Sigmomyxa, Karlsbakk & Køie, 2012

Karlsbakk, Egil & Køie, Marianne, 2012, The marine myxosporean Sigmomyxa sphaerica (Thélohan, 1895) gen. n., comb. n. (syn. Myxidium sphaericum) from garfish (Belone belone (L. )) uses the polychaete Nereis pelagica L. as invertebrate host, Parasitology Research (211) 110 (1), pp. 211-218 : 216-217

Sigmomyxa n. gen.

Coelozoic in gallbladder, sporogony disporic, plasmodia are di- to polysporic, spores smooth, spindle shaped in valvular view and sigmoid in sutural view. Valves are ellipsoid in outline, with thin walled protrusions associated with the PC tips. Polar capsules elongate pyriform, with>7 windings. Intercapsular distance is short.

Type species is S. sphaerica ( Thélohan, 1895)

Comments

Myxidium elmatboulii Ali et al., 2006 and Myxidium maamouni Abdel-Baki, 2009 are similar to S. sphaerica and likely congeners, but molecular data is lacking (cf. Ali et al. 2006; Abdel-Baki 2009). M. elmatboulii is transferred to Sigmomyxa as Sigmomyxa elmatboulii ( Ali et al., 2006) comb. n. on the basis of its morphology, the species is so similar to S. sphaerica that conspecificity is possible. The host, Tylosurus choram (Rüppell, 1837) is also related to B. belone (both belonids). The spores of M. queenslandicus appear morphologically similar to those of S. sphaerica , but these species show only 89% identity in the partial SSU rDNA sequences available. However, expansion segments in the V 7 region of M. queenslandicus are responsible for the low identity; exclusion of these gives 94% identity with S. sphaerica . M. queenslandicus is phylogenetically closest related to S. sphaerica and Ellipsomyxa spp. , but inclusion in Sigmomyxa appears to render the genus polyphyletic. However plasmodia and sporogony of M. queenslandicus are unknown and the species is therefore considered an incertae sedis. M. laticurvum Kabata 1962 (syn. Myxidium trachinorum Canning et al. 1999 , see Karlsbakk 2001) show a protruding polar capsules similar to S. sphaerica but differ in containing very prominent capsulogenic cells in mature spores, a convex spore structure and a different organisation of the polar filaments ( Kabata 1962; Canning et al. 1999). Our SSU rDNA sequences of M. laticurvum confirm that this species is not closely related to Sigmomyxa n. gen.

The erection of Sigmomyxa n. gen. removes two species from the polyphyletic genus Myxidium Bütschli, 1882 . Several species in the marine group of Myxosporea and currently assigned to Myxidium are not closely related to Myxidium sensu stricto or Sigmomyxa n. gen. on the basis of their SSU rDNA sequences, but show a related morphology and development. Redescriptions and revisions of these taxa are needed.

Life cycle

The actinosporean infection in N. pelagica and the myxosporean S. sphaerica in B. belone is considered different life cycle stages due to the high SSU sequence similarity. Sequence identity has aided the disclosure of all the marine myxosporean life cycles known so far. We observed that five specimens of N. pelagica survived and only the infected specimen died due to stress (high temperature and lack of oxygen). This indicates that an infection with actinosporean stages may affect the survival of the polychaete host. Other observations on the effects of actinosporeans on the annelid hosts are scarce. Shirakashi and El-Matbouli (2009) found feeding and fecundity of actinosporean-infected Tubifex tubifex to be reduced, but did not observe reduced survival.

Apparently only fully developed actinospores of S. sphaerica were found in the examined N. pelagica . However, the wall of the pansporocysts and younger developmental stages may have disintegrated in the decaying polychaete host.

The present actinospores differ from those of E. gobii and E. mugilis (as Zschokkella mugilis ), which also use Nereis spp. as polychaete hosts, by being nearly spherical contrary to the elongated actinospores of Ellipsomyxa spp. having nearly twice the length ( Køie et al. 2004; Rangel et al. 2009).

Nereis diversicolor and Nereis succinea from less than 1 metre depth may be infected with actinosporean stages ( Køie et al. 2004; Rangel et al. 2009). The N. pelagica specimens examined were dredged in among other a shallow sandy bay harbouring N. diversicolor and N. succinea . These two species were only infected with E. gobii , even though specimens of B. belone must have spent some time in this bay. Hence it is possible that these myxosporeans display some degree of host specificity to the invertebrate host; E. gobii uses two species of Nereis as invertebrate hosts ( Køie et al. 2004), whereas S. sphaerica apparently use one species, N. pelagica .

Actinospores of the tetractinomyxon type are the actinosporean stages of myxozoans belonging to at least three clades: the Ceratomyxa clade ( Køie et al. 2008), the Parvicapsulidae ( Bartholomew et al. 2006; Køie et al. 2007) and the Sigmomyxa / Ellipsomyxa clade. A fourth clade is represented by Ceratomyxa shasta , which also show tetractinomyxon actinospores ( Bartholomew et al. 1997), but the phylogenetic affinities of C. shasta is unclear (see Fiala and Bartosova 2010).