Comparative study on the gastrointestinal tract of mature and maturing female and male of Caspian lamprey, Caspiomyzon wagneri (Kessler, 1870) in the southern Caspian Sea (Cephalaspidomorphi: Petromyzontidae)

Present study was conducted to elucidate the nutritional value and histological events of gastrointestinal tract (GT) between mature and maturing female and male Caspian lamprey (Caspiomyzon wagneri) during their spawning migration to the freshwater rivers in southern Caspian Sea (Shirud River, Mazandaran Province, Iran). From the upstream migrating Caspian lamprey population, 60 fish specimens were randomly collected. After checking the intestine content, samples from intestine and liver tissues were removed for histological studies for two groups of mature and maturing specimens either in female or male separated based on external and internal sexual characteristics. According to the histological observation, many folds, connective tissues, interstitial fluid, goblet–shaped cells, enterocytes and numbers of intestinal villi were observed in the intestine. Also, there was an intermediate structure named “typhlosole” in the entire intestine. Furthermore, height and folds of the typhlosole were smaller at the beginning and the end of it. Significant differences were observed in the numbers of fold of intestine in mature and maturing females (P