Scale cortisol levels vary with habitat type and population in the Mexican tetra Astyanax mexicanus

This study demonstrates that long-term scale cortisol levels in the Mexican tetra (*Astyanax mexicanus*) vary significantly between surface and cave ecotypes and are influenced by habitat stability, indicating that evolutionary adaptations to diverse environments shape the species' chronic stress physiology.

The Gist

The Big Picture: Fish with a "Stress Diary"

Imagine you want to know if a person has been stressed out for the last few months. You could ask them how they feel today (which only tells you about right now), or you could look at their hair. Scientists have found that in fish, scales work like a stress diary.

Just as hair grows and traps chemicals from your body, fish scales grow and trap a stress hormone called cortisol. By measuring the cortisol in a fish's scales, researchers can see a "history" of how stressed that fish has been over a long period, not just in the last few minutes.

The Characters: The Mexican Tetra

The study focuses on a fish called the Mexican Tetra (Astyanax mexicanus). Think of this fish as having two very different "personas" living in the same family tree:

1. The Surface Fish: These live in rivers and lakes. They have eyes, they are colorful, and they live in a chaotic world with predators, changing temperatures, and lots of competition. It's like living in a busy, noisy city.
2. The Cavefish: These live in pitch-black underground caves. They have lost their eyes and pigmentation (they are pale). They live in a world with almost no food, but the temperature and water conditions are very stable. It's like living in a quiet, isolated bunker.

The Experiment: Comparing Stress Levels

The scientists wanted to answer a big question: Which fish is more stressed out in the long run?

They collected fish from:

• A busy river (Rio Choy).
• A man-made lake (Presa El Oyul).
• Four different caves (some very deep and isolated, others that get flooded by rain).

They also raised some of these fish in a lab to see if their stress levels were caused by their genes (nature) or their environment (nurture).

The Findings: What the "Diaries" Said

Here is what the "stress diaries" (the scales) revealed:

1. The Lake Fish were the Most Stressed
Surprisingly, the fish living in the artificial lake had the highest stress levels.

• The Analogy: Think of the lake as a "Goldilocks zone" that went wrong. It's a new environment (only about 45 years old) where the fish haven't had time to adapt. The water conditions there fluctuate wildly, and the fish are constantly dealing with new, unpredictable challenges. They are like people who just moved to a new country and are struggling to adjust to the culture and weather.

2. The Cavefish were the Calmest
The fish from the most isolated cave (Pachón) had the lowest stress levels.

• The Analogy: These fish are like monks who have mastered meditation. Even though they live in the dark with very little food, their bodies have evolved to be incredibly efficient. They don't panic when things are scarce; they have a "low-power mode" that keeps their stress hormones low. This is a genetic superpower they were born with, not just something they learned.

3. It's in the Genes, Not Just the Environment
The scientists raised cavefish and river fish in the same lab tank with the same food and water. Even in this perfect, stress-free environment, the cavefish still had lower stress levels than the river fish.

• The Takeaway: This proves that the "calmness" of the cavefish is written in their DNA. They are genetically programmed to handle stress differently than their surface cousins.

4. The "Flooded" Caves were Chaotic
Some caves (like Rio Subterráneo) get flooded by rain, bringing in surface fish and changing the water chemistry. The fish in these caves had higher stress levels and more variation between individuals.

• The Analogy: These caves are like a house where the door is left open during a storm. The environment is messy and unpredictable, so the fish inside are constantly on edge, leading to a wide range of stress levels among the group.

Why Does This Matter?

This study teaches us two main things:

1. Evolution is fast: Fish can evolve to handle extreme environments (like total darkness and starvation) by changing how their bodies manage stress.
2. Stability vs. Chaos: Sometimes, a "perfect" environment (like a stable cave) leads to lower stress than a "messy" one (like a fluctuating lake). The fish that can't adapt to the chaos (the lake fish) end up the most stressed.

The "Body Condition" Twist

One weird finding was that fish with better body condition (fatter, healthier fish) actually had higher stress levels.

• The Analogy: Usually, we think stress makes you lose weight. But in these fish, it's the opposite. The cavefish are famous for storing fat to survive starvation. It seems their bodies are so good at storing energy that they can afford to keep their stress hormones high without getting sick. It's like a person who is so good at saving money (fat) that they can afford to spend a lot on stress (cortisol) without going bankrupt.

Summary

The scientists used fish scales as a "stress diary" to find out that cavefish are genetically built to be chill, while lake fish are stressed out because they live in a chaotic, new environment. It's a great example of how evolution shapes not just how animals look, but how their bodies handle the pressure of life.

Technical Summary

1. Problem Statement

While acute stress responses in vertebrates are well-documented, the mechanisms governing long-term, cumulative stress in populations adapted to contrasting environments remain poorly understood. Specifically, it is unclear how evolutionary adaptations to extreme habitats (caves vs. surface waters) shape the physiological regulation of glucocorticoids (cortisol) over time.

• Context: The Mexican Tetra (Astyanax mexicanus) offers a unique model with surface-dwelling (river/lake) and cave-dwelling ecotypes. Cave environments present resource scarcity and darkness but stable abiotic conditions, whereas surface environments involve fluctuating abiotic factors and higher biotic stressors (predators, parasites).
• Gap: Previous studies focused on acute plasma cortisol spikes. There is a lack of data on chronic stress accumulation and whether differences in stress physiology are driven by immediate environmental plasticity or fixed genetic adaptations.

