Nereis pelagica, (L.), Linnaeus, 1758

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 : 213-214

Description of actinospores from N. pelagica

All actinospores were of the tetractinomyxon type ( Fig. 3 ). They all occurred free in the decaying polychaete body. No pansporocysts were found. The thick-walled actinospores were spherical to slightly ellipsoidal, length 7.0–8.0 (mean 7.6; n =10) and diameter 6.0–7.5 (6.7). The actinospores were composed of eight cells most easily identified by the presence of their nuclei; the three nuclei of the shell valve cells appeared as small thickenings internally on the spore wall, the three nuclei of the polar capsules and the two nuclei of the sporoplasm. The diameter of the three identical spherical polar capsules was 2.0–2.3 (2.1). It was not possible to provoke extrusion of the polar filaments.

SSU rDNA sequences

Partial SSU rDNA sequences were obtained from M. sphaericum -infected gall bladders of two B. belone from Denmark (sequence isolates SigBel-1 and 2, GenBank accession nos. JN033225, JN033226) and the studied infection from Norway (sequence isolate Msph, JN033227). These were identical (1696 nt compared).

Two different parts of a N. pelagica infected with tetractinomyxon actinospores produced two identical partial SSU sequences (sequence isolates Npel 1 and 2, JN033228) (1696 nt compared). These sequences were identical with the M. sphaericum sequences from B. belone , apart for four substitutions (99.8 % identity, 1696 nt compared). On this basis the actinosporean infection in N. pelagica is identified with the myxosporean M. sphaericum infecting B. belone .

Blast searches returned Ellipsomyxa spp. as most similar to the partial M. sphaericum SSU rDNA sequences. Phylogenetic analyses on the basis of the SSU rDNA sequences supported a close relationship between genus Ellipsomyxa and M. sphaericum ( Fig. 4 ). Myxidium queenslandicus Gunter and Adlard, 2008 represent a sister group to Ellipsomyxa spp. / M. sphaericum in these analyses ( Fig. 4 ). The congeneric marine clade members M. laticurvum , Myxidium incurvatum Thélohan, 1892 Myxidium gadi Georgévitch, 1916 and M. bergense are not closely related to M. sphaericum ( Fig. 4 ). A schematic illustration of the life cycle of Sigmomyxa sphaerica (syn. M. sphaericum , see below) is shown in Fig. 5 .

Figures

Fig. 3 Actinospores of S. sphaerica in naturally infected N. pelagica from northern Øresund, Denmark. Interference contrast, to same scale. a Apical and lateral views of free actino- spores. b Lateral views showing the three nuclei of the shell valve cells (arrows) and the two nuclei of the sporoplasm cells (arrowheads). Scale bar 5 μm

Fig. 4 Phylogenetic affinities of S. sphaerica among related members of the marine clade of Myxosporea. S. sphaerica is closest related to Ellipsomyxa spp., and these two genera represent a sister group to M. queenslandicus incertae sedis in a well-supported clade. Other Myxidium spp. in the marine clade are not closely related to S. sphaerica, including M. laticurvum (JN033229, new sequence) and M. bergense from the type host P. virens in Norway (JN033231, new sequence). All new sequences in bold. Clade support values: upper, MrBayes posterior probabilities (in percent); middle, maximum likelihood bootstrap (N=100) support values (Paup); lower, maximum parsimony (Mega)

Fig. 5 Schematic illustration of the life cycle of S. sphaerica. The polychaete N. pelagica acts as the invertebrate hosts and the garfish B. belone acts as the fish hosts. a Actinospore, b myxospore. Not to scale