The swimming crab Achelous spinicarpus is commonly captured as a "by catch" of the shrimp fishery. This study evaluated some aspects of the population structure of A. spinicarpus, such as sex-ratio and size classes' frequency distribution. A shrimp trawler equipped with double-rig nets was used to capture the swimming crabs at Ubatuba and Caraguatatuba areas, northern coast of São Paulo state, Brazil. A total of 1,057 individuals were collected in Ubatuba, including 598 males (525 juveniles and 73 adults) and 459 females (379 juveniles and 80 adults, including 15 ovigerous crabs). In Caraguatatuba 5,112 individuals were collected, of which 3,138 males (2,638 juveniles and 500 adults) and 1,974 females (1,746 juveniles and 228 adults, including 29 ovigerous crabs). The sex-ratio favors the number of males in both regions, probably because females have the habit of occupying deeper regions. The size classes' frequency distribution shows polymodality for both sexes, except for males in Ubatuba. This is the result of some age groups among the juveniles, which migrate to shallower areas, being affected by the fishing activity. Adults have the habit of staying in deeper areas characterized by colder waters.Keywords: Brachyura, sex-ratio, Decapoda, bycatch, size distribution. ResumoO siri candeia, Achelous spinicarpus, é comumente capturado como "by catch" da pesca camaroeira. Este estudo avaliou alguns aspectos de sua estrutura populacional, como razão sexual e distribuição de frequência em classes de tamanho. Para coletar os siris, foi utilizado um barco de pesca camaroeiro equipado com redes "double rig" nas áreas de Ubatuba e Caraguatatuba, litoral norte do estado de São Paulo, Brasil. Um total de 1.057 indivíduos foi coletado em Ubatuba, incluindo 598 machos (525 juvenis e 73 adultos) e 459 fêmeas (379 juvenis e 80 adultas, incluindo 15 ovígeras). Em Caraguatatuba, 5.112 indivíduos foram coletados, dos quais 3.138 machos (2.638 juvenis e 500 adultos) e 1.974 fêmeas (1.746 juvenis e 228 adultas, incluindo 29 ovígeras). A razão sexual favoreceu o número de machos em ambas as áreas, provavelmente devido ao hábito de as fêmeas ocuparem regiões mais profundas. A distribuição de frequência em classes de tamanho apresentou polimodalidade para ambos os sexos, exceto para machos em Ubatuba. Isso é resultado de alguns grupos etários entre os juvenis, os quais migram para áreas mais rasas, sendo afetados pela atividade pesqueira. Os adultos permanecem em áreas mais profundas, caracterizadas por águas frias. Population structure of the swimming crab Achelous spinicarpus (Crustacea, Portunoidea) in São Paulo northern coast, Brazil Palavras

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Biodiversity has become the focus of several scientific approaches in the last decade (Longo & Amado Filho, 2014). The severe depletion of populations or the extinction of species might generate uncountable damage to ecosystems (Longo & Amado Filho, 2014).According to Gaston (2000), one of the most important goals of the ecology and biogeography fields is improving the understanding of global biodiversity distribution.Compared to terrestrial and freshwater environments, marine ecosystems have always been regarded as the least damaged environments (Culotta, 1994). This is probably linked to the lower rate of documented species loss and to the extra resilience of large geographical areas (Culotta, 1994). Nonetheless, the marine environment has been currently undergoing intense changes regarding the abundance of populations, distribution patterns of species, and general structure of benthic communities (Kaiser et al., 2002). These alterations are caused by trawling activities (Kaiser et al., 2002) that decrease coral populations (Pockley, 2000) while disseminating nonindigenous species (Andrade et al., 2014;Molnar et al., 2008), and also by contamination by petroleum leaks (Pedrozo et al., 2002).Nearly, 3.2 million tons of oil annually leak into the oceans (Pedrozo