Molecular Ecology (2007) 16, 3671–3678 doi: 10.1111/j.1365-294X.2007.03421.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd Blackwell Publishing Ltd Are clownfish groups composed of close relatives? An analysis of microsatellite DNA variation in Amphiprion percula PETER M. BUSTON,* STEVEN M. BOGDANOWICZ,† ALEX WONG ‡ and RICHARD G. HARRISON † *Estación Biológica de Doñana, C.S.I.C., Avenida de Maria Luisa s/n Pabellón del Perú, 41013 Sevilla, Spain, †Department of Ecology and Evolutionary Biology, Cornell University, Corson Hall, Ithaca, NY 14853, USA, ‡Department of Molecular Biology and Genetics, Cornell University, Biotechnology Building, Ithaca, NY 14853, USA Abstract A central question of evolutionary ecology is: why do animals live in groups? Answering this question requires that the costs and benefits of group living are measured from the perspective of each individual in the group. This, in turn, requires that the group’s genetic structure is elucidated, because genetic relatedness can modulate the individuals’ costs and benefits. The clown anemonefish, Amphiprion percula, lives in groups composed of a breeding pair and zero to four nonbreeders. Both breeders and nonbreeders stand to gain by associating with relatives: breeders might prefer to tolerate nonbreeders that are relatives because there is little chance that relatives will survive to breed elsewhere; nonbreeders might prefer to associate with breeders that are relatives because of the potential to accrue indirect genetic benefits by enhancing anemone and, consequently, breeder fitness. Given the potential benefits of associating with relatives, we use microsatellite loci to investigate whether or not individuals within groups of A. percula are related. We develop seven polymorphic microsatellite loci, with a number of alleles (range 2–24) and an observed level of heterozygosity (mean = 0.5936) sufficient to assess fine-scale genetic structure. The mean coefficient of relatedness among group members is 0.00 ± 0.10 (n = 9 groups), and there are no surprising patterns in the distribution of pairwise relatedness. We conclude that A. percula live in groups of unrelated individuals. This study lays the foundation for fur- ther investigations of behavioural, population and community ecology of anemonefishes which are emerging as model systems for evolutionary ecology in the marine environment. Keywords: Amphiprion, cooperation, group living, larval dispersal, marine fish, relatedness Received 4 February 2007; revision received 30 April 2007; accepted 10 May 2007 Introduction Trying to understand why animals live in groups has been one of the major emphases of evolutionary ecology research for more than 30 years (Alexander 1974; Wilson 1975; Pulliam & Caraco 1984; Krebs & Davies 1993; Alcock 2001). The general approach has been to investigate the economics, the costs and benefits, of group living (e.g. acorn woodpeckers, Mumme et al. 1983; Koenig et al. 1995; Haydock et al. 2001; lions, Packer et al. 1990; Packer et al. 1991; Pusey & Packer 1994; cichlid fish, Taborsky 1985; Balshine et al. 1998; Brouwer et al. 2005; Stiver et al. 2005). This approach is based on the principle that natural selection will have favoured those individuals who behaved in ways that maximized their genetic contribu- tion to future generations. If this is true, then selection will have produced animals that are basically efficient, attempting to maximize their benefits while minimizing their costs. Ideally, the costs and benefits are investigated from the perspective of each individual in the group. An individual’s costs and benefits can be modulated by its behaviour, social and ecological factors and the genetic structure of the group (Keller & Reeve 1994; Emlen 1997). Thus a complete understanding of individual behaviour Correspondence: Peter M. Buston, Fax: +34 954 621125; E-mail: buston@ebd.csic.es