ALT Picture showing the journal cover of Issue 16, volume 227 of Journal of Experimental Biology. The image shows a male Danionella cerebrum fish above a female Danionella cerebrum. The males produce loud clicks by beating on their swim bladders with cartilage hammers. The fish's skeletons are picked out in red and the rest of the fish have a slightly blue tinge, although they are transparent and it is possible to see some of their internal organs

ALT A photo of a transparent male Danionella cerebrum showing the swim bladders (silver structures in the middle of the body) that produce the clicks when the hammers impact then and the muscles (yellow), around the top of the front swim bladder, that drive the hammer impacts. Photo credit: Mykola Kadobianskyi.