Sporogenesis and spore structure in Kudoa lunata (Myxosporea, Multivalvulida).

The multivalvulid myxosporean Kudoa lunata Lom, Dyková and Lhotáková, 1983 forms large polysporic trophozoites within the myocytes of scaldfish, Arnoglossus imperialis and A. laterna. The spores arise by the division of generative cells that produce a number of sporogonic cells necessary to complete a sporoblast. The development of some of the sporogonic cells can take place in isolation from other sporoblast cells, as shown by aberrant cases of polar capsule formation. Pansporoblast formation does not exist in Kudoa, at variance with large polysporic trophozoites of other myxosporean genera. The genus Kudoa also includes species with small trophozoites producing just one or two spores without pansporoblasts, as in Sphaerospora. Perhaps this type of sporogenesis was preserved in the species of Kudoa that have large trophozoites. Kudoa spores have a unique type of sporoplasm, comprising an outer cell enveloping an inner one; these differ in their contents of ribosomes and glycogen granules. This type of sporoplasm is reminiscent of the basic unit of all types of myxosporean development, i.e., the primary (vegetative) cell enclosing an inner (generative) cell. The canals for filament discharge extend through the apical spore projections and are of a length unmatched in other myxosporea.