Overview

Brief Summary

Dicyemida, or Rhombozoa, 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.

Classification is controversial (Aruga et al. 2007). Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa; however, molecular phylogenies indicate that dicyemids may be more closely related to the roundworms (Pawlowski et al. 1996). The phylum is not divided in classes, but contains three families, Conocyemidae, Dicyemidae, and Kantharellidae (ITIS 2010). Molecular evidence suggests that this phylum are derived from the Lophotrochozoa (Kobayashi et al. 2009, Suzuki et al. 2010).

Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.28 in), and they can be easily viewed through a light microscope (Barnes 1982). They 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 twenty to thirty ciliated cells. The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host's renal appendages (Barnes 1982).

  • Aruga J, Odaka YS, Kamiya A, Furuya H (2007). "Dicyema Pax6 and Zic: tool-kit genes in a highly simplified bilaterian". BMC Evol. Biol. 7: 201. doi:10.1186/1471-2148-7-201. PMC 2222250. PMID 17961212.
  • Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 248–249. ISBN 0-03-056747-5.
  • Integrated Taxonomic Information System (ITIS). "Kantharellidae". Retrieved 5 April 2010.
  • Kobayashi M, Furuya H, Wada H (2009) Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury. Evol Dev 11(5):582-589
  • Pawlowski J, Montoya-Burgos JI, Fahrni JF, Wüest J, Zaninetti L. (1996) "Origin of the Mesozoa inferred from 18S rRNA gene sequences". Mol. Biol. Evol. 13 (8): 1128–32. PMID 8865666.
  • Suzuki TG, Ogino K, Tsuneki K, Furuya H (2010) Phylogenetic analysis of dicyemid mesozoans (phylum Dicyemida) from innexin amino acid sequences: dicyemids are not related to Platyhelminthes. J Parasitol 96(3):614-625
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Life History and Behavior

Life Cycle

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 within the axial cell. These mature through direct development to form more nematogens (Barnes 1982). 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 gonads developed within the axial cell. These gonads, more correctly termed infusorigens, self-fertilise to 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 (Barnes 1982).

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.

  • Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 248–249. ISBN 0-03-056747-5.
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