Comparative study on sound production in different Holocentridae species

Abstract

Background: Holocentrids (squirrelfish and soldierfish) are vocal reef fishes whose calls and sound-producing mechanisms have been studied in some species only. The present study aims to compare sound-producing mechanisms in different Holocentridae genera (Holocentrus, Myripristis, Neoniphon, Sargocentron) from separate regions and, in some cases, at different developmental stages. An accurate comparison was made by recording six species while being hand-held, by observing TEM) the sonic muscles and by dissections of the sound-producing mechanism.

Results: In all these species, calls presented harmonics, their dominant frequency was between 80 and 130 Hz and they were composed of trains of 4 to 11 pulses with gradual increasing periods towards the end of the call. In each case, the calls did not provide reliable information on fish size. The sounds were produced by homologous fast-contracting sonic muscles that insert on articulated ribs whose proximal heads are integrated into the swimbladder: each pulse is the result of the back and forth movements of the ribs. Small differences in the shape of the oscillograms of the different species could be related to the number of ribs that are involved in the sound-producing mechanism. These fish species are able to make sounds as soon as they settle on the reef, when they are 40 days old. Comparison between Neoniphon from Madagascar and from Rangiroa in French Polynesia showed a new, unexpected kind of dialect involving differences at the level of pulse distribution. Neoniphon calls were characterised by a single pulse that was isolated at the beginning of the remaining train in Madagascar whereas they did not show any isolated single pulses at the beginning of the call in Rangiroa.

Conclusion: This family cannot use the acoustic fundamental frequencies (or pulse periods) of grunts to infer the size of partners. Pulse duration and number of pulses are statistically related to fish size. However, these characteristics are poorly informative because the correlation slope values are weak. It remains other features (sound amplitude, resistance to muscle fatigue, calling frequency) could be used to assess the body size. Characteristics of the sound producing mechanisms are conservative. All species possess fast-contracting muscles and have the same kind of sound producing mechanism. They do show some change between clades but these differences are not important enough to deeply modify the waveforms of the calls. In this case, our description of the grunt could be considered as the signature for the holocentrid family and can be used in passive acoustic monitoring.

Figure 1

Means of the successive pulse periods in calls of seven pulses in Myripristis violacea adults (Black Square), in Myripristis violacea larvae at settlement (White Square) and in Neoniphon sammara (Black Circle).

Figure 2

Comparative oscillograms in two Neoniphon sammara populations. Arrows indicate the peaks within the pulse.

Figure 3

Oscillogram in Sargocentron diadema. Arrows indicate the peaks within the pulse.

Figure 4

Comparative oscillograms in different Myripristis populations. Each pulse is supported by one main peak. Myripristis kuntee were recorded in Tulear and Myripristis violacea in Rangiroa.

Figure 5

Means of the sound level in Myripristis violacea.

Figure 6

Means of the fundamental frequencies in Myripristis violacea adults (■), in M. violacea larvae at the time of settlement (□) and in Myripristis kuntee (O). Note the fundamental frequency corresponds to the pulse period.

Figure 7

Comparative power spectrum in calls of an adult (black) and a larva (red) of Myripristis violacea. Arrows indicate the different harmonics in each call. The sound pressure level appears less important in settling larvae and shows more harmonics. Power spectrum characteristics: sampling frequency 44.1 kHz, bandwith 31.5 Hz, hamming window.

Figure 8

Oscillogram in Holocentrus rufus. Arrows indicate the peaks within the pulse.

Figure 9

Left lateral view of the sound producing apparatus in Sargocentron diadema (A) and in Neoniphon sammara (B). Latine numbers refer to the vertebra positions.

Figure 10

Left lateral view of the complete (A) sound producing apparatus in Myripristis kuntee. Muscle and sonic ligaments were removed in (B) to show the swimbladder fenestra and swimbladder ligaments. In C, ventral view of the anterior part of the swimbladder, at the level of its association with the skull.

Figure 11

Different TEM pictures of transverse sections in sound producing muscle in Myripristis. These cells are characterized by their small sections, the high number of mitochondria in periphery and the well developped reticulum sarcoplasmic. mt: mitochondria; my: myofibril; re: reticulum sarcoplasmic.

Figure 12

Schematic dorsal view of the left part of the sound producing apparatus in different Holocentridae species.