Study of Histopathological Biomarkers and Their Incidence Time Pattern in Gill, Digestive Gland and Foot in Freshwater Bivalve, Anodonta cygnea, in Exposure to Sublethal Concentration of Chromium (Cr)

The aim of this study was to investigate histopathological alterations and their incidence time pattern due to chromium (Cr) exposure during 18 days in gill، digestive gland and foot in freshwater bivalve، Anodonta cygnea. 24 bivalve specimens with length range of 12. 7-13. 3 cm were collected from Semeskandeh region، Sari. They were exposed to 125 µg l-1 of Cr for 18 days. Bivalves were sampled in days 4، 9 and 18 for obtaining tissue samples of studied organs. Tissue sections of exposed and control group samples were prepared and staining was done using haematoxylin and eosin (H&E) method. Histology of all of three studied organs of exposed bivalves in comparison with control group showed significant histopathological changes. Hypoplasia and changes in shape and size of gill lamellae، granuloma، hyperplasia and atrophy of haemolymph channels were observed. In digestive gland، atrophy of digestive tubules، loss of digestive and basophilic cells from tubules، and haemocyte aggregation and granuloma in connective tissue were occurred. Hypoplasia of external epithelium، increase of mucus cells، and swelling and tissue rapture in myocyte blocks were observed in foot. Primary signs of histopathological alteration were appeared in fourth day، and in following days appearance of new signs and expansion of affected area was detected in all of investigated organs. Based on our results، histological changes due to Cr exposure in studied organs in A. cygnea are proposed as appropriate histopathological biomarkers to monitoring Cr levels in aquatic environments and their incidence time pattern can increase fidelity of these indices.