Infrastructure in the electric sense: admittance data from shark hydrogels

Area of Science:

• Marine Biology
• Sensory Physiology
• Biophysics

Background:

• Elasmobranchs (sharks, skates, rays) utilize an electrosensory system for navigation and predation.
• This system involves specialized canals filled with a hydrogel, whose function remains unclear.
• Understanding the hydrogel's properties is key to deciphering the elasmobranch electrosensory mechanism.

Purpose of the Study:

• To investigate the electrical properties of hydrogels found in elasmobranch electrosensory canals.
• To determine the functional role of this hydrogel in electroreception.
• To compare the electrical characteristics of shark-derived hydrogels with synthetic analogues and seawater.

Main Methods:

• Electrical admittance spectroscopy was performed on postmortem hydrogel samples from Triaenodon obesus and Carcharodon carcharias.
• Synthetic collagen-based hydrogels with controlled ion concentrations were created for comparison.
• Measurements were taken across a frequency range of 0.05 to 100 kHz, relevant to elasmobranch electrosensing.

Main Results:

• Shark hydrogels exhibited significantly suppressed electrical admittance compared to both seawater and synthetic collagen hydrogels.
• Collagen hydrogels with similar ionic concentrations showed approximately 2.5 times greater polarizability than the shark hydrogels.
• These findings indicate that elasmobranch hydrogels effectively localize ionic species.

Conclusions:

• The hydrogel's properties suggest it plays a crucial role in generating and maintaining voltage gradients within the electrosensory canals.
• The gel-filled canals are better suited for fostering voltage differences along their length than for direct electrical coupling with the external seawater.
• This localized ion-binding function likely enhances the sensitivity and precision of elasmobranch electroreception.