Sound production in red-bellied piranhas (<i>Pygocentrus nattereri</i>, Kner): an acoustical, behavioural and morphofunctional study

Piranhas are known to be sound-producing animals. Nevertheless, the biological significance of piranha calls remains unclear because sounds have been recorded only when specimens were held by hand or trapped in a gill net. These sounds are generated by rapid contractions of sonic muscles that insert on a broad tendon surrounding ventrally the cranial sac of the swimbladder. The piranha swimbladder is thought to play an important role in sound production as an impedance-matching device and as a resonator. However, the vibratory capacities of the cranial and caudal sacs and the exact role of both sacs in sound production remain poorly understood. In this study, three sounds were each associated to a specific behaviour. The first sound (type 1) was produced during frontal display; it had numerous pulses and lasted 140!±17 ms, with a fundamental frequency of 120±4 Hz. It corresponded to the sound made by hand-held fishes. The second sound (type 2) was produced during circling and fighting behaviour; it was a single pulse lasting 36±8 ms, with a fundamental frequency of 43±10 Hz. The third sound (type 3) corresponded to chasing behaviour and comprised three to four pulses, each lasting 3±1 ms, with a fundamental frequency of 1739±18 Hz. Using a laser vibrometer to study the swimbladder displacement when stimulated at different frequencies, it was demonstrated that the first two sounds corresponded to the swimbladder mechanism. By contrast, the third sound was associated with the jaw mechanism. The vibrometer indicated that the swimbladder is a highly damping structure, simply copying the sonic muscle contraction rate. This study provides two interesting insights. First, it shows the relationships between three kinds of piranha sound and three specific behaviours. Second, using muscle stimulation at different rates, it shows which simultaneous conditions are required for production of sound in this species. Swimbladder calls were produced by a muscle contraction rate of approximately 100 Hz because this periodicity allowed the swimbladder to vibrate. At this frequency range, the contraction–relaxation cycles of the swimbladder muscles engendered wall displacements that had short amplitudes and with only a small variability between them

Nonlinear polarization effects in optical fibers: polarization attraction and modulation instability [Invited]

We review polarization stabilization techniques based on the polarization attraction effect in low-birefringence fibers. Polarization attraction or pulling may be based on cross-polarization modulation, on parametric amplification, and on Raman or Brillouin scattering. We also review methods for laser frequency conversion based on polarization modulation instabilities in low- and high-birefringence fibers, and photonic crystal fibers. Polarization instabilities in nonlinear fibers may also be exploited for sensing applications

A universal optical all-fiber omnipolarizer

Wherever the polarization properties of a light beam are of concern, polarizers and polarizing beamsplitters (PBS) are indispensable devices in linear-, nonlinear-and quantum-optical schemes. By the very nature of their operation principle, transformation of incoming unpolarized or partially polarized beams through these devices introduces large intensity variations in the fully polarized outcoming beam(s). Such intensity fluctuations are often detrimental, particularly when light is post-processed by nonlinear crystals or other polarization-sensitive optic elements. Here we demonstrate the unexpected capability of light to self-organize its own state-of-polarization, upon propagation in optical fibers, into universal and environmentally robust states, namely right and left circular polarizations. We experimentally validate a novel polarizing device-the Omnipolarizer, which is understood as a nonlinear dual-mode polarizing optical element capable of operating in two modes-as a digital PBS and as an ideal polarizer. Switching between the two modes of operation requires changing beam's intensity

Aspects of sound communication in the pearlfish <i>Carapus boraborensis</i> and <i>Carapus homei</i> (Carapidae)

Several species of Carapidae are known to have symbiotic relationships with marine invertebrates. The two most common species in Moorea (French Polynesia), Carapus boraborensis and Carapus homei, undergo conspecific and heterospecific encounters in the same holothurian host during which they produce sounds. Another characteristic of these fish lies in their abilities to produce sounds. The objective of this study was dual: (1) to seek if there was a sexual difference in the sounds produced by C. boraborensis; (2) to seek if there was a difference in the sound emissions between heterospecific and conspecific encounters. In each trial, sounds were only recorded when one individual entered the sea cucumber that was already occupied. In encounters, sounds were structured in regular pulse emissions whose pulse lengths and periods allowed to significantly distinguish each species, as well as both sexes in C. boraborensis. In the latter species, results show for the first time that temporal features of the emitted sounds can have a functional importance in sex identification. In heterospecific encounters, sounds were reduced 68% of the time to a single pulse emission and there was a modification in the pulse length of each species: it shortens in C. homei and it lengthens in C. boraborensis. It highlights that both carapids are able to adapt their sounds to the facing species. Because a modification of the sound appears to be done at the first emission, it is supposed that recognition precedes the sound emission

Control of signal coherence in parametric frequency mixing with incoherent pumps: narrowband mid-infrared light generation by downconversion of broadband amplified spontaneous emission source at 1550 nm

We study, with numerical simulations using the generalized nonlinear envelope equation, the processes of optical parametric and difference- and sum-frequency generation (SFG) with incoherent pumps in optical media with both quadratic and third-order nonlinearity, such as periodically poled lithium niobate. With ultrabroadband amplified spontaneous emission pumps or continua (spectral widths >10 THz), group-velocity matching of a near-IR pump and a short-wavelength mid-IR (MIR) idler in optical parametric generation may lead to more than 15-fold relative spectral narrowing of the generated MIR signal. Moreover, the SFG process may also lead to 6-fold signal coherence improvements. When using relatively narrowband filtered noise pumps (e.g., spectral widths < 1 THz), the signal from optical parametric, sum-frequency, and difference-frequency generation has nearly the same spectral width as that of the incoherent pump

Weak ferrimagnetism and multiple magnetization reversal in {\alpha}-Cr3(PO4)2

The chromium(II) orthophosphate {\alpha}-Cr3(PO4)2 is a weak ferrimagnet with the Curie temperature TC = 29 K confirmed by a \lambda-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature {\Theta} = - 96 K, indicating an intermediate frustration ratio |{\Theta}|/TC ~ 3. In its magnetically ordered states {\alpha}-Cr3(PO4)2 exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e. either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 {\mu}B/Cr2+, as compared to the spin-only moment 4.9 {\mu}B/Cr2+, cannot be ascribed to the occurence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d4 Cr2+ ions. X-ray absorption spectra at the K-edge indicate divalent chromium and unravel the high-spin state of these ions at the L2,3-edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr2+ in its six crystallographically inequivalent positions.Comment: 17 pages, 9 figure