A new class and order of myxozoans to accommodate parasites of bryozoans with ultrastructural observations on Tetracapsula bryosalmonae (PKX organism)

Tetracapsula bryosalmonae, formerly PKX organism, is a myxozoan parasite th at causes proliferative kidney disease in salmonid fish. Its primary hosts, in which it undergoes a sexual phase, are phylactolacmate bryozoans. It de velops in the bryozoan coelomic cavity as freely floating sacs which contai n two types of cells, stellate cells and sporoplasmogenic cells, which beco me organised as spores. Eight stellate cells differentiate as four capsulog enic cells and four valve cells which surround a single sporoplasmogenic ce ll. The sporoplasmogenic cell undergoes meiosis and cytoplasmic fission to produce two sporoplasms with haploid nuclei. Sporoplasms contain secondary cells. The unusual development supports previously obtained data from 18S r DNA sequences, indicating that species of Tetracapsula form a clade. It div erged early in the evolution of the Myxozoa, before the radiation that gave rise to the better known genera belonging to the two orders in the single class Myxosporea. The genus Tetracapsula as seen in bryozoans shares some o f the characters unique to the myxosporean phase and others typical of the actinosporean phase of genera belonging to the class Myxosporea. However, i t exhibits other features which are not found in either phase. A new class Malacosporea and order Malacovalvulida are proposed to accommodate the fami ly Saccosporidae and genus Tetracapsula. Special features of the new class are the sac-like proliferative body, valve cells not covering the exit poin t of the polar filament, lack of a stopper-like structure scaling the exit, maintenance of valve cell integrity even at spore maturity, absence of har dened spore walls and unique structure of sporoplasmosomes in the sporoplas ms.