Sound production in piranhas is associated with modifications of the spinal locomotor pattern

Bioacoustics supports genus identification in piranhasa)

In different teleost species, sound production can utilize specific coding schemes to avoid confusion between species during communication. Piranhas are vocal Neotropical fishes, and both Pygocentrus and Serrasalmus produce similar pulsed sounds using the same sound-producing mechanism. In this study, we analysed the sounds of three Pygocentrus and nine Serrasalmus species to determine whether sounds can be used to discriminate piranha species at both the species and genus levels. Our analysis of temporal and frequency data supports the idea that the sounds of Serrasalmus and Pygocentrus species are species specific, and that different acoustic features can be used to differentiate taxa at the genus level. Specifically, the sounds of Serrasalmus species are shorter, louder, and have a shorter pulse period (as determined after correction for standard length). This suggests that sounds can be used to support taxonomy at the genus level as well as the species level.

Reliable characterization of sound features in fishes begins in open-water environmentsa)

Many fishes use sounds to communicate in a wide range of behavioral contexts. In monitoring studies, these sounds can be used to detect and identify species. However, being able to confidently link a sound to the correct emitting species requires precise acoustical characterization of the signals in controlled conditions. For practical reasons, this characterization is often performed in small sized aquaria, which, however, may cause sound distortion, and prevents an accurate description of sound characteristics that will ultimately impede sound-based species identification in open-water environments. This study compared the sounds features of five specimens of the silverspot squirrelfish Sargocentron caudimaculatum recorded at sea and in aquaria of different sizes and materials. Our results point out that it is preferable to record fish sounds in an open-water environment rather than in small aquaria because acoustical features are affected (sound duration and dominant frequency) when sounds are recorded in closed environments as a result of reverberation and resonance. If not possible, it is recommended that (1) sound recordings be made in plastic or plexiglass aquaria with respect to glass aquaria and (2) aquaria with the largest dimensions and volumes be chosen.

A new organisational design in skeletal muscle fibres

In vertebrate skeletal muscles, the architecture of myofibrils is particularly well conserved throughout the taxa. It is composed of suites of repeating functional units called sarcomeres which give the muscle its striated structure. Here, we show that the skeletal sound producing muscles of the cusk eel Parophidion vassali have a different organisation, distinct from the classical type found in textbooks. Within sarcomeres, filaments are not straight lines but have a Y-shaped structure. This looks like chicken wire, with one branch connecting to a branch from the myofibril above and the other connecting to a branch from the myofibril below. This organisation seems to be an adaptation to counteract a trade-off between the speed and force. The low ratio of myofibrils within cell muscles and the high volume of sarcoplasmic reticulum strongly suggest that these muscles are capable of fast contractions. In parallel, the Z-bands are quite wide about 30% of the sarcomere length. This extraordinary long Z-band could smooth out the tension variations found in high-speed muscle contraction, helping to produce sounds with low variabilities in the sound features. Simultaneously, the Y-shaped structure allows having more cross-bridges, increasing the force in this high-speed muscle.