Body shape and size divergence among populations of Poecilia vivipara in coastal lagoons of south-eastern Brazil F. M. N EVES AND L. R. MONTEIRO * Laborato ´rio de Cieˆncias Ambientais, Centro de Biocieˆnciase Biotecnologia, Universidade Estadual do Norte Fluminense. Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ, cep 28013-600, Brazil (Received 19 July 2002, Accepted 11 July 2003) Geometric shape analyses were used to study body shape and size variation among populations of the livebearing fish Poecilia vivipara inhabiting the recently formed coastal lagoons of Grussaı´ and Iquipari in Northern Rio de Janeiro State, Brazil. The largest components of morphological variation among females were between different habitats in the same lagoon, whereas for males there were larger differences between lagoons than between habitats. The shape differences were mostly localized in the head region and midbody, which indicated different patterns of locomotion and foraging behaviour optimized for the habitat experienced by each population. The pattern of size variation was similar to that of size-independent shape variation. # 2003 The Fisheries Society of the British Isles Key words: body shape; geometric morphometrics; locomotion; Poecilia vivipara; sexual dimorphism; spatial variation. INTRODUCTION Body shape is an important character for the analysis of fish evolution, because it influences locomotion efficiency in different environments, particularly for- aging and predator evasion. Thus, the study of body shape is fundamental for the evolutionary analysis of spatial variation among fish populations and allows for theories about adaptation to local environmental conditions (Walker, 1997; Pakkasmaa & Piironen, 2001). The molly Poecilia vivipara Bloch & Schneider is widely distributed, as are other poeciliines, throughout the South American coast, inhabiting the lower courses of large rivers (Parenti & Rauchemberger, 1989). Its distribution is not well understood but probably represents a complex of species (Bizerril & Primo, 2001). Poeciliine fishes are generally omnivorous, eating aquatic and terrestrial invertebrates, detritus and plants, but mainly insect larvae. Most studies on the *Author to whom correspondence should be addressed. Tel.: þ55 22 27261602; fax: þ55 22 27263720; email: lrmont@uenf.br Journal of Fish Biology (2003) 63, 928–941 doi:10.1046/j.1095-8649.2003.00199.x, available online at http://www.blackwell-synergy.com 928 # 2003 The Fisheries Society of the British Isles