Temperature requirements ofScytosiphon lomentaria (Scytosiphonales,Phaeophyta) from the Gulf of Thessaloniki,Greece, in relation to geographic distribution

Temperature requirements for growth, reproduction and formation of macrothalli of a day-neutral strain ofScytosiphon lomentaria from the Gulf of Thessaloniki were experimentally determined and correlated with the geographic distribution in the North Atlantic Ocean. The microthallus grew in a wider temperature interval and better at higher temperatures than did the macrothallus. Germlings acclimated to 5 or 15°C grew sufficiently (>20% of maximum rate) and developed into macrothalli at 5–25°C and 5–27°C. Macrothalli acclimated to 10 or 15°C grew sufficiently at 5–20°C. Macrothalli acclimated to 15°C survived at −1°C and reproduced at 5 to 23°C. Regardless of the acclimation temperature, germlings and macrothalli grew optimally (>80% of maximum rate) at 15–25°C and at 10–15°C. The experimental data explain only the southern distribution boundary ofScytosiphon in the North Atlantic. This boundary is composite in nature: on the European coasts it is a growth boundary, whereas on the American coasts it is a lethal one.