Fish gill chemosensing: knowledge gaps and inconsistencies.

Area of Science:

• Physiology
• Neuroscience
• Aquatic Biology

Background:

• Gill chemosensors, particularly neuroepithelial cells (NECs), are known to influence fish cardiorespiratory reflexes.
• The precise mechanisms and neurotransmitters involved in gill chemosensing are not fully understood, with evidence suggesting roles for serotonin, adrenaline, noradrenaline, acetylcholine, purines, and dopamine.

Purpose of the Study:

• To review current knowledge on gill chemosensors in fish.
• To identify inconsistencies in data and knowledge gaps regarding the function of NECs and associated neurotransmitters in cardiorespiratory control.
• To explore the polymodal nature of NECs and their role in sensing various stimuli.

Main Methods:

• Literature review of studies on fish gill chemosensing and cardiorespiratory reflexes.
• Analysis of existing data on NEC distribution, neurotransmitter involvement, and responses to different stimuli (hypoxia, CO2, ammonia, lactate).

Main Results:

• While serotonergic NECs are widely studied, other neurotransmitters are implicated in gill chemosensing.
• The exact neuronal pathways and integration sites for chemosensory information remain elusive.
• NEC location and orientation vary significantly across fish species, impacting their sensing capabilities.
• Gill chemosensors respond to various stimuli beyond hypoxia, including CO2, ammonia, and lactate.

Conclusions:

• Significant gaps exist in understanding the full spectrum of neurotransmitters and neuronal circuits involved in fish gill chemosensing.
• The polymodal sensing capabilities of NECs are increasingly recognized, broadening their known physiological roles.
• Further research is needed to link NEC distribution and function to species-specific sensitivities and lifestyles.