Spatiotemporal variability and sound characterization in silver croaker Plagioscion squamosissimus (Sciaenidae) in the Central Amazon

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.

Figure 1. Study area.

The hand-held disturbance and cage calls where recorded in a floating house laboratory (2). Wild calls were recorded from Xiboreno Channel (1); Catalão Island (3) and Catalão Lake (4). Xs indicate sites negative for sound production. A. Low-water season. B. High-water season.

Figure 2. Photographs of Sonic muscle of Plagioscion squamosissimus.

(A) Out of the mating season; note sonic muscles are translucent and thin. (B) Thicker muscle with a red color during the mating season. SwB: Swimbladder, Tes: Testes.

Figure 3. Oscillograms of advertisement calls of Plagioscion squamosissimus.

(A) dense chorus recorder during the low-water mating season. (B) recording from an individual fish during the mating season, with double and single pulses (arrows) occurring before a series of bursts. (C) chorus from three fish in the high-water season (1, 2, 3) at various distances from the hydrophone. Note the busts are shorter and further apart than in (B).

Figure 4. Complex sounds of Plagioscion squamosissimus.

(A) Call from one individual showing the single and double pulses preceding a burst. (B) expanded double pulse. (C) last individual pulse and the first pulse of a burst. Note the similarity in waveforms.

Figure 5. Frequency spectra of calls, background soundscape (inferior line) and pulse waveform (insets) from Plagioscion squamosissimus.

(A) disturbance call. (B) advertisement call from the low-water mating season. (C) advertisement call from outside the mating season (high-water), Hanning 3000 points with 50% overlap (bandwidth 10.8 Hz).

Figure 6. Longer period cycles generate lower peak frequencies in Plagioscion squamosissimus.

(A) cycle period of high amplitude pulse and (B) peak frequency. DLW: disturbance call low-water, DHW: disturbance high-water, CLW: cage low-water, WLW: wild low-water, WLW: wild high-water. Different letters indicate significant differences (p<0.01).

Figure 7. Mean + SE of temporal parameters of calls from Plagioscion squamosissimus.

(A) pulse rate, (B) pulse period, (C) pulse per burst, (D) burst duration for a calls of Plagioscion squamosissimus. DLW: disturbance call low-water, DHW: disturbance high-water, CLW: Cage low-water, WLW: wild low-water, WLW: wild high-water. Different letters indicate significant differences (p<0.01).