Influência da exposição de pradarias marinhas na maré baixa sobre as assembleias de moluscos associadas, na Costa Branca-RN

Abstract

The present study observed the tidal influence on the mollusc assemblages in the seagrass meadows of Costa Branca-RN. Using transects with 100 m in areas exposed to air and submerged areas during the low tide, in the 6 seagrass meadows found in the region, 10 equidistant samples (10m) were collected using a PVC core with 10 cm in diameter, buried at 10 cm depth. The samples were washed in seawater, using a 0.5 mm nylon mesh for macrofauna retention, tagged, frozen and transported to the Laboratório de Zoobentos do Instituto de Ciências do Mar, da Universidade Federal do Ceará (Labomar/UFC), where they were sorted, preserved in 70% alcohol and the species were identified. Posteriorly, frequency of occurrence, relative dominance and species density were obtained for the region and also the ecological descriptors (Shannon Diversity, Pielou evenness and Margalef richness) were obtained for each sample. Then, the variation of these descriptors was compared between the two areas of the meadows (exposed and submerged) using non-parametric analysis of variance (Kruskall-Wallis); similarities and dissimilarities among the meadows and between the two evaluated areas of these meadows in the Costa Branca region also were studied using a multidimensional scaling (MDS). Of the 803 molluscs collected in the meadows of the region, 251 specimens were captured in the areas exposed to air during low tide; and 411 specimens, in the submerse areas of the meadows. These specimens referred to 50 species belonging to Bivalvia (66%), Gastropoda (30%) and Scaphopoda (4%) Classes. The most frequent and dominant species in these meadows were the gastropod Benthonella tenella followed by Eulithidium affine, both species had the same density. Comparing the ecological descriptors of the mollusc assemblages and similarities between the samples collected in the areas exposed to air and submerse during the low tide, it was observed that the areas always submerse of the meadows were significantly more abundant, rich and diverse than that of the exposed areas. The stress caused by the sporadic exposure to air, sunlight and other physical factors, as wave crash, cause not only a habitat loss, with plant and associated macroalgae reduction in these areas, and also it can select the occurrence of mollusk species and their resistance to these stressors. The relationship of these species with the plant characters and environmental variables are recommended, for a better understanding of the biology of these species, in the seagrass meadows of Costa Branca.