Morphofunctional analysis of the gastrointestinal tract of an annual killifish: In search of structural traits related to the annual life history

Annual killifishes have active and voracious rates in acquisition of food resources with display of behaviors that allow them to maintain high metabolic rates to deal with the harsh and unpredictable conditions prevalent in temporary pools where they inhabit. The objective of this research was to describe histologically and ultrastructurally, the digestive system of the annual killifish Millerichthys robustus to identify morphological traits related to its annual life history and digestive physiology. Also, we quantify food items along the intestine as a proxy for rates of digestion. Millerichthys has a short digestive system, associated with a carnivorous diet, with no evidence of a stomach. Instead, the presence of pharyngeal jaws with caniform teeth was documented, related to the breakdown of invertebrate exoskeletons, allowing prey fluids to be tasted by taste corpuscles related to selection of food items, and that digestive enzymes penetrate once in the intestine. The histological morphology of the intestine showed four different regions, associated with its digestive rates: (i) reception of food from the esophagus with intact pray; (ii) digestion with enzymes from the pancreas and liver/gallbladder of simple exoskeleton prey (Entognatha), and beginning of absorption; (iii) absorption of nutrients, and digestion of large-complex exoskeleton prey (Hexanauplia, Brachiopoda, and Ostracoda); and (iv) probable absorption of intact macromolecules. The second region of the intestine presented two anatomical loops and the highest thickness that may be related to reducing the speed of food transit, allowing for more efficient digestion given the large amount of food ingested by this species.