(Rhombozoans)

Phylum: Rhomobozoa

Number of families: 3

Thumbnail description
Characteristic parasites of the kidney of benthic cephalopod mollusks; the body consists of only 8–40 cells, which are fewer in number than in any other metazoans and are organized very simply

Evolution and systematics

The phylum Rhombozoa includes three families, eight genera, and approximately 150 species. The three families are Conocyemidae, Dicyemidae, and Kantharellidae.

Édouard van Beneden proposed the name Mesozoa for these organisms as an intermediate between Protozoa and Metazoa in body organization. Later zoologists considered rhombozoans degenerated from metazoans such as trematodes because of adaptation for the parasitic lifestyle. Results of phylogenetic studies with nucleotide sequences of 18S recombinant DNA and Hox gene have suggested rhombozoans are spiralians (protostomes) such as turbellarians, nemerteans, annelids, and mollusks. Many scientists now place this group within the phylum Dicyemida.

Physical characteristics

Rhombozoans have two phases of body organization—the vermiform stages of vermiform embryo and adult (nematogen, rhombogen) and the infusoriform embryo. The body of vermiform rhombozoans consists of a central cylindrical cell called the axial cell and a layer of 8–30 ciliated external cells called peripheral cells. The number of peripheral cells is fixed and species specific. At the anterior region, 4–10 peripheral cells form the calotte, the cilia of which are shorter and denser than in more posterior peripheral cells. The calotte comprises two tiers of cells—propolar cells and metapolar cells. Calotte shape varies with species.

Infusoriform embryos consist of 37 or 39 cells, which are more differentiated than those of vermiform organisms. Inside the embryo are four large cells called urn cells, each containing a germinal cell that may give rise to the next generation. At the anterior region of the embryo is a pair of unique cells called apical cells, each containing a refringent body composed of magnesium inositol hexaphosphate. At the posterior region, external cells are ciliated.

Distribution

Temperate and subtropical continental waters. Okhotsk Sea; Sea of Japan; northern, eastern, and western Pacific Ocean; New Zealand; Australia. Mediterranean Sea; northern, eastern, western Atlantic Ocean; Gulf of Mexico; Antarctic Ocean.

Habitat

Vermiform stages of rhombozoans are restricted to the renal sac of cephalopods. In decapods, rhombozoans are also found in the renopancreatic coelom. Some rhombozoans are found in the pericardium of decapods. On the surface of the renal appendage, vermiform rhombozoans insert their heads into renal tubules and folds.

Behavior

Vermiforms and infusoriform embryos swim with their cilia. There appears to be positive thigmotaxis to renal appendages in the vermiform stages.

Feeding ecology and diet

The surface of the rhombozoan body possesses numerous cilia and has a folded structure believed to contribute to absorption of nutrients from the urine of hosts.

Reproductive biology

The vermiform embryo develops asexually from an agamete (axoblast) and grows into an adult. A high population density in the host renal sac can cause a shift from an asexual mode to a sexual mode of reproduction. The functionally hermaphroditic gonad, the infusorigen, forms at high population density. Mature spermatozoa without tails fertilize the primary oocytes. A fertilized egg develops into an infusoriform embryo. It remains to be understood how infusoriform larvae infect the new host and develop into vermiforms.

Conservation status

No species is listed by the IUCN.

Significance to humans

None known.

Species accounts

Resources

Hochberg, F. G. "Diseases Caused by Protistans and Mesozoans." In Diseases of Marine Animals. Vol. III, edited by Otto Kinne. Hamburg: Biologische Anstalt Helgoland, 1990.

Furuya, Hidetaka. "Fourteen New Species of Dicyemid Mesozoans from Six Japanese Cephalopods, with Comments on Host Specificity." Species Diversity 4 (1999): 257–319.

McConnaughey, Bayard H. "Two New Mesozoa from the Pacific Northwest." Journal of Parasitology 43 (1957): 358–61.

Nouvel, Henri. "Les Dicyémides. Systématique, générations, vermiformes, infusorigène et sexualité." Archives de Biologie, Paris 58 (1947): 59–220.

Short, Robert B., and F. G. Hochberg. "A New Species of Dicyemennea (Mesozoa: Dicyemidae) from Near the Antarctic Peninsula." Journal of Parasitology 56 (1970): 517–22. van Beneden, Édouard. "Contribution à l'histoire des Dicyémides." Archives de Biologie, Paris 31 (1882): 195–228.

[Article by: Hidetaka Furuya, PhD]

Rhombozoa/Dicyemida
Scientific classification
Kingdom: Animalia Phylum: Rhombozoa A. Krohn, 1839

Rhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name Dicyemida precedes Rhombozoa in usage, and is preferred by most contemporary authors, Rhombozoa still enjoys much popular support.

Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa; however, molecular phylogenies indicate that dicyemids may be more closely related to the Platyhelminthes (flatworms).

Adult dicyemids range in length from 0.1-9.0 mm, and they can be easily viewed through a light microscope. Dicyemids display eutely, a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character. The organism's structure is simple: a single axial cell is surrounded by a jacket of ciliated cells. The anterior region of the organism is termed a "calotte" and functions to attach the dicyemid to folds on the surface of its host's renal appendages.

Dicyemids exist in both asexual and sexual forms. The former predominate in juvenile and immature hosts, and the latter in mature hosts. The asexual stage is termed a nematogen; it produces vermiform larvae which mature through direct development to form more nematogens. Nematogens proliferate in young cephalopods, filling the kidneys.

As the infection ages, perhaps as the nematogens reach a certain density, vermiform larvae mature to form rhombogens, the sexual life stage, rather than more nematogens. This sort of density-responsive reproductive cycle is reminiscent of the asexual reproduction of sporocysts or rediae in larval trematode infections of snails. As with the trematode asexual stages, a few nematogens can usually be found in older hosts. Their function may be to increase the population of the parasite to keep up with the growth of the host.

Rhombogens contain hermaphroditic infusorigens which produce infusoriform larvae. These larvae possess a very distinctive morphology, swimming about with ciliated rings that resemble headlights. It has long been assumed that this sexually produced infusoriform, which is released when the host eliminates urine from the kidneys, is both the dispersal and the infectious stage. The mechanism of infection, however, remains unknown, as are the effects, if any, of dicyemids on their hosts.

Some part of the dicyemid life cycle may be tied to temperate benthic environments, where they occur in greatest abundance. While dicyemids have occasionally been found in the tropics, the infection rates are typically quite low, and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate fauna. Most dicyemid species are recovered from only one or two host species. While not strictly host specific, most dicyemids are only found in a few closely related hosts.

Dicyemids are divided into two families, Conocyemidae and Dicyemidae.

References

Wikispecies has information related to:

Rhombozoa

• Furuya, H. & K. Tsuneki. 2003. Biology of Dicyemid Mesozoans. Zoological Science 20: 519-532.
• Furuya, H., F. G. Hochberg, & K. Tsuneki. 2003. Reproductive traits in dicyemids. Marine Biology 143: 693-706.
• Hochberg, F.G. 1982. The "kidneys" of cephalopods: a unique habitat for parasites. Malacologia 23: 121-134.
• McConnaughey, B.H. 1951. The life cycle of the dicyemid Mesozoa. University of California Publications in Zoology 55: 295-336.
• Pawlowski, J.I. et al. 1996. Origin of the Mesozoa inferred from 18S rRNA gene sequences. Molecular Biology and Evolution 13: 1128-1132.

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