Sharks Are No Longer Silent Hunters

For many marine organisms, sound serves as an important bridge for communication both within and between species. More than a thousand species of bony fishes are known to produce sounds in various social contexts, including courtship, aggressive encounters, reproduction, and defence against predators.

In contrast, although sharks are capable of hearing underwater sounds, they have long been regarded as animals that do not actively produce sound. However, several observations have challenged this traditional view. Some reports have described sharks emitting clicking sounds when divers approach them, suggesting that sharks may possess a previously unrecognized ability to generate sound actively.

Beginning in 2022, several studies documented sound production in close relatives of sharks—rays. Species such as the mangrove whipray (Urogymnus granulatus) and the blonde ray (Raja brachyura) have been observed to produce clicking sounds when disturbed by divers. These clicks are generally interpreted as behavioural responses to perceived threats or disturbances. Despite these findings in rays, formal scientific descriptions of active sound production in sharks themselves remained absent until a study focusing on the rig shark, Mustelus lenticulatus, provided the first documented evidence of this behaviour.

Mustelus lenticulatus（圖片來源：Arnim Littek，採用 CC BY 4.0 授權。）

The rig shark is a small to medium-sized benthic species inhabiting continental shelf waters around New Zealand and the Kermadec Islands in the southwestern Pacific. Individuals typically measure between 70 and 150 centimetres in length. This species mainly feeds on crustaceans such as crabs but can also become prey for larger sharks and marine mammals. Field researchers conducting fish surveys have reported hearing clicking sounds produced by juvenile rigs when they swim together in schools. In addition, earlier observations suggested that a related species, Mustelus mustelus, may produce loud sounds during feeding, possibly generated by grinding its teeth.

Fish employ a wide variety of mechanisms to produce sound. One of the most common mechanisms involves the swim bladder. In many bony fishes, specialized sonic muscles rapidly contract and vibrate the swim bladder, thereby generating sound. Another important mechanism is stridulation, in which hard structures such as bones or teeth rub against each other to create noise. For example, some catfish produce sound by rubbing serrated pectoral spines, while other fishes generate sound by grinding their pharyngeal teeth.

Cartilaginous fishes such as sharks and rays lack a swim bladder and currently show no obvious morphological structures specialized for sound production. For this reason, the study investigated whether anatomical features of the shark's head might provide clues to a possible sound-producing mechanism.

To investigate the clicking behaviour of rigs, researchers collected ten juvenile individuals between May 2021 and April 2022. The group consisted of five females and five males with total lengths ranging from 55.5 to 80.5 centimetres. The sharks were maintained in a laboratory circular flow-through tank approximately 2.1 metres in diameter and 1.26 metres deep, supplied with filtered seawater. During the study, the sharks were fed squid and fish three times per week. Water temperature was maintained between 14.7°C and 22.5°C, and salinity ranged from 35 to 36 ppt. Before experiments began, all individuals were allowed at least one week to acclimate to the laboratory environment.

During experiments, researchers transferred each rig shark into an experimental tank and gently handled it underwater while recording sounds for twenty seconds. Acoustic analysis revealed that the sharks produced an average of about nine clicks during each twenty-second recording period. Most clicks occurred during the first ten seconds of handling, with significantly fewer sounds recorded during the second half of the trial.

Many clicks coincided with movements of the shark's head or body, such as lateral bending. However, approximately five percent of the clicks occurred without any visible movement. The interval between clicks varied widely, ranging from 19 milliseconds to 9.57 seconds, with an average interval of about 1.4 seconds. The occurrence of clicks showed no relationship with either the sex or body length of the sharks, and every individual produced clicks during the experiment.

Head and dentition of Mustelus lenticulatus（圖片來源：Nieder, C et al. (2025) .，採用 CC BY 4.0 授權。）

Morphological examination of the rig shark's head revealed that its teeth are small and blunt, arranged in multiple rows and series. The teeth interlock through small protrusions, forming a pavement-like structure well suited for crushing hard prey such as crabs. Each tooth possesses a low crown with small lateral cusplets and short ridges near its base. These dental structures may enable friction or snapping interactions between teeth that generate the clicking sounds. No other anatomical structures capable of producing sound were identified.

All individuals produced clicking sounds when touched during the experiment. The number of clicks was highest during the initial ten seconds of handling and decreased thereafter. Researchers did not observe sound production during free swimming or feeding in the tank. This pattern suggests that the initial contact may trigger a stress or startle response in the sharks, increasing click production. As the animals become accustomed to handling, the number of clicks gradually declines.

Comparable behaviour has been reported in some bony fishes. For example, cod can emit loud clicking sounds when confronted by predators such as seals or divers. These sounds may function as deterrent signals directed toward potential predators.

Because all members of the genus Mustelus possess similar pavement-like teeth, other species in this group might also produce comparable sounds. To test this possibility, researchers conducted a preliminary experiment in October 2024 using three dusky smoothhounds (Mustelus canis). In this case, however, no clicking sounds were detected. One possible explanation is that these individuals had been kept in captivity for four months and had become accustomed to human interaction. Further studies will therefore be necessary to determine whether different Mustelus species share the same sound-producing capability.

Overall, the study demonstrates that Mustelus lenticulatus can produce clicking sounds when handled underwater, representing the first documented case of deliberate sound production in a shark. The sounds are most likely generated by forceful snapping of the shark's flattened teeth, although additional research will be required to confirm this mechanism. Observations also suggest that the clicking behaviour may represent a response to disturbance or stress. Future behavioural studies will be needed to determine whether these sounds play a communicative role in natural environments.

（Author: Bai Leng）

Reference:

Nieder, C., Parmentier, E., Jeffs, A. G., Radford. C. (2025). Evidence of active sound production by a shark . Royal Society

Open Science.