Notes: Map-like Use of Earth's Magnetic Field in Sharks

Introduction

• Elasmobranchs (sharks, skates, rays) are vital to marine ecosystems.
• They are highly migratory and exhibit site fidelity.
• Sharks use electrosensory organs to interact with Earth's geomagnetic field (GMF).
• GMF provides map and compass information aiding in navigation.

Research Aims

1. Determine if sharks use magnetic cues for orientation.
2. Investigate if map-like use of GMF explains genetic variation patterns in sharks.

Experiment Details

• Species: Bonnethead shark (Sphyrna tiburo).
• Location: Turkey Point Shoal, Florida.
• 20 juvenile bonnetheads captured and tested.
• Sharks exposed to three magnetic field conditions:
  1. Control (site field).
  2. 600km south field (weaker GMF).
  3. 600km north field (stronger GMF).

Methodology

• Magnetic conditions altered using Merritt coils.
• Experimental tank setup with GoPro for movement tracking.
• Tested orientation in different fields to see navigation responses.

Results

• Control Field: No preferred orientation (random movement).
• Southern Field: Significant northward orientation.
• Northern Field: No orientation preference, similar to control.

Discussion

• Sharks can differentiate GMF components for orientation.
• Southern field results imply learned magnetic maps.
• Northern field lack of response suggests maps are not innate but learned through experience.
• Variations in genetic structure among populations linked to GMF differences.

Implications

• Magnetic cues may influence shark migrations and genetic diversity.
• Findings align with similar studies on sea turtles and other marine animals.
• GMF's role in marine navigation extends to fully marine species.

Conclusion

• Sharks possess a map-like understanding of GMF aiding in navigation.
• This aids in understanding migration patterns and population structures in marine environments lacking physical barriers.
• Future research should further investigate GMF's influence on marine species.

Method Details

• Experimental Design: Utilized Merritt coils to simulate different magnetic fields.
• Behavioral Trials: Sharks acclimated before trials, recorded using GoPro.
• Genetic Analysis: Explored genetic distances related to magnetic, temperature, and coastal differences.

Key Findings

• Magnetic differences more predictive of mtDNA variation than temperature or coastal distance.
• Consistent results with previous studies on migratory marine species.

Visuals

• Figures and Tables: Included for reference on experimental setup and results.
• Figure Descriptions:
  • Figure 1: Merritt coil setup and tracking procedure.
  • Figure 2: Orientation results in different magnetic fields.
  • Table 1: Details on magnetic field conditions and shark responses.

Additional Resources

• Supplementary Information: Available for detailed experimental data and genetic structure analysis.
• Contact Information: Lead contact Bryan Keller for further details.