COMPREHENSIVE STUDY OF THE DIGESTIVE TRACT IN THE EUROPEAN HAKE (Merluccius merluccius)

The European hake (Merluccius merluccius) is an important commercial fish that is widespread in the Adriatic Sea. It is a carnivorous fish whose diet consists mainly of fish. The aim of this study was to perform a detailed analysis of the digestive tract in the European hake. Therefore, the anatomy and histology of the posterior part of the digestive tract (from the esophagus to the rectum) were described. The fish were caught by longline fishing in the middle part of the Adriatic Sea along the Croatian coast. A total of 33 adult specimens were analyzed. Tissue components were visualized using hematoxylin-eosin, Mallory's trichrome, Verhoeff-Van Gieson, Alcian Blue-PAS kit and a reticular fiber staining kit. To investigate cellular digestion, the localization and activity of alkaline phosphatase, acid phosphatase, non-specific esterase and aminopeptidase were measured. All parts of the digestive tract were composed of mucosa, submucosa, muscularis and serosa. The type of epithelium varied from part to part. The muscular layer of the mucosa was not identified in the esophagus, the intestine proper and the rectum. The muscularis consisted of smooth muscle cells, except in the esophagus, where it consisted of striated muscle fibers. All parts of the digestive tract are involved in the digestion and absorption of nutrients. Results observed from optical density of enzymes highlighted that although the intestine was the main site for lipid and protein digestion, the stomach and rectum were also important locations for protein digestion. Although the anatomy and histology of the digestive tract in European hake have been partially described, there is no data on the optical density of the enzymes in the available literature. To achieve objective results that allow for precise data comparison, the optical density of the enzymes was measured in this study. This research provide comprehensive findings and introduced new knowledge that significantly expands and partly differs from what is known so far, highlighting the necessity for further studies in this area. Celostna študija prebavil evropskega osliča (Merluccius merluccius) Izvleček: Evropski oslič (Merluccius merluccius) je pomembna komercialna riba, ki je razširjena v Jadranskem morju. Je mesojeda riba, ki se prehranjuje predvsem z ribami. Namen raziskave je bil opraviti podrobno analizo prebavil evropskega osliča, zato sta bili opisani anatomija in histologija prebavil (od požiralnika do danke). Ribe so bile ulovljene s parangalom v Jadranskem morju ob hrvaški obali. Skupaj je bilo analiziranih 33 odraslih osebkov. Komponente tkiva so bile vizualizirane z uporabo hematoksilin-eozina, Malloryjevega trihroma, Verhoeff-Van Giesona, kompleta Alcian Blue-PAS in kompleta za barvanje z mrežastimi vlakni. Da bi raziskali celično prebavo, smo izmerili lokalizacijo in aktivnost alkalne fosfataze, kisle fosfataze, nespecifične esteraze in aminopeptidaze. Vsi deli prebavnega trakta so sestavljeni iz sluznice, podsluznice, mišične plasti in seroze. Vrsta epitelija se razlikuje od dela do dela. V požiralniku, črevesu in danki nismo ugotovili mišične plasti sluznice. Mišična plast je sestavljena iz gladkih mišičnih celic, razen požiralnika, kjer jo tvorijo prečno progasta mišična vlakna. Vsi deli prebavnega trakta sodelujejo pri prebavi in absorpciji hranil. Glavno mesto za prebavo lipidov in beljakovin je sicer črevo, vendar sta želodec in danka prav tako pomembna za prebavo beljakovin. Čeprav sta anatomija in histologija prebavnega trakta pri evropskem osliču delno opisani, v razpoložljivi literaturi ni podatkov o optični gostoti encimov. Opisana raziskava, opravljena na 33 osebkih, prinaša izčrpne ugotovitve in nova spoznanja, ki bistveno razširjajo doslej znane in se delno razlikujejo od njih, ter poudarja potrebo po nadaljnjih študijah na tem področju. Ključne besede: evropski oslič; Merluccius merluccius; histologija; histokemija; prebava; encimi