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The aim of this study was to determine the lethal salinity (LC 50 ) for the yellow clam Mesodesma mactroides (Bivalvia: Mesodesmatidae) and identify histopathological alterations that could be used to diagnose structural changes in clam tissue. Clams in two size classes (adults and juveniles) were placed in 10 L chambers and exposed to salinities of 35, 30, 25, 20, 15, 10, and 5 g/L. There were triplicate chambers with seven clams each for each salinity. The LC 50 values for a 48 h exposure were 6.5 g/L and 5.7 g/L for adults and juveniles, respectively. For a 96 h exposure, the LC 50 values were 10.5 g/L for adults and 8.8 g/L for juveniles. The histological examination of yellow clams exposed to 10 g/L for 96 h showed intercellular oedema and necrotic foci in the epithelium of the digestive gland and occlusion of the lumen of the digestive gland. In conclusion, M. mactroides can be characterised as a moderately euryhaline species, tolerating salinities from 35 to 15 g/L.Keywords: yellow clam, lethal salinity, LC 50 , histology, extreme southern Brazil. Efeito da salinidade reduzida no marisco branco Mesodesma mactroidesResumo O objetivo deste estudo foi determinar a salinidade letal (CL 50 ) para o marisco branco Mesodesma mactroides (Bivalvia: Mesodesmatidae) e as alterações histopatológicas que poderiam ser úteis para o diagnóstico de mudanças estruturais no tecido dos bivalves. Mariscos de duas classes etárias de tamanha (juvenis e adultos) foram colocados em recipientes de 10 L e expostos a salinidades de 35, 30, 25, 20, 15, 10 e 5 g/L. Os tratamentos foram realizados em triplicata com sete bivalves em cada recipiente, A CL 50 para 48 h de exposição foi 6,5 g/L e 5,7 g/L para adultos e juvenis, respectivamente. Para 96 h de exposição, a CL 50 foi 10,5 g/L para adultos e 8,8 g/L para juvenis. O exame histológico dos mariscos expostos à salinidade de 10 g/L por 96 h revelou edema intracelular e focos necróticos no epitélio da glândula digestiva e oclusão da luz da glândula digestiva. Em conclusão, M. mactroides pode ser considera uma espécie eurialina moderada, tolerando salinidades de 35 até 15 g/L.Palavras-chave: marisco branco, salinidade letal, CL 50 , histologia, extremo sul do Brasil.

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The yellow clam Mesodesma mactroides is a valuable shellfish occurring from the southeastern coast of Brazil to the northern coast of Argentina. Populations of yellow clams are disappearing from their entire range, and the cause is still unknown. The objective of this paper was to search for World Organisation for Animal Health (OIE)-listed pathogens and their relatives in the genera Marteilia, Bonamia, and Perkinsus as well as Mikrocytos mackini and the virus OsHV-1 μ var the yellow clam population in southern Brazil using molecular techniques and classic histology protocols. A total of 180 clams were manually collected in the intertidal region at six sampling points covering the entire coast of Rio Grande do Sul State (length, 622 km) in 2013. Tissue samples were tested by OIE-recommended single-step conventional polymerase chain reaction assays. The screening showed no evidence of the specific sequences of the protistan parasites and viral pathogens at any site within the six zones under study. We recommend continuous monitoring of the mollusks in the region. Received July 3, 2015; accepted February 3, 2016.

This paper reports on the acute inflammatory and cellular process in the yellow clam, Mesodesma mactroides, induced by injection of India ink into the muscular foot. Histological observations with optical and electronic microscopy were made at 24 and 48 h after injection. The induced cellular inflammatory response consisted of a general hemocyte infiltration without necrosis and apoptotic activity. Migration of ink-laden phagocytes across the intestinal epithelium was recorded. It appeared that the yellow clam "excreted" ink particles through the gill and kidney. The positive staining for apoptosis was observed in the digestive gland. Electronic microscopy revealed ultrastructural changes of endoplasmic reticulum stress and apoptotic bodies in the digestive gland. The mechanism by which the India ink particles induce apoptosis remains unknown, but might possibly be associated with the endoplasmic reticulum stress. This work has highlighted features that require further discussion in the restricted field the inflammatory responses of mollusks.