254 J. Phycol. 35, 254–269 (1999) REPRODUCTIVE SUCCESS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) IN THE BALTIC SEA 1 Ester A. Serra ˜o 2 Department of Plant Biology and Pathology, University of Maine, 5722 Deering Hall, Orono, Maine 04469-5722 Susan H. Brawley School of Marine Sciences and Department of Plant Biology and Pathology, University of Maine, 5722 Deering Hall, Orono, Maine 04469-5722 Jenny Hedman Department of Plant Physiology, Umea ˚ University, S-90187 Umea ˚, Sweden Lena Kautsky Department of Botany, Stockholm University, S-10691 Stockholm, Sweden and Go ¨ran Samuelsson Department of Plant Physiology, Umea ˚ University, S-90187 Umea ˚, Sweden Marine organisms colonizing brackish habitats such as the Baltic Sea must cope with the negative effects of low salinities on reproductive success be- cause these may reduce gamete viability and/or in- crease polyspermy. Reproductive characteristics of the marine seaweed Fucus vesiculosus L. were studied in several brackish habitats, particularly in the north- ern Baltic Sea, to understand its ability to reproduce where few other marine species survive. Polyspermy and fertilization success were variable at the bound- ary of the continuous distribution of F. vesiculosus in the Baltic Sea, and polyspermy was high (10%–30%) when fertilization was successful. A strong female bias (80%–86%, ca. 5.5:1) was found at the north- ernmost limit of Baltic F. vesiculosus. Electrophysio- logical studies showed that many eggs have a high input resistance (519  150 M [mean  SE, n  14] at Drivan, 1995), which may be helpful in pre- venting polyspermy in this brackish habitat. The polyspermy block remains sodium-dependent in the northern Baltic. Sperm bound quickly to northern Baltic eggs in natural water, but fertilization was de- layed compared to marine F. vesiculosus. A subset of northern Baltic eggs studied during an optimal re- productive period (7–11 July 1995) had a membrane potential (E m ) of ca. -100 mV and an effective fer- tilization potential (FP) 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 occurred more slowly in Baltic than in ma- rine zygotes. The reproductive success of these 1 Received 23 September 1998. Accepted 6 January 1999. 2 Author for reprint requests; e-mail eserrao@ualg.pt. Present address: Centro de Cie ˆncias do Mar-CCMAR, U.C.T.R.A., Uni- versidade do Algarve, Campus de Gambelas, P-8000 Faro, Portu- gal. boundary populations may be dependent upon win- dows of opportunity when there are favorable com- binations of the levels of salinity, water motion, pop- ulation density, and sex ratio. These factors and the short duration of the reproductive season in the northern Baltic Sea may result in reproductive fail- ure in some years. Key index words: Baltic Sea; estuaries; fertilization success; fucoids; Fucus; polyspermy; reproductive ecology Abbreviations: AW, artificial water; E m , membrane po- tential; E m-bf , membrane potential before fertiliza- tion; FP, fertilization potential; psu, practical salinity unit Marine organisms colonizing brackish habitats (e.g. estuaries, the Baltic Sea) must be able to cope with the negative effects of low salinities on repro- ductive success. Low salinities may decrease the suc- cess of fertilization by reducing the motility and lon- gevity of gametes (e.g. Serra ˜o et al. 1996a). In- creased polyspermy is another effect that low salin- ities might have on the reproductive success of marine organisms (Brawley 1992). Polyspermy is le- thal for most species (Jaffe and Gould 1985); there- fore, it is not only essential for an egg to be fertil- ized, but also to avoid being fertilized by more than one sperm. Many organisms have several types of polyspermy blocks to prevent the entry of additional sperm after the first sperm-egg fusion, and these in- clude the fast block (i.e. an electrical block) and a slow block (e.g. secretion of a fertilization envelope or cell wall) (see review by Jaffe and Gould 1985). Fast blocks against polyspermy act on scales of mil- liseconds after the first sperm entry and are usually