Life history of Acrosiphonia (Codiolales, Chlorophyta) in southwestern British Columbia, Canada

This study establishes the phenology of the alternate life history phases o f the green alga Acrosiphonia in British Columbia, Canada. Free-living, fil amentous plants are seasonal, March-July, with peak percent cover (10%) in April. Plants are fertile immediately after establishment. The unicells, pr eviously identified as Chlorochytrium inclusum and Codiolum petrocelidis, a re the sporophyte phase of Acrosiphonia. 'Chlorochytrium,' spherical and 16 0-280 mum in diameter, colonizes the foliose red alga Mazzaella splendens 1 mc, after filamentous Acrosiphonia plants appear. Maximum density (53 'Chl orochytrium' cells/cm(2) of blade) was recorded in May. 'Codiolum,' on the other hand, is stalked (the vesicle measures 150 x 50 mum) and colonizes th e red algal crust Petrocelis. Peak density (22400 'Codiolum' cells/cm(2) of crust) was recorded 2 mc, after 'Chlorochytrium' density peaked. The endop hytes survive high summer temperatures, which correlate with death of the f ree-living plants, and overwinter in their hosts. Zoospore release in late winter corresponds to decreased host abundance, suggesting the endophytes h ave evolved a strategy whereby duration in the host is synchronized with ho st seasonality. A bet-hedging strategy is proposed for Acrosiphonia's life history: two morphologically different phases have adapted to a seasonally variable environment, with the sporophyte phase capable of colonizing two d ifferent hosts.