Overview on the diversity of sounds produced by clownfishes (Pomacentridae): importance of acoustic signals in their peculiar way of life

Abstract

Background: Clownfishes (Pomacentridae) are brightly colored coral reef fishes well known for their mutualistic symbiosis with tropical sea anemones. These fishes live in social groups in which there is a size-based dominance hierarchy. In this structure where sex is socially controlled, agonistic interactions are numerous and serve to maintain size differences between individuals adjacent in rank. Clownfishes are also prolific callers whose sounds seem to play an important role in the social hierarchy. Here, we aim to review and to synthesize the diversity of sounds produced by clownfishes in order to emphasize the importance of acoustic signals in their way of life.

Methodology/principal findings: Recording the different acoustic behaviors indicated that sounds are divided into two main categories: aggressive sounds produced in conjunction with threat postures (charge and chase), and submissive sounds always emitted when fish exhibited head shaking movements (i.e. a submissive posture). Both types of sounds showed size-related intraspecific variation in dominant frequency and pulse duration: smaller individuals produce higher frequency and shorter duration pulses than larger ones, and inversely. Consequently, these sonic features might be useful cues for individual recognition within the group. This observation is of significant importance due to the size-based hierarchy in clownfish group. On the other hand, no acoustic signal was associated with the different reproductive activities.

Conclusions/significance: Unlike other pomacentrids, sounds are not produced for mate attraction in clownfishes but to reach and to defend the competition for breeding status, which explains why constraints are not important enough for promoting call diversification in this group.

Figure 1. Behavioral postures associated with vocalizations and exhibited by Amphiprion frenatus during agonistic interactions.

A) Dominant individual chasing subordinate while producing aggressive sounds. B) Head shaking movements displayed by subordinate while producing submissive sounds in reaction to aggressive act by dominant. Note that wiggly lines indicate the sound-producing individual, and arrows point out the receiver of the aggressive act.

Figure 2. Example of agonistic sounds produced by Amphiprion frenatus during interactions.

A) Oscillogram (top) and spectrogram (bottom) of submissive sounds produced by subordinate during head shaking movements. B) Oscillogram (top) and spectrogram (bottom) of aggressive sounds produced by dominant while displaying charge and chase. Note the differences in (1) pulse duration and (2) pulse period. The acoustic variable measured in (3) represents the sound duration in the case of submissive sounds, and the train duration in the case of aggressive sounds. The colour scale corresponds to the intensity associated with the different frequencies.

Figure 3. Influence of fish size (SL) on acoustic features of submissive sounds in Amphiprion frenatus.

Correlations of (A) dominant frequency, (B) pulse duration, (C) pulse period, (D) number of pulses per sound, (E) sound duration, (F) sound period and (G) number of sounds per train against SL. Fish ranged from 44 to 112 mm in SL (N = 9). Results are expressed as mean values of all recorded pulses for each individual (○ = rank 4, □ = rank 3, ▵ = rank 2).

Figure 4. Fish size (SL) and size ratios of individuals adjacent in rank within each group of Amphiprion frenatus.

A) The observed distribution of fish size (SL) within each group. B) Distribution of body size ratios between individuals adjacent in rank within each group. Results are expressed as mean ± S.D. values (□ = group 1, ▵ = group 2, ○ = group 3).