Ecologia populacional do caranguejo chama-maré Minuca mordax (SMITH, 1870) (Decapoda, Ocypodidae) em um estuário da costa amazônica brasileira

Abstract

The fiddler crabs are characteristic of the intertidal zone of tropical and subtropical regions, especially in estuarine environments, more then they perform an important role in nutrient cycling due to sediment bioturbation. The study aimed to describe the population structure of fiddler crab Minuca mordax in areas with different salinities in an estuary on the Amazon coast of the state of Pará, Brazil. Population aspects (sex ratio, size, density, biomass and reproductive period) were studied in four areas (E1, E2, E3 and E4) along a salinity gradient and in seasonal periods (rainy and dry). The crabs were collected during twelve quarterly sampling campaigns, six in the rainy season (May/2013, February/2014, May/2014, February/2015, May/2015 and February/2016), and six in the dry season (August/2013, November/2013, August/2014, November/2014, August/2015 and November/2015). In each location and campaign, all crabs were manually sampled in six quadrants (1m2 ) in the intertidal, distributed along two transects (100 m, each) perpendicular to the low tide line. Furthermore, were collected abiotic factors: air and water temperature, pH, salinity, dissolved oxygen and electrical conductivity of the water. In the laboratory, sex was identified and carapace width (CW) measured for each individual. The crabs were classified as mature or immature, that is, individuals larger or smaller than the smallest ovigerous female, respectively. A total of 1001 specimens were collected, 527 males and 474 females (29 ovigerous). The total sex ratio was 1.1:1 (male: female), with no significant deviation between the sexes. Males were larger and more frequent in the larger size classes (>15 mm), while females predominated in the smaller size classes (<13 mm). The density and total biomass of species were higher in the upper areas of the estuary in both periods, where salinity averaged between 2.8 and 19.2 (rainy season) and 20.2 and 33.9 (dry season). In general, the groups of ovigerous, mature and immature females also had their highest numbers in the upper estuary, in both seasonal periods. Our results suggest that population density is influenced by environmental variables related mainly to the spatial gradient, especially salinity. The establishment of the population in different salinity conditions confirms its wide osmoregulatory plasticity.