Oecologia (Berlin) (1986) 71:38-40 Oecologia 9 Springer-Verlag1986 The site of settlement indicates commensalism between bluemussel and its epibiont P. Laihonen and E.R. Furman Department of Biology, Laboratory of Ecological Zoology, University of Turku, Finland Tvfirminne Zoological Station, University of Helsinki, Finland Summary. The site of settlement of barnacles (Balanus im- provisus) attached on shells of bluemussels (Mytilus edulis) was mapped from a sample of mussels collected in the Baltic Sea. Most barnacles had settled near the siphonal apertures of the mussel. An experiment was made to measure the disadvantages and advantages that living in close associa- tion brings to barnacles and mussels. The barnacles on shells of living mussels were shown to grow significantly faster than those on empty mussel shells. Presence of barna- cles had no effects on growth of mussels. The two-species assocation under study was demonstrated to be a case of commensalism. Key words: Mytilus Balanus - Commensalism - Baltic Sea - Brachish water Gregariousness, grouping together at settlement, is a com- mon and well reported phenomenon among several sessile marine invertebrates (see Crisp 1979). Experimental ecology has used gregarious animals widely in the study of competi- tion and predation. In fact several classical, though some- times controversial, studies of community ecology have been made using barnacles, mussels, and their predators as test animals (Connell 1961 a, b; Harger 1972; Paine 1969, 1971, 1974). However, the study of gregariousness from the point of view of evolutionary ecology has been largely neglected. Most gregarious invertebrates are highly selective in choosing settlement site, because after settlement the capa- bility to move is lost or at least becomes limited. This phe- nomenon offers opportunities to make preliminary assump- tions on prevailing circumstances at settlement sites of these animals. Individuals can be expected to settle on the most favourable sites available. Hard-bottom invertebrate communities in the northern Baltic consist almost entirely of two highly gregarious spe- cies, the barnacle (Balanus improvisus) and the bluemussel (Mytilus edulis). The empty space available on the surface of mussels' shells is frequently used by barnacles as a sub- stratum. The "host" mussels with attached barnacles form a two-species unit ideally suited for this study. Our field observations suggested that the barnacles generally settle near the anterior end of mussel shells. Since barnacle cy- Offprint requests to: P. Laihonen prids are so selective at settlement (Yule and Walker 1985) it could be assumed that the anterior region of the mussel shell was more favourable to adult survival than the posteri- or region. Experiments were performed to test the disadvan- tages or advantages of this two-species association. Material, Methods, and Results The first part of the study is concerned with the pattern of settlement of barnacles on mussels' shells. In the second part an experiment was made to measure the effects that the animals have on one another through simultaneous use of a common food resource. Bluemussels (Mytilus edulis) were collected from the outer parts of Tvfirminne arcipelago on the SW-coast of Finland. Shells of 224 mussels were considered in three equal zones (see Fig. i), each consisting of about 30% of the total shell area. The number of barnacles (Balanus im- provisus) in each of the zones was counted and compared to equal settlement to each zone by a chi-squared equality test. From 224 bluemussels, 50% had barnacles on their shells. Figure 1 shows that 76% of the barnacles had at- tached close to the siphonal apertures in zone 1. The differ- ence between expected and observed numbers of barnacles in the three areas was highly significant 0(2= 185.6, df=2, P<0.001). For the second part of the study over 300 mussels were cleaned of epibionts, placed in plastic cages (mesh size 5 ram, volume 350 cm 3) and suspended in the midwater (1-2 m depth) to allow new barnacle larvae to settle on the shells. After an adequate barnacle settlement 180 mus- sels were separated into three test groups as follows: (1) 60 mussels each having one barnacle attached close to the siphonal apertures. (2) As in (1), but adductor muscles of the 60 mussels were cut thus killing the animal. (3) 60 mussels with no barnacles on their shells. Mussels were placed individually in plastic cages (mesh size 5 mm, volume 62 cm 3) and suspended in midwater. The shell length of the live mussels (anterior - posterior) and barnacles (carina - rostrum) was measured every two weeks to an accuracy of 0.1 mm. Dead mussels from groups 1 and 3 and mussels on which the barnacle had died were removed when discovered. Observations were made for barnacles from September 1984 to November 1984 and for mussels from September 1984 to July 1985.