Record of predation by microgastropods Heleobia australis in stromatolites from Lagoa Salgada, State of Rio de Janeiro, Brazil

This work aimed to characterize the shells found in stromatolites from Lagoa Salgada, Rio de Janeiro, and to understand the relationship and influence of these small organisms with the development of the biosfructure in which they occur. A recent sample of domal stromatolite was divided from the bottom to the top into five facies, with the thrombolite facies having a pervasive occurrence of bioclasts identified as micro gastropod shells of the species Heleobia australis. Five hundred specimens of valves of white color, sometimes translucent, with a pearly luster, showing no signs of traction or transport but being mostly intact. The shells vary in size from 1.0 mm to 4.0 mm in length with diameters ranging from 1.0 mm to 2.0 mm. The specimens of different sizes occur together in the cavities of the strata. The life cycles of H. australis were estimated by analyzing the size frequency distribution, in which a young population was identified with a probable short life cycle during stromatolitic development. The variation in the sizes of the shells found, representing different stages of the life of the micro gastropods, suggested that these organisms were not transported and trapped by the stromatolite, but lived in that ecosystem during the development of the stromatolitic structure, and also preying the microorganisms of the stromatolite. H. australis were also responsible for the intense bioerosion present in this sample, culminating in an increase in the porosity of that facies. Finally, the valves are one of the carbonate sources in Lagoa Salgada, due to the biochemical dissolution caused by microbial activity.