Rhombozoa

(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

List of Species

No common name

Dicyema acuticephalum

family

Dicyemidae

taxonomy

Dicyema acuticephalum Nouvel, 1947, Japan.

other common names

None known.

physical characteristics

Dicyema acuticephalum is characterized by the presence of four propolar and metapolar cells. In this genus, cells of the propolar tier are opposite cells in the metapolar tier. Adults are up to 0.03 in (800 µm) long. Vermiform embryos are approximately 0.002 in (50 µm) long. There are 16–18 peripheral cells. The calotte is bell shaped. Infusoriform embryos consist of 37 cells and are approximately 0.001 in (30 µm) long.

distribution

Japan.

habitat

Renal sacs of Octopus vulgaris.

behavior

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

feeding ecology and diet

Absorption of nutrients from the urine of hosts.

reproductive biology

Adults have a single hermaphroditic gonad, the infusorigen. Approximately nine egg-line cells (oogonia primary oocytes) and 16 sperm-line cells (spermatogonia, primary spermatocytes, secondary spermatocytes, sperm) usually are found in an infusorigen.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Dicyemennea antarcticensis

family

Dicyemidae

taxonomy

Dicyemennea antarcticensis Short and Hochberg, 1970, Antarctic Ocean.

other common names

None known.

physical characteristics

Dicyemennea antarcticensis is characterized by the presence of four propolar and five metapolar cells. The body length of adults is up to 0.2 in (5,500 µm). The body length of vermiform embryos is approximately 0.01 in (300 µm). There are 34–36 peripheral cells. The calotte is triangular in the side view and pointed to rounded in the anterior view. Infusoriform embryos consist of 39 cells. The body length of infusoriform embryos is approximately 0.002 in (51 µm).

distribution

Antarctic Ocean.

habitat

Renal sacs of the Antarctic octopus (Pareledone turqueti).

behavior

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

feeding ecology and diet

Absorption of nutrients from the urine of hosts.

reproductive biology

Adults have a single hermaphroditic gonad, the infusorigen. Approximately 10 egg-line cells (oogonia primary oocytes) and four sperm-line cells (spermatogonia, primary spermatocytes,

secondary spermatocytes, sperm) usually are found in an infusorigen.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Dicyemodeca deca

family

Dicyemidae

taxonomy

Dicyemodeca deca McConnaughey, 1957, northeastern Pacific Ocean (Washington, United States).

other common names

None known.

physical characteristics

Dicyemodeca deca is characterized by the presence of four propolar and six metapolar cells. The body length of adults does not exceed 0.04 in (1,000 µm). The body length of vermiform embryos is approximately 0.002 in (40–60 µm). There are 23 or 24 peripheral cells. The calotte is disk shaped. Infusoriform embryos consist of 35 cells and are approximately 0.001 in (33 µm) long.

distribution

Northeastern Pacific Ocean.

habitat

Renal sacs of Octopus dofleini.

behavior

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

feeding ecology and diet

Absorption of nutrients from the urine of hosts.

reproductive biology

Adults have two infusorigens. Approximately 16 egg-line cells (oogonia primary oocytes) and 15 sperm-line cells (spermatogonia, primary spermatocytes, secondary spermatocytes, sperms) usually are found in an infusorigen.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Microcyema vespa

family

Conocyemidae

taxonomy

Microcyema vespa van Beneden, 1882, English Channel.

other common names

None known.

physical characteristics

Microcyema vespa is characterized by an irregular shape in adults; they lack external cilia and have a syncytiumal peripheral cell. The body length of adults is up to 0.03 in (800 µm). The body length of vermiform embryos is approximately 0.001 in (25 µm). There are 10 peripheral cells.

In vermiform embryos the calotte is a syncytium containing six nuclei. Infusoriform embryos consist of 39 cells. The body length of infusoriform embryos is approximately 0.001 in (26 µm).

distribution

English Channel, Mediterranean Sea.

habitat

Renal sacs of Sepia officinalis.

behavior

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

feeding ecology and diet

Absorption of nutrients from the urine of hosts.

reproductive biology

Adults have approximately 10 infusorigens. Approximately 9 egg-line cells (oogonia primary oocytes) and 8 sperm-line cells (spermatogonia, primary spermatocytes, secondary spermatocytes, sperms) usually are found in an infusorigen.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Hidetaka Furuya, PhD