Constructing a Fish’s Complete Thermal Tolerance Polygon: Effective Practices for Polygon Experiments using Mottled Catfish Corydoras paleatus

Critical Thermal Methodology (CTM) and Chronic Lethal Methodology (CLM) experiments were utilized to construct a complete thermal tolerance polygon for the South American fish species, Mottled catfish (Corydoras paleatus). Mottled catfish showed Chronic Lethal Maximum (CLMax) of 34.9±0.52°C and Chronic Lethal Minimum (CLMin) of 3.8±0.08°C. Fish were chronically acclimated (~2 weeks) to 6 temperatures ranging from 7.2±0.05°C →32.2±0.16°C (7°C, 12°C, 17°C, 22°C, 27°C, and 32°C), and CTM used to estimate upper and lower acute temperature tolerance. Linear regression of Critical Thermal Maximum (CTMax) and Minimum (CTMin) data with each acclimation temperature were used along with CLMax and CLMin to create a complete thermal tolerance polygon. The highest CTMax was 38.4 ±0.60°C for fish acclimated to 32.2±0.16°C, and the lowest CTMin was 3.36±1.84°C for fish acclimated to 7.2±0.05°C. Mottled catfish have a polygon measuring 785.7°C2, and the slope of the linear regressions showed the species gained 0.55°C and 0.32°C of upper and lower tolerance, respectively. We compared slopes of CTMax or CTMin regression lines to each other using a set of comparisons between 3, 4, 5, or 6 acclimation temperatures. Our data demonstrated that 3 acclimation temperatures were as sufficient as 4→6 to pair with estimates of chronic upper and lower thermal limits for accurately determining a complete thermal tolerance polygon.