Life history regulation and phenology of the red alga Bonnemaisonia hamifera

Bonnemaisonia hamifera Hariot (Rhodophyceae, Bonnemaisoniales) from Galway Bay, Ireland has been studied in the field and in laboratory culture. The reproductive behaviour of tetrasporophytes and gametophytes in the field appeared to be strictly regulated by their temperature/daylength responses as observed in culture. Tetrasporangia were abundant in early autumn when short days (<12 h of light per day) coincided with seawater temperatures over about 11°C, the lower limit for sporangium formation. Spermatangia were observed in very young gametophytes between mid-December and February, and in adult plants from late March until the end of May. They were absent in mid-winter when low temperatures of about 2°C inhibited their formation. Carpogonia were first observed at the end of April as seawater temperatures had by then risen to the required value of around 10°C. Carpogonia were fertilised and plants with mature cystocarps were present until early July. The onset of reproduction was accompanied by a cessation of growth and led to senescence within 2–3 months. Thus, gametophytes were absent in summer in spite of persistently favourable seawater temperatures. In various parts of the North Atlantic Ocean, annual temperature regimes are such as to cause a certain lack of synchronisation in the occurrence of reproductive male and female plants. This may account for the many anomalous reports of reproductive plants in the wild