Pablo D. Ribeiro

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m−2) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m−2 d−1 and 4.80 mg m−2 d−1 of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.

Population Structure of the SW Atlantic Estuarine Crab Neohelice granulata Throughout Its Range: a Genetic and Morphometric Study

Phylogeographic patterns of the SW Atlantic estuarine crab Neohelice granulata were examined using mitochondrial DNA cytochrome oxidase I sequences and analyzed together with morphometric data. Specimens were sampled during a 4-year period (2001–2004) from 11 localities encompassing the full distributional range of this species along the SW Atlantic (22°57′ S to 42°25′ S). DNA sequences were obtained from 69 individuals belonging to seven localities, and morphometric variation in 12 continuous characters was analyzed for 646 crabs from ten localities. Strong genetic differentiation, consistent with a pattern of isolation-by-distance, was detected among all localities indicating that gene flow occurs mainly between neighboring populations. Analyses of molecular variance showed genetic subdivision between the southern (Argentina) and the northern (Brazil) sites, suggesting restricted gene flow at a regional scale. The genetic structure of this species could be divided into two distinct groups due to a limited gene flow between southern and northern regions as a consequence of larval dispersal patterns. Coastal currents in the vicinity of the Rio de la Plata likely act as a barrier to dispersal within the species range. Moreover, genetic data indicate that populations of N. granulata might have undergone a northward demographic expansion since the late Pleistocene. The morphometric analysis showed no geographical pattern of morphological differentiation, although there were differences among sampling sites.

Density affects mating mode and large male mating advantage in a fiddler crab

Fiddler crabs show two different mating modes: either females search and crabs mate underground in male burrows, or males search and crabs mate on the surface near female burrows. We explored the relationship between crab density, body size, the searching behavior of both sexes, and the occurrence of both mating modes in the fiddler crab Uca uruguayensis. We found that crabs change their mating mode depending on their size and crab density. Crabs mated mostly on the surface at low densities, and underground at high densities. The proportion of wandering receptive females but not courting males accounted for the variation in mating modes. This suggests that whether crabs mate underground (or on the surface) is determined by the presence (or absence) of searching females. We found that the change in the mating mode affected the level of assortative mating; males mating underground were bigger than those mating on the surface, suggesting active female choice. Given that fiddler crabs experience multiple reproductive cycles, they are prone to showing behavioral plasticity in their mating strategy whenever the payoffs of using different mating modes differ between reproductive events. Our results suggest that the incorporation of different levels of environmental variability may be important in theoretical models aimed at improving our understanding of the evolution of alternative mating tactics and strategies.

Hood-building dynamics and mating mode in the temperate fiddler crab Uca uruguayensis Nobili, 1901

Courting males of 18 species of fiddler crabs ( Uca Leach, 1814) are known to build mud or sand structures at the entrances of their burrows. Females orient to these structures when seeking mates and, in some species, males sometimes orient to their own structures as well to relocate their burrows. We studied hood building in the temperate species Uca uruguayensis Nobili, 1901, the southernmost fiddler crab species, which mates both underground in males’ burrows, especially at high densities, and on the surface at the entrance to females’ burrows, a more common mode at low densities. Uca uruguayensis is relatively inactive during the winter and it was expected that the intensity of hood building would vary seasonally, with more hoods built when underground mating was more common. Courting male U. uruguayensis built nearly symmetrical cupped hoods of muddy sand, approximately half as high and two-thirds as deep as wide. Male courtship and mating occurred in summer from November 2001 to January 2002, but hood building was largely restricted to the last semi-monthly cycle, when the maximum number of matings were coincident with the maximum occurrence of hoods. The predominance of hood building at the end of the season may reflect the amount of time following winter inactivity that males need to feed before they exceed a threshold in the trade-off between allocation of resources to growth or reproduction. Contrary to expectations, males built more hoods at low densities where inter-burrow distances were greater. Males more often build hoods at lower densities because hoods enable them to venture further from their burrows to court both passing and burrow resident females. The temporal pattern of hood building by male U. uruguayensis may therefore reflect the mechanisms courting males use to relocate their burrows as well as variation in the social and spatial context of courtship and mate choice.

Mudflat Use and Predation on Male Southwestern Atlantic Fiddler Crabs (Uca uruguayensis) by Gull-billed Terns (Gelochelidon nilotica)

Abstract. Gull-billed Terns (Gelochelidon nilotica) comprise a group of cosmopolitan subspecies that make use of a wide range of aquatic ecosystems and have a broad dietary spectrum. This study documents the use of a southwestern Atlantic mudflat in Argentina and depredation of the southwestern Atlantic fiddler crab (Uca uruguayensis) by Gull-billed Terns. Gull-billed Terns preyed exclusively on male fiddler crabs. This may have been due to the observed presence of Gull-billed Terns mainly between 2 and 3 hr after low tide, when more than 80% of fiddler crabs active on the surface were males. Gull-billed Terns spent 66.9% of the time flying over and feeding on fiddler crabs. Gull-billed Terns landed without feeding 9.3% of the time. Gull-billed Terns succeeded in capturing prey in 29.8% of cases. Food capture rate of Gull-billed Terns was 68.9 items/hr, which is higher than rates reported for the species feeding on fiddler crabs in Mauritania and Guinea Bissau.