Histology and ultrastructure of the gastrointestinal tract in four temperate marine herbivorous fishes

Abstract

While alimentary tract anatomy in many terrestrial herbivorous vertebrates is well documented, the digestive systems of marine herbivorous fishes are poorly characterised. The gastrointestinal tract (GIT) of four species of marine herbivorous fishes from northeastern New Zealand, butterfish Odax pullus (Labridae), marblefish Aplodactylus arctidens (Aplodactylidae), notch-head marblefish A. etheridgii (Aplodactylidae) and silver drummer Kyphosus sydneyanus (Kyphosidae), were examined using histology and transmission electron microscopy (TEM) to provide a detailed histological and ultrastructural description of gut anatomy. Gastric glands were distributed over rugae in the stomach of A. arctidens, A. etheridgii and K. sydneyanus. The luminal surface of the stomach of these three species was lined by columnar mucous cells, and oxynticopeptic cells lined the glands in the stomach. Villi were present along the length of the intestine in all four species. The anterior intestine had thin musculature, and was lined by absorptive cells with long microvilli and numerous small vesicles in the apical cytoplasm. The posterior intestine was lined by absorptive columnar cells with long microvilli, invaginations between microvilli with electrondense membranes, and pinocytotic vesicles. Surface area generally decreased from the anterior to posterior intestine. Histological and ultrastructural results were consistent with lipid absorption occurring in the anterior GIT and protein absorption in the posterior GIT. The results of this study indicate clear differences in GIT structure among the study species, and digestion models based on chemical reactor theory were developed to characterise these differences.

Distinct microbiota composition and fermentation products indicate functional compartmentalization in the hindgut of a marine herbivorous fish

“…Fish were removed immediately from the water and processed aboard the University of Auckland research vessel RV Hawere. The gut was removed and divided into five segments, numbered I–V, as described previously (Johnson & Clements, 2021 ). The samples used for the total microbiota counts were three adult K .…”

Fine scale transitions of the microbiota and metabolome along the gastrointestinal tract of herbivorous fishes

“…It has been suggested that microbial community differentiation along the gut facilitates distinct processes that aid in the stepwise digestion and utilization of algal biomass, yielding distinct chemistries in each of the gut sections [ 21 ]. Histological data from marine herbivorous fishes including members of Kyphosidae ( Kyphosus sydneyanus ) reveal direct evidence of morphological specialization along the gastrointestinal tract and altered absorptive modes indicative of the prominent roles of microbial metabolism of algae in the posterior gastrointestinal tract [ 25 ]. In parallel to morphological evidence, carbohydrase enzyme activity assays have revealed spatial variation in microbial contributions to the breakdown of starch, laminarian, carrageenan, alginate, and agarose, all of which were elevated in the posterior portions of the gut [ 26 ], as well as functional partitioning between endogenous breakdown of starch and exogenous (microbial) breakdown of structural carbohydrates [ 27 ].…”

Tannockella kyphosi gen. nov., sp. nov., a member of the family Erysipelotrichaceae, isolated from the hindgut of the marine herbivorous fish Kyphosus sydneyanus

“…Fish were processed aboard the University of Auckland research vessel RV Hawere. The gut was removed and divided into five segments, numbered I–V, as described previously [20]. The contents of section IV of the hindgut of a freshly captured fish were transported to the laboratory under anaerobic conditions.…”