Agustina Méndez Casariego

Abiotic stress mediates top-down and bottom-up control in a Southwestern Atlantic salt marsh

Increasing evidence has shown that nutrients and consumers interact to control primary productivity in natural systems, but how abiotic stress affects this interaction is unclear. Moreover, while herbivores can strongly impact zonation patterns in a variety of systems, there are few examples of this in salt marshes. We evaluated the effect of nutrients and herbivores on the productivity and distribution of the cordgrass Spartinadensiflora along an intertidal stress gradient, in a Southwestern Atlantic salt marsh. We characterized abiotic stresses (salinity, ammonium concentration, and anoxia) and manipulated nutrients and the presence of the herbivorous crab Neohelice (Chasmagnathus) granulata, at different tidal heights with a factorial experiment. Abiotic stress increased at both ends of the tidal gradient. Salinity and anoxia were highest at the upper and lower edge of the intertidal, respectively. Nutrients and herbivory interacted to control cordgrass biomass, but their relative importance varied with environmental context. Herbivory increased at lower tidal heights to the point that cordgrass transplants onto bare mud substrate were entirely consumed unless crabs were excluded, while nutrients were most important where abiotic stress was reduced. Our results show how the impact of herbivores and nutrients on plant productivity can be dependent on environmental conditions and that the lower intertidal limits of marsh plants can be controlled by herbivory.

Stage-dependent interactions between intertidal crabs: from facilitation to predation

Large parts of the south-western Atlantic soft bottom intertidals are inhabited by the burrowing crab Neohelice granulata (previously known as Chasmagnathus granulatus ) and the mud crab Cyrtograpsus angulatus , but adults rarely coexist in the same microhabitat. We describe the influence of burrows of N. granulata on the recruitment dynamic of C. angulatus and the effects of different ontogenetic stages of N. granulata on survival of recruits of C. angulatus . A two summer sampling shows that N. granulata burrows facilitate settlement of both species. To evaluate the mortality of recruits inside burrows, we performed a field experiment with juvenile and adult crab exclusion cages and inclusion of juveniles or adults of N. granulata . The results showed differences in crab sizes between treatments, due to predatory interactions that depended on prey size. When only juveniles of N. granulata were present, the higher mortality of C. angulatus was observed in smaller crabs. However when adults of N. granulata were present, larger juvenile crabs are the ones that suffered the highest mortality. These results show that adults are preying upon larger juveniles (of both species) reducing the mortality of the smaller ones that are preyed by the larger ones. We also measured emigration from these burrowing assemblages using bidirectional pitfall traps which showed that C. angulatus juveniles are leaving the burrows towards the subtidal. Here we demonstrate that N. granulata have positive and negative effects on the settlement of C. angulatus . These results highlight the importance of including stage-dependence relationships to analyse species interaction in marine ecology field works.

Differential coloration patterns in the intertidal crab Neohelice (=Chasmagnathus) granulata

Many crustaceans with spatial segregation may present differential body coloration depending on the habitat they inhabit. Since crustaceans are unable to synthesize carotenoids, individuals must acquire them from their diet, so diet changes can modify individual coloration. The burrowing crab Neohelice (=Chasmagnathus) granulata inhabits the intertidal, from the uppermost parts of salt marshes to the lowest mudflat zones with a spatial segregation of different size-classes. They are primarily deposit feeders in mud flats and herbivorous–detritivorous in the salt marsh. We analysed the effect of intermoult length on the carapace colour of N. granulata , and the changes in carapace colour with crab size and zone. Crab colour varied between unvegetated and vegetated areas. Some of these differences were in part caused by colour changes related to crab size. Larger crabs show less intensive colours, and given that they inhabit the upper part of the intertidal a differential coloration pattern occurred between zones. In field experiments intermoult duration affected crab colour and some body regions analysed showed differences between zones unrelated to size.

Spatial variability of larval parasites harboured by two crab species in an estuarine environment in Argentina

Fil: Mendez Casariego, Maria Agustina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales; Argentina