MORPHOLOGY AND ULTRASTRUCTURE OF TERATOLOGICAL FORMS OF THE DIATOMS STEPHANODISCUS NIAGARAE AND S-PARVUS (BACILLARIOPHYCEAE) FROM HAMILTON HARBOR (LAKE-ONTARIO, CANADA)

Teratological forms of Stephanodiscus niagarae Ehrenb. and S. parvus S toermer & Hakansson were observed during a study of diatoms preserved in a radiometrically dated core from Hamilton Harbour (Lake Ontario, C anada), Morphological features and ultrastructures of both species wer e studied under the light and scanning electron microscope. The valve structure of abnormal forms of S. niagarae appears to be weakly silici fied, especially in the central area. The shape of satellite pores are very irregular in comparison with the round shape of the normal speci men. The central fultoportulae are characterized as small tubes extend ing out of the external valve. Two types of abnormal frustules are pre sent in the population of S. papvus studied. In one type valves are '' underdeveloped'' and only the siliceous layer and ribs are present, an d in the other type the valves are strongly silicified and the areolae are almost completely occluded. The teratological forms of both speci es appeared in the core sediments after 1911, and both became the domi nant components of diatom assemblages after 1970. Their occurrence and increased abundance coincides with records of heavy metal pollution i n the harbour.