Habitat shifts and spatial distribution of the intertidal crab Neohelice (Chasmagnathus) granulata Dana Agustina Mendez Casariego ⁎, Juan Alberti, Tomás Luppi, Pedro Daleo, Oscar Iribarne Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, CC 573 Correo Central. B7600WAG, Mar del Plata, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Rivadavia 1917, CP C1033AAJ, Ciudad de Buenos Aires, Argentina abstract article info Article history: Received 20 October 2010 Received in revised form 27 April 2011 Accepted 1 May 2011 Available online 12 May 2011 Keywords: Cannibalism Spatial segregation Differential mortality Intertidal Crabs Intertidal zones of estuaries and embayments of the SW Atlantic are dominated by the semiterrestrial burrowing grapsid crab, Neohelice (Chasmagnathus) granulata, and characterized by extensive mud flats surrounded by salt marshes. In this work we examined spatial patterns of distribution of N. granulata during two years to explain their movement patterns. The results of the population sampling showed segregation by sex and size throughout the intertidal, with seasonal variations in densities and different condition indices for adults and juveniles at the different zones. The comparison of seasonal activity (ambulatory activity outside burrows) between marshes and mudflats shows that short term (e.g. daily) variations in activity were controlled by tides. Crabs were active at high tides but increased their activity on days with higher tidal amplitude. Seasonal activity showed that at both areas, females remain with low activity except for a peak in winter, while males showed the highest activity during summer in the mudflat zone, but not so in the marsh. This pattern can be the response to differences in stress tolerance, suggesting that high temperatures are limiting the performance of adult crabs during summer, especially at the marsh where physical conditions can be more critical. The spatial size segregation can be explained by differential mortality in each zone (estimated with tethered crabs), and by the juvenile movement between these zones (estimated with movement traps). Juvenile mortality is higher at the mudflat, while adult mortality is higher in the marsh. Smaller juveniles moved to the marsh, where the mortality is lower, and the larger juveniles moved towards the mudflat. This mortality is due almost exclusively to cannibalism, so our results suggest that this movement of different size classes between zones is controlled, at least in part, by intraspecific predation. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Zonation patterns in intertidal habitats are imposed by physical and/or biological factors. The upper limit is mainly defined by factors such as salinity (problems of osmoregulation), temperature, oxygen availability and humidity which cause dissecation or respiration problems (Newell 1979, Halperin et al., 2000) which define the upper limit of species distributions (Newell 1979, Bertness & Leonard 1997, Bertness 1999). The lower distribution limits, instead, are most likely established by biological factors like food abundance, prey avoidance or competition (e.g. Bertness & Leonard 1997, Bertness 1999). These factors control distribution limits at species levels, especially with sessile organisms. Mobile species, instead, can deal with these physical or biological forces leading in some species to spatial segregation by sex and size in different zones (Giménez 2004), particularly for marine invertebrates with complex life cycles (Koga 1995). Ontogenetic changes in osmorregulatory ability (Charmantier et al., 2002), air exposure resistance, diet choices, predator avoidance (Hunt & Scheibling 1997) and reproductive behavior (Carr et al., 2004) may cause variations in habitat use during life cycle of organisms (Etherington et al., 2003, Lipcius et al., 2005). In addition, structured habitats may affect these habitat choices influencing possible in- teractions with other species or ages (Hines et al., 1987). Generally, vegetated intertidal areas, such as mangroves, sea grass beds or marshes, are important refuges or nursery habitats for many species (Orth & van Montfrans 1987). Several species with complex life cycles depend upon these areas because they enhance survival by diminishing predation and increasing food availability in comparison with non vegetated areas (Heck & Thoman 1984, Beck et al., 2001, Heck et al., 2003, Minello et al., 2003). However, in some cases, these areas support such a concentration of individuals that lead to competition, predation or cannibalism (Lipcius et al., 2005). In this case, emigration or differential mortality can lead to spatial segregation as well. Neverthe- less, marsh plants ameliorate the thermal stress at higher intertidal zones (Callaway 1995, Nomann & Pennings 1998) which may enhance the intertidal distributions of species (Bortolus et al., 2002). High temperatures cause dehydration, protein denaturalization, and cellular Journal of Sea Research 66 (2011) 87–94 ⁎ Corresponding author at: Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, CC 573 Correo Central. B7600WAG, Mar del Plata, Argentina. E-mail address: mendezc@mdp.edu.ar (A.M. Casariego). 1385-1101/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2011.05.001 Contents lists available at ScienceDirect Journal of Sea Research journal homepage: www.elsevier.com/locate/seares