Thermal tolerance and routine oxygen consumption of convict cichlid, Archocentrus nigrofasciatus, acclimated to constant temperatures (20 °C and 30 °C) and a daily temperature cycle (20 °C → 30 °C).

Organismal temperature tolerance and metabolic responses are correlated to recent thermal history, but responses to thermal variability are less frequently assessed. There is great interest in whether organisms that experience greater thermal variability can gain metabolic or tolerance advantages through phenotypic plasticity. We compared thermal tolerance and routine aerobic metabolism of Convict cichlid acclimated for 2 weeks to constant 20 °C, constant 30 °C, or a daily cycle of 20 → 30 °C (1.7 °C/h). Acute routine mass-specific oxygen consumption ([Formula: see text]O2) and critical thermal maxima/minima (CTMax/CTMin) were compared between groups, with cycle-acclimated fish sampled from the daily minimum (20 °C, 0900 h) and maximum (30 °C, 1600 h). Cycle-acclimated fish demonstrated statistically similar CTMax at the daily minimum and maximum (39.0 °C, 38.6 °C) but distinct CTMin values, with CTMin 2.4 °C higher for fish sampled from the daily 30 °C maximum (14.8 °C) compared to the daily 20 °C minimum (12.4 °C). Measured acutely at 30 °C, [Formula: see text]O2 decreased with increasing acclimation temperature; 20 °C acclimated fish had an 85% higher average [Formula: see text]O2 than 30 °C acclimated fish. Similarly, acute [Formula: see text]O2 at 20 °C was 139% higher in 20 °C acclimated fish compared to 30 °C acclimated fish. Chronic [Formula: see text]O2 was measured in separate fish continually across the 20 → 30 °C daily cycle for all 3 acclimation groups. Chronic [Formula: see text]O2 responses were very similar between groups between average individual hourly values, as temperatures increased or decreased (1.7 °C/h). Acute [Formula: see text]O2 and thermal tolerance responses highlight "classic" trends, but dynamic, chronic trials suggest acclimation history has little effect on the relative change in oxygen consumption during a thermal cycle. Our results strongly suggest that the minimum and maximum temperatures experienced more strongly influence fish physiology, rather than the thermal cycle itself. This research highlights the importance of collecting data in both cycling and static (constant) thermal conditions, and further research should seek to understand whether ectotherm metabolism does respond uniquely to fluctuating temperatures.

Temperature tolerance and oxygen consumption of two South American tetras, Paracheirodon innessi and Hyphessobrycon herbertaxelrodi.

Temperature is a primary factor affecting species' ability to thrive in a particular ecological niche, but thermal conditions have changed dramatically in recent decades. Fishes shift their thermal tolerance range with a maximum and minimum temperature correlated to their recent thermal acclimation history, and species can show a reduced temperature quotient (Q10) following chronic thermal acclimation. Neon tetra (Paracheirodon innesi) and Black Neon tetra (Hyphessobrycon herbertaxelrodi) are popular hobbyist aquarium fishes, and both species are examples of freshwater teleosts native to South American river systems that are potentially affected by changing thermal conditions. We acclimated these species to three different constant temperatures (26 °C, 29 °C, and 31 °C) for 15.4 ±â€¯2.1 days, then measured acute critical thermal maxima (CTMax) and acute oxygen consumption rate (Mo2) at each acclimation temperature. We also estimated chronic lethal thermal maximum (CLT) for both species following a 2-week acclimation to 30.4 °C. Mean CTMax of both species were found to increase with acclimation temperature from 38.5 to 39.6 °C for Neon tetra and from 39.5 to 41.0 °C for Black Neon tetra, gaining 0.24 (Neon tetra) or 0.29 °C (Black Neon tetra) of tolerance per 1 °C of acclimation. However, Black Neon tetra demonstrated consistently higher CTMax (1.0-1.4 °C). CLT was lower for Neon tetra (33.5 °C), compared to Black Neon tetra (35.9 °C). Mean Mo2 were statistically similar across acclimation temperatures within species; Q10 between 26-31 °C were 1.92 and 1.22 for Neon and Black Neon tetra, respectively. Neon and Black Neon tetras physiologically acclimated to changing thermal demands, and although they demonstrate robust CTMax responses, CLT responses indicated both species are unable to survive temperatures 4-5 °C above current average natural values. The demonstrated metabolic plasticity and CTMax values provide a moderate cushion for both species to combat changing temperatures due to climate change, but CLT values suggest vulnerability to projected climate trends.

Scaling of major organs in hatchling female American alligators (Alligator mississippiensis).

Allometric equations represent relationships between morphological/physiological traits and body mass Y = aMb , where Y is the trait, a is elevation, b is the exponent describing the shape of the line, and M is body mass. We measured visceral organ masses in hatchling alligators (Alligator mississippiensis) from five clutches from approximately 45 to 500 g wet body mass. The interaction between initial egg mass and clutch identity was significant for initial hatchling mass, but only egg mass, not clutch, had a significant effect on initial snout-vent and head length. Kidney and liver mass showed biphasic scaling with body mass, as determined by "breakpoint" analyses, with the breakpoint at 120 g wet body mass. Kidney and liver wet mass showed slopes b > 1.0 as animals increased approximately 45-120 g, with significantly lower b approximately 0.8-0.9 for alligators 120-500 g. Within kidney and liver mass, below and above the breakpoint, organ mass slopes tended to be similar across clutches. Lung and heart wet mass did not show biphasic scaling, with b approximately 0.8-0.9. Within lung and heart mass, clutches had statistically identical slopes. Combined clutch data for wet mass showed distinct regressions with b > 1.4 for approximately 45-120 g alligators' kidney and liver mass, compared with approximately 120-500 g alligators' kidney, liver, lung, and heart mass b < 1.0. Alligators show rapid kidney and liver growth following hatching, with higher rates than lung or heart tissue. Clutch, egg mass, and hatchling size influence organ size, and each factor should be accounted for in future studies exploring reptile morphology and physiology to assess environmental versus clutch contributions.