Feeding ecology of the snake eel pisodonophis boro (Hamilton, 1822) in Pranburi river estuary, Prachuap Khiri Khan province

Abstract

The snake eel, Pisodonophis boro (Hamilton, 1822), one of the economically important species, is a potential aquaculture fish candidate at Pranburi River estuary. However, the roles in the food web of the ecosystem and the habitat utilization of this fish are still lacking. Hence, feeding ecology, morpho-histological structures of the digestive tract and gonadal histological study are required. Seventy-seven of P. boro were collected from the Pranburi River estuary, during March 2015 to March 2016 for the study of the gut content and morpho-histological structures of the digestive tract. Gonadal tissues of 105 specimens (77 former specimens and 28 additional specimens) were collected for the gonadal histological study. The resulted of gut content and morpho-histological structures of the digestive tract revealed that P. boro is a carnivore and it is considered as a specialist feeder rather than an opportunistic feeder. The results of the gut content demonstrated that the important prey item was crabs: Metaplax elegans, Perisesarma bidens, Sarmatium germaini, and Uca perplexa. The index of relative importance revealed that the most important prey was S. germaini followed by M. elegans, U. perplexa and P. bidens respectively. Morpho-histological study of the digestive tract of P. boro demonstrated the overall anatomical morphology of the digestive tract was elongated, and the stomach appeared as Y-shaped. The digestive tract consisted of four layers: mucosa, submucosa, muscularis and serosa. The histology of the esophagus demonstrated the longitudinal folds and the mucosal epithelium, which was lined with simple squamous epithelium inserting with numerous mucous cells. The mucosal layers in both anterior and posterior regions of stomachs were similarly formed as gastric rugae. Histology of the intestine regions divided into three regions: anterior intestine, middle intestine, and posterior intestine. Numerous intestinal folds were exclusively presented in the anterior region, which was lined by simple columnar cells. Goblet cells were also seen among the epitheliums and positively reacted to Periodic Acid Schiff (PAS) and alcian blue (AB) staining throughout the intestine tract. It indicated that the intestine tract contained mucopolysaccharides and glycoprotein. All specimens (n=105) had the similar gonadal morphology of ovarian structure which contained perinucleolar stage oocyte. The snake eel was considered as a synchronous developmental oocyte based on the histological observation. The morpho-histological study of P. boro digestive tract supported the index of relative importance of the diets of P. boro which implied that the digestive tract of P. boro has undergone evolutionary modifications to optimize digestion and absorption of crab-derived metabolites while also being able to breakdown and then eliminates hard body parts (e.g., carapace, chelipeds) that are not digestible. This study suggested that the role of estuary ecosystem in Pranburi River is important in providing food sources and habitat for snake eel, P. boro. The snake eel did not use the estuary of Pranburi River for spawning purpose but rather as a spawning migration route to the spawning ground.