Inhibitory effects of red algal extracts on larval settlement of the barnacle Balanus improvisus Received: 19 December 2002 / Accepted: 8 April 2003 / Published online: 11 July 2003 Ó Springer-Verlag 2003 Abstract We examined the chemical antifouling prop- erties of four sublittoral red algae, Chondrus crispus, Delesseria sanguinea, Osmundea ramosissima, and Polyides rotundus, which are all rarely fouled in the field. Two different approaches were used. Firstly, we tested the effects of lipophilic crude extracts on the settlement behaviour of cyprid larvae of the co-existing barnacle Balanus improvisus. Secondly, in a settlement preference experiment, we tested whether B. improvisus cyprid larvae settle on living algae when given a choice between natural algal surfaces and control surfaces. With this procedure, we were able to test both if the algae inhibit recruitment of cyprids, and if this inhi- bition is a result of chemistry. The settlement of B. improvisus larvae was strongly inhibited at concen- trations estimated to be potentially ecologically rele- vant for all of the tested extracts. However, only C. crispus significantly inhibited settlement in the preference experiment, even though there was also a tendency for settlement inhibition on P. rotundus and O. ramosissima. In contrast, D. sanguinea seemed to stimulate settlement. This contradiction probably re- sulted from an extraction of metabolites that naturally occur only inside the alga. However, as this study shows, a combination of settlement assays with whole- cell extracts and preference tests of ecologically rele- vant fouling organisms on natural algal and control surfaces may be a useful procedure to avoid erroneous conclusions regarding natural antifouling roles of compounds based on settlement assays with only whole-cell extracts. Furthermore, this study also shows that production of inhibitory metabolites may explain the low degree of fouling, especially by B. improvisus, on C. crispus. Introduction In marine environments, the availability of unoccupied surfaces is essential for recruitment of a wide array of benthic organisms. One important source of substrata for colonization of organisms is benthic marine mac- roalgae (seaweeds), which due to their three-dimen- sional structure offer a significant proportion of surface in hard subtidal habitats (Seed 1985). Besides surfaces for attachment, seaweeds provide an array of resources for epibiota including food supply, permanent or tem- porary shelter, and sediment traps (Williams and Seed 1992). Fouling may impose a significant cost for the host algae by reducing rates of photosynthesis, growth, and reproduction due to increased shading (D’Antonio 1985; Brawley 1992). Additionally, some fouling organisms may damage host algal tissues directly by mechanical anchoring or indirectly by increasing the attractiveness of the host to herbivores (D’Antonio 1985; Gonzalez and Goff 1989; Wahl and Hay 1995). Fouling may also increase mortality by increasing the drag on the host and its susceptibility to being torn off the substrata (Dixon et al. 1981; D’Antonio 1985; see also review by de Nys and Steinberg 1999). Whereas some seaweeds are heavily fouled, other species in the same habitat are rarely epiphytized, indi- cating the presence of defence mechanisms against fouling. Antifouling mechanisms may be physical, such as sloughing of surface layers of host algae (e.g. Johnson and Mann 1986; Keats et al. 1997) or chemical, that is, production of secondary metabolites that prevent set- tlement and growth of fouling organisms. Seaweeds are particularly rich in secondary metabolites (see Tringali 1997) and several studies have shown that algal metab- olites may deter certain fouling organisms. For example, Lau and Qian (1997) showed that phlorotannins from Sargassum tenerrimum inhibited metamorphosis of Marine Biology (2003) 143: 875–882 DOI 10.1007/s00227-003-1093-9 G. M. Nylund Æ H. Pavia Communicated by L. Hagerman, Helsingør G. M. Nylund (&) Æ H. Pavia Tja¨rno¨ Marine Biological Laboratory, Department of Marine Ecology, Go¨teborg University, 452 96 Stro¨mstad, Sweden E-mail: goran.nylund@tmbl.gu.se Tel.: +46-526-68732 Fax: +47-526-68607