Spatiotemporal Variability and Sound Characterization in Silver Croaker Plagioscion squamosissimus (Sciaenidae) in the Central Amazon Alfredo Borie 1 , Hin-Kiu Mok 2 , Ning L. Chao 3 , Michael L. Fine 4 * 1 Department of Fishery, Amazon Federal University, Manaus, Brazil, 2 Department of Oceanography and Asia-Pacific Ocean Research Center, National Sun Yat-sen University, Kaoshiung, Taiwan, 3 National Museum of Marine Biology and Aquarium, Checheng, Pingtung, Taiwan, 4 Department of Biology, Virginia Commonwealth University, Richmond, Virginia, United States of America Abstract Background: The fish family Sciaenidae has numerous species that produce sounds with superfast muscles that vibrate the swimbladder. These muscles form post embryonically and undergo seasonal hypertrophy-atrophy cycles. The family has been the focus of numerous passive acoustic studies to localize spatial and temporal occurrence of spawning aggregations. Fishes produce disturbance calls when hand-held, and males form aggregations in late afternoon and produce advertisement calls to attract females for mating. Previous studies on five continents have been confined to temperate species. Here we examine the calls of the silver croaker Plagioscion squamosissimus, a freshwater equatorial species, which experiences constant photoperiod, minimal temperature variation but seasonal changes in water depth and color, pH and conductivity. Methods and Principal Findings: Dissections indicate that sonic muscles are present exclusively in males and that muscles are thicker and redder during the mating season. Disturbance calls were recorded in hand-held fish during the low-water mating season and high-water period outside of the mating season. Advertisement calls were recorded from wild fish that formed aggregations in both periods but only during the mating season from fish in large cages. Disturbance calls consist of a series of short individual pulses in mature males. Advertisement calls start with single and paired pulses followed by greater amplitude multi-pulse bursts with higher peak frequencies than in disturbance calls. Advertisement-like calls also occur in aggregations during the off season, but bursts are shorter with fewer pulses. Conclusions and Significance: Silver croaker produce complex advertisement calls that vary in amplitude, number of cycles per burst and burst duration of their calls. Unlike temperate sciaenids, which only call during the spawning season, silver croaker produce advertisement calls in both seasons. Sonic muscles are thinner, and bursts are shorter than at the spawning peak, but males still produce complex calls outside of the mating season. Citation: Borie A, Mok H-K, Chao NL, Fine ML (2014) Spatiotemporal Variability and Sound Characterization in Silver Croaker Plagioscion squamosissimus (Sciaenidae) in the Central Amazon. PLoS ONE 9(8): e99326. doi:10.1371/journal.pone.0099326 Editor: Melissa J. Coleman, Claremont Colleges, United States of America Received August 26, 2013; Accepted May 13, 2014; Published August 6, 2014 Copyright: ß 2014 Borie et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors acknowledge the Bio-Amazon Conservation for the financial support and the Brazilian Research and Technology National Council (CNPq) for the MSc. scholarship during the study. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: Michael L. Fine is on the editorial board. This does not alter the authors’ adherence to PLOS ONE policies on sharing data and materials. * Email: mfine@vcu.edu Introduction Sciaenid fishes, with common names such as croakers and drums, produce advertisement calls during the reproductive season [1,2,3,4,5,6]. The sounds are produced by contraction of a pair of sonic muscles typically present in males or occasionally in both sexes [7]. Sonic muscles may be extrinsic and run from an aponeurosis on the dorsal bladder to a tendon on the ventral midline [8] or intrinsic and confined to the swimbladder walls as in black drum [9,10]. The sonic muscles form during puberty when the juvenile gonads differentiate into testes [11] and in weakfish have the classic morphology of superfast fish sonic muscles with a core of sarcoplasm surrounded by a radially-arranged contractile cylinder consisting of alternating ribbons of sarcoplasmic reticu- lum and myofibrils [12]. Under androgenic control [13], the sonic muscles progress through a yearly hypertrophy-atrophy cycle in which the muscles increase in mass, thickness and fiber diameter to a peak for the mating season and atrophy afterward [14]. Muscle protein, glycogen and lipids also increase seasonally [14]. Using passive-acoustics, fishery biologists have mapped the distribution of vocalizing aggregations of numerous soniferous sciaenids, e.g., Bairdiella chrysoura, Cynoscion nebulosus, Pogo- nius cromis [10,15], Protonibea diacanthus [4], Micropogonias furnieri [16], Cynoscion regalis [1], to name a few. Passive- acoustics is a non-visual, non-invasive and non-destructive tool that provides information about the distribution, daily and seasonal activities patterns of fishes [17]. South American freshwater sciaenids are represented by four genera: Pachypops, Pachyuru, Plagioscion, and Petilipinni [18]. Plagioscion contains about 15 species [19], which occur in the PLOS ONE | www.plosone.org 1 August 2014 | Volume 9 | Issue 8 | e99326