# Acoustic Characterization for The Feeding Activities of _Haliotis_ _discus_ _Hanmai_
## 4 Discussions  
### Eating-Based Sound Production Mechanism  
The digestive system of abalone is on the left side of the adductor muscle that is in the middle of the body. The digestive organ includes mouth, gullet, stomach, intestine rectum, and anus. In the mouth, there are jaws, radula, and salivary glands. The radula is the main tool for abalone to take food. The front part of the radula has abrasion during the long-term eating behavior, related to the way abalone scrapes food [41]. When the abalone eats, the mouth is firstly opened. Under the action of the muscles and cartilage, the radula sticks out of the mouth and the back of the radula faces the food. When the radula retracts into the mouth, the back of the radula fully contacts the food surface and starts to scrape the food. The ingested food is first chopped by the jaws, then ground by the radula, mixed with saliva, and finally sent to the esophagus through the pharyngeal flap.  
During the experiments, the acoustic signal disappeared immediately when the abalone left the food after feeding. Taking account of the physiological structure of the abalone, the digestion of the food is a continuous process. If there is sound in the process of digesting food, the acoustic signal can still be collected after the abalone stops eating, which is contrary to the experimental results. Therefore, the possibility that the abalone produces sound when digesting food is ruled out.  
Through the audio and video data collected in the experiments, it can be found that the appearance time of the acoustic signal is almost synchronized with the action of the abalone ingesting food, indicating that the sound is mainly produced during the process of ingesting food. The main activities of abalone during ingesting are scraping and grinding of the radula and chopping food with the jaw. In the experiments of Group B, it was observed that the jaw was exposed after feeding, while the acoustic signal was still produced. Thus, the possibility of the sound produced by the jaw chopping is also ruled out.  
Therefore, it is considered that the eating-based acoustic signal is mainly produced by scraping and grinding the food with the radula. Because there was a significant difference in the eating signal of abalones in Group A and Group B, it indicates that the generation of abalone ingesting sound is also related to the type of food.  
Marine animals have different mechanisms to produce eating-based sounds, but few are similar to the eating-based sound mechanism of the abalone. A \"click\" sound is produced by the maxipellids and mandibles of shrimps when they cut and tear food [20; 42]. A \"click\" sound is also produced by the movement of supraocyipital-corner articulation  
Figure 8: Changes in abalone eating rate over time. It shows the exponential relationship (R2 = 0.6345) between time and the eating rate.  
of seahorse during feeding [43; 44]. Sounds are produced when clownfish lower the hyoid apparatus rapidly and close the mouth at the same time [45; 46]. Sounds are produced as an exaptation during feeding because of the movement of the upper and lower pharyngeal jaws of _Haemulon flavolineatum_[47]. Sounds and a head snap are produced during feeding of _Syngnathus fuscus Syngnathus floridae_[48]. The cerato-mandibular (c-md) ligament plays a sound maker role in both feeding and communication in damselfishes [49].