2. Methodology

The study employed a comparative approach across six populations (four cave, two surface) over two consecutive years (2023–2024), integrating field sampling with laboratory controls.

• Study Sites & Populations:
  • Surface: Rio Choy (river) and Presa El Oyul (artificial lake).
  • Cave: Pachón (highly isolated, stable), Los Sabinos, Tinaja, and Rio Subterráneo (hydrologically dynamic, prone to surface flooding/hybridization).
• Sampling Strategy:
  • Wild: Fish were captured in the field using dip nets (caves) or angling (surface). Scales were collected, flash-frozen, and stored at -20°C.
  • Laboratory Control: To disentangle genetic vs. environmental effects, wild-caught Rio Choy (surface) and Pachón (cave) fish were compared against laboratory-reared offspring from the same genetic lines (maintained for decades).
• Abiotic Characterization: Water parameters (temperature, pH, dissolved oxygen, conductivity, nutrients, heavy metals) were measured at each site to characterize habitat variability.
• Cortisol Extraction & Analysis:
  • Scales were pulverized and extracted using methanol.
  • Cortisol concentrations were quantified via ELISA (Neogen kit), validated for fish scales with ~90% recovery.
• Statistical Analysis:
  • Generalized Linear Models (GLMs) with Gamma distribution and log-link were used due to non-normal, right-skewed cortisol data.
  • Models tested effects of habitat (river, lake, cave), population × year, and origin (wild vs. lab).
  • Covariates included Fulton's K-index (body condition) and sex.
  • Bootstrap resampling (1,000 iterations) was used to assess inter-individual variation.

3. Key Contributions

• Validation of Scale Cortisol: Confirms scale cortisol as a robust biomarker for long-term stress in A. mexicanus, capable of detecting differences between ecotypes and populations.
• Genetic vs. Plasticity: Demonstrates that differences in long-term stress physiology between surface and cave ecotypes persist even in identical laboratory environments, suggesting a genetic basis for stress adaptation rather than purely plastic responses.
• Habitat Variability Link: Establishes a correlation between environmental variability (hydrological dynamics, abiotic fluctuations) and both the magnitude of cortisol levels and the degree of inter-individual variation.

4. Key Results

A. Habitat and Population Differences

• Lake vs. River/Cave: Contrary to the hypothesis that caves are more stressful, lake surface fish (Presa El Oyul) exhibited the highest scale cortisol levels. River surface fish (Rio Choy) and cavefish generally showed lower levels.
• Cave Population Variation:
  • Pachón (most isolated/stable) had the lowest cortisol levels.
  • Rio Subterráneo (dynamic, flooded) showed intermediate levels in 2023 but significantly increased levels in 2024, correlating with higher environmental variability.
  • Tinaja and Los Sabinos showed intermediate to low levels, generally lower than lake fish.
• Wild vs. Lab: No significant difference was found between wild and laboratory-reared fish within the same population (Rio Choy or Pachón). This confirms that the lower cortisol in Pachón cavefish is a genetically determined trait, not a result of immediate environmental conditions.

B. Individual Variation

• Populations in stable, isolated environments (Pachón, Los Sabinos) exhibited low inter-individual variation in cortisol.
• Populations in variable environments (Presa El Oyul lake, Rio Subterráneo cave) showed high inter-individual variation.
• This suggests that stable selective pressures lead to uniform stress phenotypes, while fluctuating environments maintain diverse stress responses within a population.

C. Covariate Effects

• Body Condition (K-index): A significant positive correlation was found between body condition and cortisol levels. This contrasts with typical stress literature (where stress lowers condition) but aligns with cavefish biology: cavefish accumulate fat (high K-index) and may have altered metabolic cortisol regulation linked to energy storage.
• Sex: No significant effect of sex on cortisol levels was detected.

5. Significance and Implications

• Evolutionary Adaptation: The study provides evidence that stress physiology is a target of natural selection. Cavefish have evolved a "stress-coping" phenotype characterized by lower basal long-term cortisol, likely linked to metabolic adaptations (e.g., insulin resistance, fat storage) required for survival in resource-poor caves.
• Environmental Monitoring: The findings highlight that environmental variability (not just the presence of stressors) drives physiological heterogeneity. Populations in dynamic habitats (like the lake or flooded caves) show greater physiological diversity, potentially aiding resilience to changing climates.
• Biomarker Utility: The research reinforces the utility of scale cortisol for assessing chronic stress in wild fish populations, offering a non-invasive alternative to blood sampling which only captures acute spikes.
• Future Directions: The authors suggest investigating the specific genetic loci (QTL mapping) controlling cortisol regulation and further exploring the link between cortisol, fat metabolism, and longevity in cavefish.

In summary, this paper demonstrates that long-term stress physiology in Astyanax mexicanus is shaped by a complex interplay of genetic adaptation to specific habitat stability and environmental variability, with cavefish evolving a distinct, low-cortisol phenotype that persists regardless of immediate environmental conditions.