Correlation of temperature and daylength response ofSphaerotrichia divaricata(Phaeophyta, Chordariales) with field phenology in Nova Scotia and distribution in Eastern North America

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Abstract
Sphaerotrichia divaricata from the southern Gulf of St Lawrence, Canada has been studied in the field and in laboratory culture. Juvenile macrothalli occurred on eelgrass (Zostera marina) in early summer when water temperature was rising from 5° to 20°C. Macrothalli bore unilocular sporangia from August to late November. In culture, unispores developed into haploid microthalli that tolerated -1° to 28°C and were asexual under most conditions, but functioned as dioecious gametophytes at 0° to 15°C in long days. Microthalli in the field presumably replicated themselves asexually during summer months when water temperature exceeded 15°C. Juvenile macrothalli were again found in early October, after initiation in September when water temperature dropped below 15°C and daylength still exceeded 12 h. Fully formed macrothalli tolerated temperatures of 5 to 24°C, but macrothallus protonemata also survived 0°C. The species extends along the Atlantic coast of North America from northern Labrador to New Jersey. The northern boundary marks a summer temperature limit for maturation of macrothalli; the southern boundary marks the limit of coincidence of low temperatures and long daylength.