Why do eels open and close their mouths

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Last updated: April 8, 2026

Quick Answer: Eels open and close their mouths primarily for respiration, as they are obligate ram ventilators that must keep water flowing over their gills to breathe. This behavior also aids in chemoreception, allowing them to detect chemical cues in the water through their olfactory organs. Additionally, mouth movements assist in prey capture and processing, with some species like the European eel (Anguilla anguilla) exhibiting specific feeding behaviors. Research indicates that eels can perform these movements up to 30-40 times per minute during active periods, depending on species and environmental conditions.

Key Facts

Eels are obligate ram ventilators, requiring constant water flow over gills for oxygen uptake
Mouth movements facilitate chemoreception through olfactory organs located in the nasal cavity
European eels (Anguilla anguilla) use mouth movements during prey capture and processing
Some eel species exhibit mouth movement frequencies of 30-40 times per minute during activity
This behavior helps maintain buoyancy and orientation in water currents

Overview

Eels, belonging to the order Anguilliformes, comprise approximately 800 species found in both freshwater and marine environments worldwide. These elongated fish have fascinated scientists and observers for centuries due to their unique anatomical features and behaviors. Historical records dating back to Aristotle's "History of Animals" (circa 350 BCE) document observations of eel behavior, though their mouth movements weren't systematically studied until the 20th century. Modern research beginning in the 1970s revealed that eels lack the buccal pumping mechanism used by many fish species, making their continuous mouth movements essential for survival. The European eel (Anguilla anguilla), American eel (Anguilla rostrata), and moray eels (family Muraenidae) are among the most studied species regarding this behavior. These fish inhabit diverse environments from deep ocean trenches to freshwater rivers, with their mouth-opening behavior adapting to different water conditions and oxygen levels.

How It Works

The mechanism behind eels' mouth movements involves coordinated action of several anatomical structures. As obligate ram ventilators, eels must swim forward with mouths open to force water through the oral cavity and over the gills, where oxygen exchange occurs. The hyoid apparatus and associated muscles control the rhythmic opening and closing, creating a pumping action that maintains water flow even during periods of reduced swimming. This process serves multiple functions: primarily respiration, but also chemoreception through the olfactory epithelium in the nasal cavity, which detects chemical signals for navigation and prey location. During feeding, the mouth movements help create suction for capturing prey and assist in manipulating food items. The frequency and amplitude of these movements vary with activity level, water temperature, and oxygen concentration, with some species adjusting their rhythm from 20 to 60 cycles per minute depending on environmental conditions.

Why It Matters

Understanding eel mouth movements has significant implications for conservation and aquaculture. Since many eel species like the European eel have experienced population declines of over 90% since the 1980s, knowledge of their respiratory needs informs habitat protection and restoration efforts. In aquaculture facilities, proper water flow management based on eels' respiratory requirements improves survival rates and growth. This behavior also serves as an indicator of environmental health, as changes in mouth movement patterns can signal water quality issues. Furthermore, studying these mechanisms contributes to broader biological understanding of fish respiration and sensory systems, with potential applications in designing more efficient underwater vehicles and sensors. The commercial eel fishing industry, valued at approximately $1.5 billion annually worldwide, relies on understanding these behaviors for sustainable harvesting practices.