Calcified gill filaments increase respiratory function in fishes

Area of Science:

• Ichthyology
• Comparative Physiology
• Biomechanics

Background:

• Fish gill morphology is vital for aerobic capacity and stress tolerance.
• Gill surface area is studied, but tissue mechanical properties' role is unclear.
• Calcified sheaths on gill filaments in some fish have an unknown function.

Purpose of the Study:

• To investigate the function of calcified gill filaments in fish.
• To test if calcification enhances water flow or supports gills out of water.
• To understand the mechanical role of gill tissues in respiratory function.

Main Methods:

• Surveyed over 100 species of killifishes and related orders for filament calcification.
• Studied calcification changes in the killifish *Kryptolebias marmoratus* under terrestrial acclimation.
• Investigated the effect of calcification removal on branchial resistance in *K. marmoratus*.

Main Results:

• Filament calcification is widespread in killifishes, predating amphibious lifestyles.
• Calcification did not differ between amphibious and fully aquatic species.
• Terrestrial acclimation induced calcium deposition, suggesting a structural role out of water.
• Acclimation to hypoxia or high temperatures induced filament calcification.
• Removing calcification decreased branchial resistance, disrupting gill basket positioning.

Conclusions:

• Gill filament calcification plays a significant role in enhancing aquatic respiratory function.
• Mechanical properties of gill filaments are important for fish respiration.
• Calcification may provide structural support for gills, particularly in amphibious species.