Bats are exceptionally loud and elephants trumpet, but some of the noisiest creatures, including colossal sperm whales and booming blue whales, live in water. Yet the loudest aquatic creature per kilo of muscle has to be the diminutive Danionella cerebrum. At home in the streams of Myanmar's Bago Yoma mountains, the transparent 1.1 cm long male fish produce clicks in excess of 140 dB when submerged, approaching the volume of bottlenose and Atlantic dolphins. But when do the tiny males develop their ear-splitting clicks? After recently discovering that the extraordinary fish pound on their buoyant swim bladders with cartilage hammers to produce each 2.5-ms-long click, Antonia Groneberg and Benjamin Judkewitz from Charité Universitätsmedizin Berlin, Germany, decided to monitor the sounds produced by the fish as they grew from microscopic larvae to fully fledged adults to find out how the sounds and the structures that produce them develop as the youngsters age.
After collecting freshly fertilized D. cerebrum eggs, Groneberg – with Lena Dressler, Mykola Kadobianskyi and Julie Müller (also from Charité Universitätsmedizin Berlin) – began listening to the minute larvae at 3 weeks old, continuing to record any sounds produced by the youngsters each week as they grew until fully grown at 14 weeks old. Impressively, the transparent fish began producing their distinctive clicks at around 6 weeks of age, when they were just ∼8 mm long. Most surprisingly, even the tiniest youngsters were already able to hammer on their swim bladders at the same rate as fully developed adults, producing 60 clicks s−1 when using only one swim-bladder hammer, or 120 clicks s−1 when pounding alternately with the two. ‘The fact that young fish produce adult click types shows that they don't need to practice to get their muscles up to speed, meaning that their brain circuits must have fully developed by the time they start making sounds’, says Groneberg. In addition, as the fish grew, they clicked more, each producing a few clicks at the earliest age of 6 weeks, rising to over 500 clicks per outburst at 14 weeks. The volume of the fish's clicks also increased as they grew, initially registering ∼135 dB at 6 weeks, but increasing to exceed 140 dB by adulthood, effectively doubling in volume.
Having charted how the fish's clicks change as they grow, the team also monitored how the structures that work together to produce the sounds develop over time: they include the cartilage hammers, the drumming rib (which is pulled by the drumming muscle to lift the hammer in preparation for a blow), and the bone structure that holds the swim bladder in place. Using specialised dyes that highlight cartilage and bone, the team saw the drumming rib begin to grow thicker and the swim-bladder-supporting bone begin to develop the distinctive hook that differentiates male fish from the clickless females when the young males were ∼5 weeks old. Shortly after, the team saw the first faint signs of the cartilage hammer beginning to appear and, a few days after, the swim-bladder-supporting bone fused to the spine to hold the swim bladder in place. ‘All the little bones and cartilage pieces that the males need to produce clicks develop between 5 and 7 weeks. Once all these pieces are there, we start hearing them click away’, says Groneberg.
In addition, the team took a closer look at the bones that contribute to the fish's hearing, and they were well established before the swim bladder hammers and thickened rib on each side appear, meaning that the fish can probably hear well before they begin clicking at each other.
