Consistent seasonal flexibility of the gut and its regions across wild populations of a winter-quiescent fish

Here, we investigated phenotypic flexibility in the size of the gastrointestinal (GI) tract across three northern populations of a winter-dormant warm-water fish, pumpkinseed sunfish (Lepomis gibbosus). The dried masses of all gut regions (stomach, pyloric caeca, intestine) were measured approximately biweekly between January and August 2021. In all populations, pumpkinseed exhibited pronounced structural flexibility in the GI tract, aligned with winter and the timing of reproduction. The dry mass of the GI increased by 1.3- to nearly 2.5-fold in the early spring. The pyloric caeca demonstrated the greatest capacity for flexibility, increasing by up to 3.7-fold prior to reproduction. In all populations, minimum dry GI mass was consistently achieved during winter and mid-summer. This capacity for gut flexibility may represent a novel mechanism for facilitating rapid adaptive responses (e.g., metabolic plasticity) to future environmental change.

Methods
Data are reported for female pumpkinseed sunfish from 60 to 110 mm total length from three pond populations in Southern Ontario (GSP - Pond A, PP - Pond B, KSS - Pond C). Ponds are located on private property in King City, Ontario at the Koffler Scientific Reserve and Stouffville, Ontario. All fish were captured using rod-and-reel angling or clover fish traps baited with chicken-flavoured dog kibble. Fish were measured (0.1 cm), weighed (0.001 g), and fully dissected to process and weigh all internal organs (digestive tract, liver, gonads, stomach contents). Data have been trimmed to include only information included in the analysis.