Although clownfish sounds were recorded as early as 1930, the mechanism of sound production has remained obscure. Yet, clownfish are prolific "singers" that produce a wide variety of sounds, described as "chirps" and "pops" in both reproductive and agonistic behavioral contexts. Here, we describe the sonic mechanism of the clownfish Amphiprion clarkii.

Fourteen individuals of the skunk clownfish Amphiprion akallopisos of different sizes and of different sexual status (non-breeder, male or female) were analysed for four acoustic features. Dominant frequency and pulse duration were highly correlated with standard length (r = 0.97), and were not related to sex. Both the dominant frequency and pulse duration were signals conveying information related to the size of the emitter, which implies that these sound characteristics could be useful in assessing size of conspecifics.

Elucidating the origins of complex biological structures has been one of the major challenges of evolutionary studies. Within vertebrates, the capacity to produce regular coordinated electric organ discharges (EODs) has evolved independently in different fish lineages. Intermediate stages, however, are not known. We show that, within a single catfish genus, some species are able to produce sounds, electric discharges or both signals (though not simultaneously). We highlight that both acoustic and electric communication result from actions of the same muscle. In parallel to their abilities, the studied species show different degrees of myofibril development in the sonic and electric muscle. The lowest myofibril density was observed in Synodontis nigriventris , which produced EODs but no swim bladder sounds, whereas the greatest myofibril density was observed in Synodontis grandiops , the species that produced the longest sound trains but did not emit EODs. Additionally, S. grandiops exhibited the lowest auditory thresholds. Swim bladder sounds were similar among species, while EODs were distinctive at the species level. We hypothesize that communication with conspecifics favoured the development of species-specific EOD signals and suggest an evolutionary explanation for the transition from a fast sonic muscle to electrocytes.

BackgroundClownfishes (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 FindingsRecording 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/SignificanceUnlike 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.

BackgroundMany coral reef fishes undergo habitat and diet shifts during ontogeny. However, studies focusing on the physiological and morphological adaptations that may prepare them for these transitions are relatively scarce. Here, we explored the body shape variation related to ontogenetic shifts in the ecology of the surgeonfish Acanthurus triostegus (Acanthuridae) from new settler to adult stages at Moorea Island (French Polynesia). Specifically, we tested the relationship between diet and habitat shifts and changes in overall body shape during the ontogeny of A. triostegus using a combination of geometric morphometric methods, stomach contents and stable isotope analysis.ResultsAfter reef settlement, stable isotope composition of carbon and nitrogen revealed a change from a zooplanktivorous to a benthic algae diet. The large amount of algae (> 75% of stomach contents) found in the digestive tract of small juveniles (25–30 mm SL) suggested the diet shift is rapid. The post-settlement growth of A. triostegus is highly allometric. The allometric shape changes mainly concern cephalic and pectoral regions. The head becomes shorter and more ventrally oriented during growth. Morphological changes are directly related to the diet shift given that a small mouth ventrally oriented is particularly suited for grazing activities at the adult stage. The pectoral fin is more anteriorely and vertically positioned and its basis is larger in adults than in juveniles. This shape variation had implications for swimming performance, manoeuvrability, turning ability and is related to habitat shift. Acanthurus triostegus achieves its main transformation of body shape to an adult-like form at size of 35–40 mm SL.ConclusionMost of the shape changes occurred after the reef colonization but before the transition between juvenile habitat (fringing reef) and adult habitat (barrier reef). A large amount of allometric variation was observed after diet shift from zooplankton to benthic algae. Diet shift could act as an environmental factor favouring or inducing morphological changes. On the other hand, the main shape changes have to be achieved before the recruitment to adult populations and start negotiating the biophysical challenges of locomotion and feeding in wave- and current-swept outer reef habitat.

BackgroundClownfishes are colorful coral reef fishes living in groups in association with sea anemones throughout the Indo-Pacific Ocean. Within their small societies, size hierarchy determines which fish have access to reproduction. These fishes are also prolific callers whose aggressive sounds seem to play an important role in the social hierarchy. Agonistic interactions being involved in daily behaviour suggest how acoustic communication might play an important role in clownfish group. Sounds were recorded and compared in fourteen clownfish species (some of which have never been recorded before) to evaluate the potential role of acoustic communication as an evolutionary driving force.ResultsSurprisingly, the relationship between fish size and both dominant frequency and pulse duration is not only species-specific; all the specimens of the 14 species are situated on exactly the same slope, which means the size of any Amphiprion can be predicted by both acoustic features. The number of pulses broadly overlaps among species, whereas the pulse period displays the most variation even if it shows overlap among sympatric species. Sound comparisons between three species (A. akallopisos, A. ocellaris and A. frenatus) having different types of teeth and body shape do not show differences neither in the acoustic waveform nor in the power spectrum.ConclusionSignificant overlap in acoustic features demonstrates that the sound-producing mechanism is highly conservative among species. Differences in the calls of some species are due to size dimorphism and the sound variation might be in this case a by-product. This morphological constraint does not permit a consideration of acoustic communication as the main driving force in the diversification of clownfishes. Moreover, calls are not produced to find mate and consequently are less subject to variations due to partner preference, which restricts the constraints of diversification. Calls are produced to reach and defend the competition to mate access. However, differences in the pulse period between cohabiting species show that, in some case, sounds can help to differentiate the species, to prevent competition between cohabiting species and to promote the diversification of taxa.

SUMMARYClownfish live in social groups in which there is a size-based dominance hierarchy. In such a context, sonic cues could play a role in social organisation because dominant frequency and pulse length of sounds are strongly correlated with fish size. Data on the hearing ability of these fish are, however, needed to show that they have the sensory ability to detect the frequencies in their sounds. The present study determines the hearing sensitivity in three different anemonefish species (Amphiprion frenatus, Amphiprion ocellaris and Amphiprion clarkii), and compares it with the frequencies in their calls. The frequency range over which the three species can detect sounds was between 75 and 1800 Hz, and they were most sensitive to frequencies below 200 Hz. During sound production, dominant frequency is clearly related (R=0.95) to the fish size, whatever the species. Dominant frequency extends from 370 to 900 Hz for specimens having a size between 55 and 130 mm. The best hearing sensitivity of small specimens were found to be lower than the dominant frequency of their own calls. However, they were found to be close to the dominant frequency of larger fish calls. The interest of juveniles lies in localising the adults and thus their location on the reef.

auditory sensitivity may potentially facilitate eavesdropping by sneaker males and their assessment of vocal type I males for the selection of cuckoldry sites during the breeding season.