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

Teratological forms of Stephanodiscus niagarae Ehrenb. and S. parvus Stoermer & Håkansson 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.The teratological forms of both species appeared in the core sediments after 1911, and both became the dominant components of diatom assemblages after 1970. Their occurrence and increased abundance coincides with records of heavy metal pollution in the harbour.