Pathologies affecting the Yellow Clam Mesodesma mactroides (Bivalvia: Mesodesmatidae)

Author: Yuri Bovi Morais Carvalho (Currículo Lattes)
Supervisor: Dr Luis Alberto Romano
Co-supervisor: Dr Luis Henrique da Silva Poersch

Abstract

The present work analyzed the health status of Mesodesma mactroides sand mollusk from the coast of the State of Rio Grande do Sul, Brazil. Histopathological analyzes were performed on adult molluscs collected during the four seasons of 2012 at Praia do Cassino (Chapter 1). In Chapter 2, tissue samples were analyzed, using optical microscopy and transmission electron microscopy, of dying juveniles of M. mactroides during a massive mortality event in 2011. In Chapter 3, diseases were screened of mandatory notification by the World Organization for Animal Health (OIE) (marteiliosis, perkinsose, bonamiosis, mikrocytosis and infection by the herpes ostreides virus - OsHV μ-var) with the use of molecular analysis using the polymerase chain reaction (PCR) technique ) in M.mactroides collected from Capão da Canoa (northern coast of Rio Grande do Sul) to Chuí (southern border of the state). In Chapter 4, the effect of low salinity on the survival of juveniles and adults of M. mactroides and the associated pathological condition were analyzed. Finally, in Chapter 5, an experiment was carried out evaluating the ability of white seafood to react to the presence of a foreign body (carbon particles) injected into the muscular foot. The most common parasites were the ciliate Trichodina sp. located in the gill and Paravortex turbodelary mesodesma in the intestinal lumen. Despite no record of damage to the hosts, trichodines when in high density can obstruct the gills and the turbellaria can cause the occlusion of the light from the intestinal tube. Parasites of intermediate importance,as coccids similar to Pseudoklossia and gregarines Nematopsis sp. were found rarely and at low intensity, causing a low inflammatory response in the hosts. Indeterminate metacercariae of Digenea located at the tip of the siphons causing the rupture of muscle fibers and slight hemocyte infiltration were observed in low prevalence and intensity. Indeterminate sporocysts of Digenea have been recorded rarely between the digestive tubules causing severe hemocyte response. Prokaryotes of the Rickettsia genus were rarely observed in adult clams, however these organisms were registered in 100% of dying juveniles, soon being associated with the mortality event. Through the PCR analysis, no disease was mandatory to be reported by the OIE in M. mactroides.The average lethal salinity for 48h of exposure was 6.5 and 5.7 for adults and juveniles, respectively. For 96h of exposure, mean lethal salinity was 10.5 and 8.8 for adults and juveniles, respectively. Shellfish exposed to salinities below 15 showed intracellular edema and necrotic foci in the epithelium of the digestive tubules and occlusion of the digestive tubules' lumen. Through the experiment carried out in Chapter 5, it was possible to demonstrate important morphological changes related to inflammation and cell death resulting from the injection of ink in white seafood, which were efficient in eliminating foreign material through phagocytosis by hemocytes, diapedesis by intestinal epithelium and also through the gills and kidney.Electron microscopy analysis showed apoptosis and endoplasmic reticulum stress in cells in which the injected carbon particles accumulated.

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