A novel method for measuring acute thermal tolerance in fish embryos.

Area of Science:

• Aquatic Ectotherm Biology
• Environmental Physiology
• Developmental Biology

Background:

• Aquatic ectotherms face thermal stress, with embryos being particularly vulnerable.
• Accurate measurement of embryonic thermal tolerance is crucial for understanding species vulnerability to climate change.
• Existing methods for measuring critical thermal maximum (CTmax) in fish embryos are challenging and may yield incomparable data.

Purpose of the Study:

• To introduce a novel, reliable method for measuring CTmax in fish embryos.
• To compare the new 'last movement' metric with the 'last heartbeat' metric for embryonic CTmax.
• To enable robust comparisons of thermal tolerance across fish life stages and species.

Main Methods:

• Developed a novel method to determine fish embryo CTmax based on the temperature of cessation of movement.
• Compared this 'last movement' CTmax with CTmax determined by the temperature of the last heartbeat.
• Validated the embryonic CTmax measurements against established CTmax measurements in larval and adult fish.

Main Results:

• Fish embryos exhibit a period of increased activity before reaching CTmax, similar to other life stages.
• CTmax measured by last movement is a conservative and practical metric for all embryo sizes.
• Embryonic CTmax based on last movement aligns with larval and adult CTmax, unlike the last heartbeat method which likely overestimates tolerance.

Conclusions:

• The 'last movement' method provides a standardized and comparable measure of embryonic CTmax.
• This new method allows for accurate assessment of thermal vulnerability in fish embryos.
• The findings facilitate better predictions of species' responses to warming aquatic environments.