Reproductive success of Fucus vesiculosus (Phaeophyceae) in the Baltic Sea

Marine organisms colonizing brackish habitats such as the Baltic Sea must c ope with the negative effects of low salinities on reproductive success bec ause these may reduce gamete viability and/or increase polyspermy, Reproduc tive characteristics of the marine seaweed Fucus vesiculosus L, were studie d in several brackish habitats, particularly in the northern Baltic Sea, to understand its ability to reproduce where few other marine species survive , Polyspermy and fertilization success were variable at the boundary of the continuous distribution of F, vesiculosus in the Baltic Sea, and polysperm y was high (10%-30%) when fertilization was successful. A strong female bia s (80%-86%, ca, 5.5:1) vas found at the northernmost limit of Baltic F, ves iculosus. Electrophysiological studies showed that many eggs have a high in put resistance (519 +/- 150 M Omega [mean +/- SE, n = 14] at Drivan, 1995), which may be helpful in preventing polyspermy in this brackish habitat. Th e polyspermy block remains sodium-dependent in the northern Baltic, Sperm b ound quickly to northern Baltic eggs in natural water, but fertilization wa s delayed compared to marine F, vesiculosus. A subset of northern Baltic eg gs studied during an optimal reproductive period (7-11 July 1995) had a mem brane potential (E-m) of ca, -100 mV and an effective fertilization potenti al (EP) of ca, 2 min with a plateau of -25 mV, but repolarized too rapidly for the FP to be protective. Pronuclear migration and cell wall secretion o ccurred more slowly in Baltic than in marine zygotes, The reproductive succ ess of these boundary populations may be dependent upon windows of opportun ity when there are favorable combinations of the levels of salinity, water motion, population density, and sex ratio. These factors and the short dura tion of the reproductive season in the northern Baltic Sea may result in re productive failure in some years.