Comparing glucose acquisition strategies between two ancient fish species: Lake sturgeon (Acipenser fulvescens) and North Pacific spiny dogfish (Squalus suckleyi)

We assessed the functional role of the spiral valve in carbohydrate digestion and glucose acquisition during different feeding states in Acipenser fulvescens and Squalus suckleyi. For S. suckleyi, maltase activity was highest in the anterior and mid spiral valve, while sodium-glucose linked transporter 1 (sglt1) transcripts peaked in the mid spiral valve. Alongside these metrics, glucose tissue uptake demonstrated decreased maximal transport rates from 24 h to 7+ days post feeding, demonstrating a putative means for energy conservation. A. fulvescens showed the highest maltase activity and sglt1 abundance in regions anterior to the spiral valve (pyloric ceca and anterior intestine). Additionally, glucose transport did not reach saturation in the spiral valve and anterior intestine of A. fulvescens over the measured concentrations, suggestive of a heightened capacity in these regions for glucose uptake. Overall, the spiral valve played a primary role in glucose digestion and transport in S. suckleyi, whereas A. fulvescens utilized the anterior intestine in addition to the spiral valve. Combined, these results also suggest regional functionality of carbohydrate acquisition within the spiral intestine in S. suckleyi but not in A. fulvescens. This demonstrates that the spiral valve may not always be the primary region of nutrient absorption in all species as suggested in the literature, highlighting the importance of comparing morphological and functional studies.