The relationship between growth rate and ossification is not consistent among fish. In some species, fast-growing individuals ossify earlier, while in other species, a trade-off between growth rate and ossification exists. This research was conducted to test the hypothesis about the existence of a trade-off mechanism in common carp offspring. For this purpose, specimens from two groups of common carp offspring (96 in total) with different growth rates were sampled at multiple time points between 5 and 29 days post-hatching. The standard length was measured and presented in correlation with the days post-hatching. To evaluate the skeletal system development, whole specimens were fixed in 10% neutral buffered formalin and stained for bone and cartilage. Particular emphasis was placed on the onset of ossification in the clearly visible bones of the head, vertebral column, and fins. According to our findings, larger, fast-growing specimens ossify earlier. The hypothesis on the existence of a trade-off between fast growth and the onset of ossification in common carp was not confirmed in our study.

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

The small-spotted catshark and the smooth-hound are cartilaginous, carnivorous fish with similar depth ranges in their habitats. These two species are among the most abundant elasmobranchs in the Adriatic Sea and are frequently caught by local fishermen using longline fishing. Despite their ecological similarities, little is known about the physiological differences in their digestive processes. The study of enzymatic digestion in these ecologically relevant species helps to fill the knowledge gap in the understanding of nutrient processing in cartilaginous fish. Therefore, the aim of this study was to determine, measure and compare the enzymatic activity of alkaline phosphatase, acid phosphatase, non-specific esterase and aminopeptidase. Fish were caught in the central part of the Adriatic Sea between 2021 and 2023. A total of 60 adult individuals were analyzed, with samples taken from six parts of the digestive tract. Histochemical analysis of 1440 slides revealed clear differences in enzyme activity between the two species. In the small-spotted catshark, cellular protein degradation was most pronounced in esophagus, posterior stomach and rectum, whereas in the smooth-hound, it was concentrated in posterior stomach and spiral intestine. Cellular digestion of lipids in the small-spotted catshark appears to occur primarily in the stomach. The results of this study provide new insights into the distribution of cellular digestive enzymes in cartilaginous fish and emphasize the importance of studying the entire digestive tract as an integrated system rather than focusing on individual parts. This study fills an important knowledge gap and contributes to a deeper understanding of digestive physiology, which in turn has implications for species conservation and biological variability.

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