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Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into “variable” (e.g., sperm and eggs) and “fixed” (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the “fixed” cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating., As formalized by Charnov (1982), mating group size (MGS) is one of the most important factors affecting sex allocation of simultaneous hermaphrodites. Although the original concept assumes that the MGS does not differ between male and female functions, we found that sex allocation of a barnacle was affected differently by MGS in the male function and that in the female function.

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Balanus rostratus is one of the few species of barnacles that can form large, multi-levelled aggregations of differently aged individuals. It was found that individuals in aggregations were smaller than solitary barnacles. Most probably, these smaller sizes can be accounted for by (1) competition for substrate and food by surrounding barnacles, (2) form of the substrate, which restricted the barnacle's base growth in at least one direction, as they mainly survive settling in the area of the opercular aperture of a host barnacle, and (3) suboptimal oxygen and food supply, as the water current is weakened in the lower part of the aggregation. The growth of barnacles in differing situations (solitary or in aggregations) was not isometric. Their ratio of rostro-carinal basal length to carina height decreases with age. Solitary individuals living under favourable conditions were mostly conical (flattened). In the multi-levelled aggregations and in polluted areas with muddy bottoms the barnacles proved to be more oblong. It is likely that these individuals respond to deviations in parameters of environmental conditions from the optimum by increasing their shell height. Balanus rostratus est l'une des quelques especes de balanes qui peuvent former des agregats a multiples niveaux d'individus d'âge different. Il a ete trouve que les individus en agregats etaient plus petits que les solitaires. Tres probablement, ces tailles plus petites pouvaient etre mises sur le compte de (1) la competitition pour la nourriture et le substrat avec les balanes environnantes, (2) la forme du substrat, qui restreignait la croissance de la base des balanes dans une direction au moins, comme elles survivent principalement en se fixant dans la region de l'ouverture operculaire d'une balane-hote et (3) l'apport reduit d'oxygene et de nourriture, le courant d'eau etant affaibli dans la partie inferieure de l'agregat. La croissance des balanes dans les differentes situations (solitaires ou en agregat) n'etait pas isometrique. Le rapport de la longueur du basal rostro-carenal a la hauteur de la carene decroit avec l'âge. Les individus solitaires vivant dans des conditions favorables etaient surtout coniques (applatis). Dans les agregats a multiples niveaux et dans les zones polluees avec des fonds vaseux, les balanes etaient plus oblongues. Il est probable que ces individus repondent aux deviations (par rapport a l'optimum) dans les parametres des conditions d'environnement en accroissant la hauteur de la coquille.

The fatty acid composition of the barnacles Hesperibalanus hesperius and Balanus rostratus, taken from the epibiosis of the bivalve mollusc Patinopecten (Mizuhopecten) yessoensis, collected at two different sites in the Sea of Japan, was analysed. Fatty acid markers were used to determine the food sources of these barnacles. Differences in the fatty acid contents of barnacles of different sizes (ages) arise from the increasing variety of food sources. The distribution of fatty acid markers in the young of both species reflects the detrital origin of their food and additionally indicates that small barnacles are predominantly scavengers (necrophagous). With increasing size of the small species H. hesperius, the proportion of the fatty acids, which are characteristic for diatoms, increases. Adult B. rostratus contained large amounts of fatty acids, typical of diatoms, indicating a strong evidence of a dietary input of diatoms. The resemblance of the fatty acid compositions of both species of similar size and from the same environment, thus is the result of the similarity of the food supplied.La composition en acides gras des balanes Hesperibalanus hesperius et Balanus rostratus, prelevees sur des epibioses du mollusque bivalve Patinopecten (Mizuhopecten) yessoensis collecte en differents sites de la mer du Japon, a ete analysee. Des marqueurs en acides gras ont ete utilises pour determiner les sources alimentaires de ces balanes. Les differences dans les teneurs en acides gras chez les balanes de differentes tailles (âges) resultent de la variete croissante des sources de nourriture. La distributions des marqueurs en acides gras chez les jeunes des deux especes reflete l'origine detritique de leur nourriture et, de plus, que les petites balanes sont surtout detritivores (necrophages). Avec l'accroissement de taille de la petite espece, H. hesperius, la proportion d'acides gras, typiques des diatomees, s'accroit. Les B. rostratus adultes contenaient de fortes proportions d'acides gras, typiques des diatomees, ce qui indique manifestement une consommation alimentaire de diatomees. La ressemblance des compositions en acides gras chez les deux especes, a taille similaire et dans le meme environnement, resulte le plus probablement de la similarite des nourritures.

1. Barnacles of the species Balanus eburneus Gould were found to orient themselves with their long axes parallel to the grooves in the substrate, while oysters (Ostrea virginica Gmel.) were found to orient themselves at random.2. The grooving in the substrate was found to affect the topography of the compartments and the basal plate of the barnacle Balanus eburneus Gould. The grooves are duplicated upon the compartments and the ridges upon the basal plate. Any one groove on the record causes a ridge to occur on the basal plate and a groove to appear on the compartments of the barnacle. These grooves and ridges are initiated at the early growth stages and persist irrespective of the ultimate size of the organism.3. In a similar manner the grooved surface of the phonograph records affects the shells of the mollusc Ostrea virginica Gmel. both externally and internally by causing grooves and ridges to appear on the upper and lower shells.4. The living species of barnacles, Balanus rostratus apertus Pilsbry, B...