Dicyemida, Van Beneden, 1876

Catalano, Sarah R., 2012, A review of the families, genera and species of Dicyemida Van Beneden, 1876, Zootaxa 3479, pp. 1-32 : 5

Phylum Dicyemida

The dicyemids, along with the orthonectids, which parasitise a number of marine invertebrate phyla including Platyhelminthes (turbellarians), nemerteans, annelids (polychaetes), molluscs (gastropods, bivalves, but not cephalopods), echinoderms (ophiuroids), and chordates (ascidians), have long been considered a class within the Phylum Mesozoa (see Stunkard 1972; Hochberg 1983; McConnaughey 1983a, b). However in light of the dissimilarities between dicyemids and orthonectids, including differences in internal features and stages of their respective lifecycles, it is now accepted to treat these two groups as separate phyla, Dicyemida and Orthonectida Giard (see Hochberg 1983). The Dicyemida contains three families: Dicyemidae Van Beneden, 1882 , Conocyemidae Stunkard, 1937 and Kantharellidae Czaker, 1994 (see Fig. 1), although the validity of the Kantharellidae is questioned and uncertain because its single species is inadequately described ( Furuya et al. 2007). Nine genera are recognised within the three families: Dicyemidae — Dicyema von Kölliker, 1849 , Dicyemennea Whitman, 1883 , Dicyemodeca ( Wheeler, 1897) Bogolopova, 1957 , Pseudicyema Nouvel, 1933 , Pleodicyema Nouvel, 1961 and Dodecadicyema Kalavati & Narasimhamurti, 1980 ; Conocyemidae — Conocyema Van Beneden, 1882 and Microcyema Van Beneden, 1882 ; and Kantharellidae — Kantharella Czaker, 1994 .

Of the 112 currently described species, the largest number are in Dicyema (61), followed by Dicyemennea (40). Other genera are monotypic or contain a small number of species: Pseudicyema (3), Dicyemodeca (3), Dodecadicyema (1), Pleodicyema (1), Conocyema (1), Microcyema (1) and Kantharella (1) (see Table 1). The number and orientation of cells in each tier of the calotte, the presence or absence of abortive axial cells and the presence or absence of syncytial stages determines the genus ( Hochberg 1982, 1983). Calotte morphology is discussed in detail by Furuya et al. (2007). Briefly, the Dicyemidae have four propolar cells, but different numbers of metapolar cells: Dicyema (four metapolar cells, propolar cells opposite to metapolar cells), Pseudicyema (four metapolar cells, propolar cells alternate with metapolar cells), Dicyemennea (five metapolar cells), Dicyemodeca and Pleodicyema (six metapolar cells), Dodecadicyema (six metapolar cells plus three micropolar cells: small cells which form the anterior tip of the calotte, only found in Dodecadicyema species ) (see Fig. 1). The Conocyemidae are characterised by having no metapolar cells, but either parapolar cells or a syncytial cell: Conocyema (four parapolar cells) and Microcyema (syncytial cell: a single cell with six nuclei which is only found in Microcyema species ) (see Fig. 1). The Kantharellidae is different from the other families because there is no cell constancy, with three to nine propolar cells, two to seven metapolar cells and two to four parapolar cells ( Czaker 1994) (see Fig. 1).

For species classification, the size of the adult stages, the number of cells comprising the body, the shape of the calotte, the anterior extension of the axial cell, the presence or absence of verruciform cells and the structure of the infusoriform larvae are distinguishing characters ( Hochberg 1982, 1983). New species descriptions should include measurements from all lifecycle stages (nematogen, rhombogen, vermiform embryo and infusoriform embryo), along with line drawing and light micrograph images of distinguishing characters for comparison with previously described species (see Furuya 2009 as a guide for when undertaking new species descriptions).

Table 1 lists the described and valid dicyemid species separated into family and genera, and includes characteristic information used to classify genera and species (such as cells in each tier of the calotte, size of the adult stages, number of cells comprising the body and host species infected). The authority is given following each parasite’s scientific name. Host species lists the hosts the dicyemid parasite was recorded from as in the original publication along with the authority, but if cephalopod name changes have since occurred, the current accepted name is presented in parenthesis (as accepted by Gofas 2011). Type collection locality, other localities and microhabitat are stated as in the original description and refer to the type and other localities the host cephalopod was collected from and the site in the host, respectively. Rows are left blank if no other localities were recorded in the original record apart from the type locality. Calotte and peripheral cell numbers as well as largest nematogen and rhombogen lengths are presented as in the original description. The most common peripheral cell number observed is underlined (unless it was not acknowledged in the original record) and length measurements are presented in micrometers (µm). Reference highlights the published original description along with records of any subsequent redescriptions, unless indicated otherwise. The total number of described species is currently recognised as 112, although the validity of some species is uncertain (annotated in Table 1; see section below).