Life cycle studies of Myxobolus parviformis sp. n. (Myxozoa: Myxobolidae) from bream.

We experimentally followed the life cycle of Myxobolus parviformis sp. n., a myxozoan parasite from the gills of common bream Abramis brama L. Establishing the development of both stages (myxospore and actinospore) in 2 consecutive, full transmission trials, we were able to separate plasmodia of a common genotype from sympatric Myxobolus spp. occurring in naturally infected gill lamellae. Therefore, isolated gill plasmodia representing individual myxosporean 'clones' were used for subsequent infection of oligochaetes after molecular and morphological identification. The plasmodia of this species are located in median to distal regions of the primary gill filaments, forming small spherical pseudocysts. The comparatively small myxospores share some uniform characteristics: they taper posteriorly, have 2 inward inclining polar capsules that occupy half of the spores' interior space, and usually show 4 posterior sutural edge markings. The corresponding actinosporean has already been described morphologically and molecularly. The 18S rDNA sequence of the actinosporean stage was identical in all our samples, including myxospores. The triactinomyxon had a stout style, 32 sporozoites and short tapering caudal processes, and was shed by the tubificid oligochaete Limnodrilus hoffmeisteri. The ellipsoid sporoplasm was covered by a soft sheath which was emitted after valve shell opening upon stimulation by agitation or fish mucus. The molecular data (unique restriction fragment length polymorphism pattern and a 1586 bp 18S rDNA sequence) clearly differ from those for similar species and, together with the morphological data, justify the description of this parasite as a new species.