The histochemistry of gastric secretion has been little studied in fish, although there is agreement on the presence of only one type of secretory cell in fish which stains positively for indicators of pepsinogen (pepsin precursor) cells. There is some question whether there may be more than one pepsin present in some fish, but no chromatographic or other tests have been done to investigate this. Several attempts have been made to identify acid-secreting cells, but results were either negative or confusing.
Other gastric enzymes have been proposed, but not firmly identified. Chitinolytic activity with an optimum at pH 4.5 was claimed for the stomach of Salmo irideus, but in most cases is probably from exogenous sources. If fish are like higher vertebrates, then the stomach wall also produces the hormone gastrin which stimulates gastric secretion. A lipase may also be present.