In lieu of an abstract, here is a brief excerpt of the content:

The phylum Dicyemida, also referred to as Rhombozoa, consists entirely of parasites, even though no harm to their hosts has been quantified. Consequently, they also could be considered symbionts in the wide sense. Few people are aware of the group because its members are internal and restricted to cephalopods, at least for the known stages. They also are seldom recognized because only a few over 110 species have been described; however, to date over 120 different host species have been examined. They have been found at all depths in all the world’s oceans. In temperate waters, nearly 100% of benthic host cephalopods are infected. Certainly, many undescribed species exist. For example, 41 species, not all named, have been found in 19 of over 70 potential cephalopod hosts in Japanese waters (Furuya and Tsuneki 2003). In the Gulf of Mexico, only 7 described species have been reported. Biology of Dicyemida The unusual group consists of 0.1–9.0-mm long adults of species that infect the excretory organs, or “kidneys,” of cephalopods. They occur attached to the appendages within the renal coelom (sacs). In decapod cephalopods , they also occur attached to appendages in the renopancreatic and branchial heart coeloms (Hochberg 1982, 1983, Furuya et al. 2004). The dicyemid life cycle consists of both asexual and sexual stages, hence the name “Dicyema,” meaning two types of embryos. The asexual vermiform stage, “nematogen ,” is abundant in young hosts, and it produces vermiform larvae that undergo direct development forming additional nematogens. In contrast with asexual reproduction in trematodes that occurs in different parasitic larval stages, the reproduction in dicyemids occurs within the cytoplasm of the adult’s axial cell. Once the nematogens proliferate and occupy most of the kidney tissue, the vermiform larva produces a “rhombogen.” The rhombogen , the sexual vermiform stage, contains hermaphroditic “infusorigens.” The young infusorigen consists of an axial cell containing a few spermatids and other sperm-forming cells surrounded by a jacket containing an approximate number of oogonia and oocytes equal to the amoeboid spermatozoa. Ultimately the sperm fertilizes an ovum, producing a zygote that develops into an “infusoriform ” larva. The larva is quite conspicuous, consisting 523 25 Dicyemida of the Gulf of Mexico, with Comments on the Orthonectida Robin M. Overstreet and F. G. Hochberg  Dicyemida. After Short 1991. 524 ~ Dicyemida, with Comments on the Orthonectida and when two species co-occur, one has a conical shaped calotte and attaches to the convolutions, crypts, or folds and the other has a flat discoidal calotte, which attaches to the surface of the renal appendage (Furuya, Hochberg, and Tsuneki 2003). When cuttlefishes or bobtail squids (sepioids) and squids (teuthoids) have two co-occurring species, the second is usually in the renal pancreatic coelom. Some hosts, mostly oceanic species, also have chromidinid ciliates in renal and pancreatic sites, either occurring together or separately (Hochberg 1990, Furuya et al. 2004). These apostome ciliates superficially appear like dicyemids. Benthic cephalopods, primarily octopods and sepioids, include hosts most heavily infected with dicyemids and infected with the most species (Hochberg 1990, Furuya, Hochberg, and Tsuneki 2003). However , a few pelagic squids are infected, and part of their life history probably includes a period associated with the substratum (Furuya et al. 2004). In contrast, even octopuses that inhabit holes or gaps in rocks or corals are not infected (Furuya et al. 2004, Furuya 2006). Systematics The phylum Dicyemida contains the single class Dicyemida (Hochberg 1983, Short 1991) with two families Dicyemidae and Conocyemidae. Czaker (1994) proposed a third family, Kantharellidae, that has been neither recently accepted nor discussed. Until relatively recent (e.g., Kozloff 1969, Hochberg 1983, Brusca and Brusca 1990, Pawlowski et al. 1996), the superficially similar Dicyemida and Orthonectida had been placed in the phylum Mesozoa, a name referring to the linkage of members between protozoans and metazoans (Beneden 1876, Stunkard 1982). Classifiers differed on whether members , all parasitic, of Mesozoa or Dicyemida constituted highly evolved multicellular protists, with affinities to ciliates (Cavalier-Smith 1993), intermediates between protists and animals (Beneden 1876, Lapan and Morowitz 1974, Margulis and Schwartz 1988), or degenerate parasitic platyhelminths (Nouvel 1947, Stunkard 1954, 1982). None of these appears to be the case; however, a relationship with acoel turbellarians may exist. Data from 18S rDNA of three dicyemids and one orthonectid suggest that those two groups indicate a polyphyletic origin of the Mesozoa, and both phyla branch close to Myxozoa and Nematoda (Hanelt et al. 1996, Pawlowski et al. 1996) or to those and acoel turbellarians (Katayama et al...

Share