ABSTRACT The gastroenteropancreatic system in elasmobranchs presents a number of firsts in the evolutionary history of vertebrates. This group of fishes were the first to have evolved an acid–pepsin-secreting stomach and demonstrate a glandular pancreas with exocrine and endocrine cells. They additionally were the first to have evolved a well-developed spiral or scroll-like intestine in comparison to the precursor typhlosole – an infolding of the intestinal wall – found in lampreys. Coupled with these features, the unusual metabolic strategy of elasmobranchs is characterized by a lack of reliance on fatty acid oxidation in muscle tissue, and a heavy reliance on ketones and amino acids as metabolic fuels. Further, the ureosmotic strategy of marine elasmobranchs makes them particularly nitrogen limited and thus reliant on regular high-protein meals. Central to these physiological oddities is the gastrointestinal tract (GIT), with absorption of macronutrients and micronutrients being key to maintaining appropriate osmotic balance alongside ensuring continued energy production to fuel somatic growth. Interestingly, the relationship between the physiology of the GIT, energy balance and osmoregulation is only just being explored in elasmobranchs. Here, we review the known processes involved in the assimilation of dietary nutrients in elasmobranchs, from acid digestion to uptake across the spiral intestine. We provide an understanding of the physiological role of accessory organs (gall bladder and pancreas), and we suggest a key role for the (thus far) barely recognised intestinal microbiome in the nitrogen cycle of these fish.
Drummond et al. (Tue,) studied this question.
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