Deep down, fishes use acoustic signals rather than visual ones

Autors:

Raick X, Campisi J, Under The Pole Consortium, Bertucci F, Lecchini D, Di Iorio L, Parmentier E. Coastal and Shelf Science (2025) 

Article summary:

Mesophotic coral ecosystems (MCEs, 30-150 m depth) represent approximately 80% of the habitable space in tropical reef ecosystems. However, technical and technological constraints limit scientific advances in these ecosystems.  

The mesophotic zone can be divided into two main levels (“upper zone” between 30 and 60 meters, “lower zone” between 60 and 150 meters) with different coral communities. Similarly, the fish communities living in MCEs differ between these two zones. If depth influences the composition of fish communities, what about the soundscape associated with these fish? This is the question the authors sought to answer.  

In ecology, the word “niche” encompasses various parameters that describe a species: its diet, preferred temperature or light conditions, availability of breeding areas, interactions with other species, etc. The term “acoustic niche” is used to describe how a species uses sound to communicate: what frequency (high or low), at what time of day/night, what types of sounds (short, rapid sounds or spaced-out sounds), etc.  

This study aims to compare the day/night dynamics and acoustic niches of fish sounds at two depths (60 and 120 m) in the mesophotic coral reefs of the Tuamotu Archipelago (French Polynesia) over a period of 62 hours.  The authors analysed 52,255 fish sounds recorded using passive acoustic method, a non-intrusive technique used to collect different types of sounds with high temporal resolution.   

The results reveal that fish sound signals vary significantly between depths of 60 and 120 m. Species that live at night and those that live at great depths (120 m) produce more stereotypical sounds than species that live during the day and those that live at shallower depths (60 m). The authors explain these results by the need for very precise sound for fish species that do not benefit from light (nocturnal species and species living at 120 m) to send their vocal message. On the contrary, species that benefit from light can supplement their sound signal with a visual signal (e.g., visual behaviour). In addition, each sound occupies its own acoustic niche: no two sounds are alike, and when they are, they are produced at different times. 

This characteristic of fishes living in low-light conditions (at night or in deeper waters) reflects the need for more stereotypical acoustic signals and suggests that these species have adapted to low-light environments.  

This work, based on a non-invasive method, has shown that the sound behaviour of fishes varies with depth and daily cycle. It leads to new questions, such as better understanding the auditory capacity of fish, or associating each sound with the species that produces it.