Effects of seasonal growth rate on morphological variation of Undaria pinnatifida (Alariaceae, Phaeophyceae)

Undaria pinnatifida (Harvey) Suringar is currently divided into two morphol ogical forms, f. typica Yendo. and f. distans Miyabe & Okamura. The objecti ve of this study was to determine the effects of seasonal variation in grow th rate on the morphology of U. pinnatifida, and to define the form of U. p innatifida growing in Otago Harbour, New Zealand. Morphological variables ( stipe length, blade length, blade width, sporophyll length and degree of bl ade incision), growth rates (frond, blade and stipe) and blade erosion were measured each month from August 1993 to February 1995, and compared using correspondence analysis. Variation in the morphology of U. pinnatifida was largely accounted for by varying growth rates. Definition of the form of U. pinnatifida growing in Otago Harbour is equivocal because morphological ch aracteristics of both f. typica and f. distans were exhibited at different times of the year.