Two reproductive strategies in Baltic Fucus vesiculosus (Phaeophyceae)

Marked seasonal reproduction is a widespread phenomenon in seaweeds in temp erate waters strongly influenced by seasonal variations. In the Baltic Sea, however, the perennial brown alga Fucus vesiculosus exhibits two periods o f reproduction: early summer (May-June) or late autumn (September-November) . In this study, we compared the geographical distribution, the reproductiv e biology and the recruitment success for F. vesiculosus that reproduce dur ing either of these two periods on the east coast of Sweden. Both monocultu res and mixed populations of summer- and autumn-reproducing F. vesiculosus occurred along the mainland coast from the southern parts of the Stockholm archipelago to the southernmost part of Sweden. F. vesiculosus reproduced o nly during summer around the coasts of the large islands of bland and Gotla nd. After reproduction, in July and January respectively, plants that repro duced in summer abscised their receptacles and part of their branches, whil e plants reproducing in autumn abscised only their receptacles. In laborato ry experiments, short-day treatments (light: dark 8: 16 h) were shown to in itiate receptacles in summer-reproducing plants, whereas plants reproducing in autumn initiated receptacles independently of short- or long-day (light : dark 16: 8 h) treatment. Plants reproducing during summer produced more e ggs (21.0 x 10(4) eggs/g(-1) frond mass) of smaller size (0.067 mm) than pl ants reproducing during autumn (8.9 x 10(4) eggs/g(-1) frond mass, eg size 0.070 mm). However, recruitment success in the field was similar irrespecti ve of the period of reproduction. The differences in the photoperiodic resp onse, the differences in reproductive output and egg size between summer an d autumn plants, and the observation that a single individual kept the same pattern of reproduction over a period of at least three reproductive seaso ns, may indicate that the two strategies of reproduction in F. vesiculosus are expressions of two distinct genotypes.