The effect of Anonchocephalus chilensis Riggenbach (Eucestoda: Bothriocephalidea) on infracommunity patterns in Genypterus maculatus Tschudi (Osteichthyes: Ophidiidae) S.A. Mun ˜ oz* and M. George-Nascimento Departamento de Ecologı ´a Costera, Facultad de Ciencias, Universidad Cato ´lica de la Santı ´sima Concepcio ´ n, Casilla 297, Concepcio ´n, Chile Abstract The use of parasite body size (i.e. body mass) is a promising proxy to improve the study of patterns in parasite infracommunities, which are usually analysed using only numerical descriptors. This study deals with the importance and effect of the presence of a large endoparasite species, the cestode Anonchocephalus chilensis in a marine fish species, Genypterus maculatus, on the structure of the parasite infracommunities. Numerical and volumetric measures of aggregated properties of parasite infracommunities were compared and their correlation examined. The highly dominant presence of A. chilensis by volume causes a dramatic change in the patterns observed, including a smaller total volume of the remaining species when this volumetrically dominant species is present. However, C-scores and V-ratios, both indices based on null models of species occurrence, do not support the idea of communities structured by interspecific competition. Analyses reveal that numerical and volumetric community descriptors are complementary ways to search for patterns and to reveal processes within these systems. Introduction When analysing patterns of relative abundance of parasite species among host individuals, parasite body size can be used as a proxy for productivity (Mouillot et al., 2005) and estimates of parasite energetic require- ments (George-Nascimento et al., 2004; Poulin & George- Nascimento, 2007). However, patterns in parasite communities are commonly detected using only numeri- cal descriptors, which may not hold when parasite mass is used. For example, in the southern pomfret, Brama australis Valenciennes, parasite infracommunities did not show significant correlations between numerical and volumetric descriptors, nor between numerical abundance and the body size of parasites (George- Nascimento et al., 2002). The host whose infracommunities are studied here, Genypterus maculatus Tschudi, is a marine fish species that harbours one of the largest parasite species reported along the coast of Chile, the cestode Anonchocephalus chilensis Riggenbach. Although this host is said to harbour species- poor infracommunities (George-Nascimento & Huet, 1984; Mouillot et al., 2005), other analyses suggest structured infracommunities (Mouillot et al., 2005). Some authors have interpreted patterns in species distribution and/or abundance as a result of interspecific interactions (Gotelli & Rohde, 2002; Mouillot et al., 2005), while others have considered that a species assemblage is structured if it diverges in a repeatable or predictable way from a random assortment (Poulin, 1998). In any case, most parasite infracommunities in marine fishes seem to be unsaturated with either individuals or species, and therefore intra- and interspecific competition would not be important, but only sporadic (Rohde, 1984, 2005; Poulin, 1998; Mouillot et al., 2003, 2005; Mun ˜ oz et al., 2006). Among infracommunity patterns found to hold across host species, a consistent, decreasing relation- ship between evenness and abundance, both measured *E-mail: smunoz_ucsc@yahoo.es Journal of Helminthology (2008) 82, 221–226 doi:10.1017/S0022149X08960788