From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla)

Area of Science:

• Ichthyology
• Comparative Anatomy
• Evolutionary Biology

Background:

• The European eel (Anguilla anguilla) undergoes significant phenotypic changes during its migration, including the yellow to silver eel transformation.
• While early life stages are well-documented, cranial morphology during silvering remains poorly understood.
• Existing literature suggests major musculoskeletal alterations during this critical migratory phase.

Purpose of the Study:

• To investigate the cranial musculoskeletal morphology of European eels during the silvering process.
• To analyze changes in jaw closing and respiratory muscles, as well as skull dimensions.
• To compare morphological differences between naturally and artificially matured silver eels.

Main Methods:

• Utilized 3D-reconstructed micro-computed tomography (µCT) data from 11 male European eels at various silvering stages.
• Assessed cranial musculoskeletal anatomy, focusing on jaw and respiratory muscles.
• Used eye size as a quantitative indicator of the silvering stage.

Main Results:

• Jaw muscle volume showed an insignificant increase, with a near-significant rise in bite force during silvering.
• Respiratory muscle size significantly increased, potentially enhancing oxygen uptake for migration.
• Skull and orbit dimensions enlarged, accommodating the growing eyes.
• Artificially matured eels exhibited wider, taller skulls and larger jaw muscles compared to wild counterparts.

Conclusions:

• Significant increases in respiratory muscle mass during silvering may support energy demands during long migrations in oxygen-limited environments.
• Skull and orbital remodeling are crucial for accommodating eye enlargement during transformation.
• Artificial maturation leads to distinct cranial and muscular morphologies compared to natural maturation, possibly due to environmental factors or hormonal treatments.