Abstract
Two new taxa within the proposed genus Porannulus are described from the body wall of Antarctic sponges. These diatoms are small organisms with either rhombic to linear (P. contentus) or nearly circular (P. contentus var. circulus) valves. The diagnostic character of the new genus is a linear row of simple, unoccluded pores (not portulae) present along the valve margins. Such pores are typically absent from the valve face. Frustules are, for their size, heavily silicified and are initially formed as transapical extensions from a primary sternum. Systematically, the new genus belongs to the order Fragilariales based on the araphid nature of the valve, the presence of a sternum in early stages of silicification and the rare presence of pores on the valve face. One or possibly two parietal plastids are associated with the pore openings around the valve margins. Of 54 Porifera (20 species) collected in Maxwell Bay, Antarctica, 52 samples contained specimens from the new genus. Porannulus dominated the diatom flora from the two sponges Axociella flabellata and Sphaerotylus antarcticus. The absence of areolae on the valve face and copulae of Porannulus may be a special adaptation to living in the mesohyl of sponges. Additional records were also observed from Synascidae gut samples.
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