Marine teleostean fish are hypo‐osmotic to seawater. As part of a multiorgan osmoregulatory strategy they drink seawater and selectively absorb water and minerals across the intestinal epithelium. Notably, divalent cations (Ca2+ and Mg2‒) are left behind. We report here that in the gulf toadfish, Opsanus beta, the ionic by‐products of osmoregulation in the intestine contribute to de novo formation of a carbonate mineral, tentatively identified as calcian kutnohorite. Our data suggest that intestinal mineralization is a general feature of osmoregulation in marine teleosts and that this process is an unrecognized and possibly substantial source of marine carbonate sediments.

Abstract. Under long-term (24 h) control measurements, significant urea was excreted (15 to 90% of excreted nitrogen) by Opsanus beta (Goode and Bean) collected in 1989 from Biscayne Bay, Florida, USA. Urea excretion rates and plasma urea concentrations were not affected by antibiotic treatments which decreased intestinal microbe populations. These results suggest that nitrogen recycling by gut microbe urease is probably not significant in this species. Urea excretion rates increased significantly following 8 h of air-exposure and in response to high levels of NH4C1. These results suggest that urea is synthesized and excreted by the toadfish primarily in situations that limit ammonia excretion. Thus, the ornithineurea cycle in the toadfish liver primarily maintains low concentrations of ammonia within the fish. High levels of variability in urea excretion rates and variation in response to air-exposure suggest that urea synthetic rates are affected by immediate past environmental conditions.

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The study was made to determine if enzymatic degradation of chitin occurs in the digestive tract of the cod, Gadus morhua. The method employed corresponds to the end product measurement of Jeuniaux (1966), using 'native' chitin as the substrate. The following results were obtained.( I ) Chitinolytic enzyme of high activity is present in enzyme solutions from the stomach contents, gastric mucosa and intestinal contents.(2) Lower chitinase activities are found in samples of the intestinal mucosa and the pyloric caeca.(3) The optimum pH ranges for the action of the enzymes in the stomach and the intestine differ: 4.5-5.1 and 5.1-6.5, respectively. (4) The role of chitin-decomposing bacteria is discussed, based on bacterial numbers and pH conditions in the digestive tract. The existence of two different enzyme systems is indicated.

Groups of cod, Gadus niorhuo (L.), were fed exclusively on fish, crustaceans, or crustacean shells for a period of 3 weeks. Chitinase and P-glucosidase activities were measured in enzyme extracts of stomach contents, stomach tissue, pyloric caeca, intestinal contents, and intestine tissue, and compared to the enzyme activities of control fish starved over the same period. Fulton's condition factor K, liver lipid content and liver water content were determined to estimate the effects of the diets on the condition of cod.In general, the highest chitinase activities were measured in samples of cod that had been fed on whole crustaceans. In this feeding group, there was also a remarkable increase of activity by a factor of four to eight in the pyloric caeca, compared to the group fed on fish and the control group, respectively.Measurements of P-glucosidase activity revealed no similar dependence on food quality. 0-glucosidase occurrence seemed to be mainly restricted to the pyloric caeca and the intestine of cod.