their hairlike fibers. Adults have two sex organs that contain about sixteen egg-related cells and fifteen sperm-related cells.

Dicyemodeca deca and people: D. deca has no known importance to people.

Conservation status: D. deca is not threatened or endangered. ∎



Barnes, R. S. K., Peter Calow, and Peter Olive. The Invertebrates: A Synthesis. 3rd ed. Malden, MA: Blackwell, 2001.

Brusca, Richard C., Gary J. Brusca, and Nancy Haver. Invertebrates. 2nd ed. Sunderland, MA: Sinauer, 2002.

Web sites:

Furuya, Hidetaka, and Kazuhiko Tsuneki. "Biology of Dicyemid Mesozoans." Zoological Science. (accessed on January 23, 2005).

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