For wild animals, being able to hear is paramount to survival. It helps animals sense danger, find each other and sometimes find food. Things under the water are no different. Organisms such as fish can sense sounds moving through the water, though they do so slightly differently from most land-dwelling animals. Fish have three small ‘stones’ in their ears called otoliths, which – when combined with the cells that respond to sound – form ‘endorgans’, allowing the fish to hear and maintain their balance. Scientists believe that one of these endorgans – the saccule – is what fish rely on for hearing and that another – the utricle – is used for balance, enabling them to hold themselves upright in the water. However, the function of the third endorgan – the lagena – remains unclear. Because of the lagena's relatively large size, Ieng Lau and Raquel Vasconcelos of the University of Saint Joseph, Macao, China, think that this mysterious endorgan might help zebrafish (Danio rerio) to hear louder sounds when their saccule is already at its capacity for noise.
In an attempt to answer this question, the researchers placed the zebrafish in an aquarium and played them white noise for 24 h at 168 dB through an underwater speaker. This noise level is louder than a small boat travelling at 40 km h−1 if you're standing 1 m away. Lau and Vasconcelos then checked each endorgan to see whether there was any damage. Inside each endorgan are tiny, hair-like cells that bend when they feel the pressure from soundwaves. But, if the sound is too loud for too long, these cells can be damaged or even die. Unsurprisingly, the saccule's hair cells were devastated by the loud noise, showing damage in up to 59% of the hair cells. Not only were the cells damaged, most were dead. The complete opposite was true of the utricle. The duo didn't find any damage to the hair cells after 24 h of loud white noise. When they looked at the damage to the lagena, they found that loud noise damaged up to 23% of the hair cells, suggesting that this endorgan does indeed respond to loud sounds and helps the fish hear. The hair cells in both the saccule and the lagena were obviously impacted by the loud noise, but was there also other damage to the endorgans?
Lau and Vasconcelos stained each endorgan for the protein ribeye b, which is a marker of how well nerve cells can communicate with each other. If there is more ribeye b, then the nerve cells are communicating normally, but after 24 h of loud noise, the saccule had lost up to 69% of this protein, suggesting that the nerve cells in the saccule had lost their ability to communicate. Similarly, the lagena also experienced losses in nerve communication, though less so than the saccule. Taken together with the damage to the hair cells in this endorgan, the researchers concluded that the lagena must be helping the zebrafish hear sounds. With worldwide shipping lanes through the oceans, rivers and lakes, the likelihood of fish experiencing some kind of hearing loss is almost inevitable. Luckily for zebrafish at least, they can rely upon their lagena to hear the louder sounds even if their main method of hearing is damaged.