Ecossistemas mesofóticos e a malacofauna do extremo norte do Brasil: um estudo das expedições oceanográficas GEOMAR II E III

Abstract

Studies on the continental shelf of northern Brazil have shown increasing promise as the understanding of environmental complexity improves, particularly regarding the influence of the Amazon River discharge along the entire coastal zone. In this region, the existence of mesophotic ecosystems plays a significant role in ocean biodiversity and productivity, as they harbor a wide variety of marine species, many of which remain unknown to science. Research conducted during the 1960s and 1970s revealed considerable diversity in the faunal assemblages of the northern Brazilian continental shelf, attributing their characteristics to different sediment types, such as sand, clay, and mud, as well as the presence of geological features like submarine canyons and sandbanks shaped by the Amazon River system. The discovery of mesophotic ecosystems in submarine areas extending from 30 to 150 meters in depth—primarily composed of algae, rhodoliths, sponges, lateritic bottoms, soft corals, and black corals—has enhanced the understanding of the region's faunal richness. These systems are considered essential refuges and breeding grounds for many fish and invertebrate species, as they are less exposed to human impacts and other stressors associated with coastal environments. This study characterized the mollusk assemblages of the Amapá continental shelf based on samples collected during the GEOMAR II and GEOMAR III oceanographic expeditions conducted in the 1970s, contributing to biodiversity assessments and analyzing species distribution patterns in light of historical oceanographic data and current information on mesophotic ecosystems. The studied samples were obtained through dredging operations conducted by the oceanographic vessel Almirante Saldanha and are currently housed in the “Professor Henry Ramos Matthews” Malacological Collection at the Federal University of Ceará. A total of 56 sampling stations were analyzed, covering various sedimentary facies, including arenaceous-biodetritic, external platform biodetritic, muddy, transitional, and terrace facies, reflecting the environmental heterogeneity of the region. In total, 311 mollusk taxa were identified, distributed among the classes Gastropoda, Bivalvia, and Scaphopoda, with bivalves being the most predominant, especially in arenaceous-biodetritic facies associated with consolidated substrates. The highest species diversity was observed in areas under the influence of the Great Amazon Reef System (GARS), suggesting a positive relationship between biogenic reefs and mollusk richness, with notable species such as Arca domingensis, Arca zebra, and Nuculana acuta. The collector’s curve indicated that the sampling effort was insufficient to fully characterize the region's molluscan fauna, highlighting the need for further studies to achieve a more comprehensive assessment. The analyzed material was incorporated into the Professor Henry Ramos Matthews Malacological Collection (CMPHRM), contributing to the preservation and expansion of knowledge about the marine biodiversity of the Amazon shelf. This study underscores the importance of environmental monitoring initiatives, especially in light of the growing economic activities in the region, such as oil exploration, which may threaten the integrity of these unique reef ecosystems.