Fine structure of swimming, encysting, and germinating spores of Haliphthoros milfordensis

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Abstract
Electron microscope observations were made of swimming, encysting, and germinating zoospores of two strains of Haliphthorus milfordensis Vishniac isolated from gills of the white shrimp, Penaeus setiferus, and from the American lobster, Homarus americanus. Although structurally similar to zoospores of other Oomycetes, those of H. milfordensis have certain distinguishing features including: mitochondria grouped anteriorly around the nucleus, the presence of electron-dense vesicles which become transformed into unique "scalloped-fringed" vesicles, and an anterior arrangement of most cell organelles with the posterior of the zoospore being filled with numerous fibrous vesicles. During spore encystment, flagellar retraction and rounding up of the spore is accompanied by a central positioning of most cell organelles (central nucleus surrounded by concentric bands of rough endoplasmic reticulum), and ends with formation of a unique, spined, trilaminate wall. Cyst germination occurs via fracturing of the spined wall, with repeated emergence of zoospores from the spore cysts noted in addition to the more normal germ tube formation.