Morphology and ultrastructure of teratological forms of the diatoms Stephanodiscus niagarae and S. parvus (Bacillariophyceae) from Hamilton Harbour (Lake Ontario, Canada)

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Teratological forms of Stephanodiscus niagarae Ehrenb. and S. parvus Stoermer & Hakansson were observed during a study of diatoms preserved in a radiometrically dated core from Hamilton Harbour (Lake Ontario, Canada). Morphological features and ultrastructures of both species were studied under the light and scanning electron microscope. The valve structure of abnormal forms of S. niagarae appears to be weakly silicified, especially in the central area. The shape of satellite pores are very irregular in comparison with the round shape of the normal specimen. The central fultoportulae are characterized as small tubes extending out of the external valve. Two types of abnormal frustules are present in the population of S. parvus studied. In one type valves are “underdeveloped” and only the siliceous layer and ribs are present, and in the other type the valves are strongly silicified and the areolae are almost completely occluded.