Vocal rhythms in nesting Lusitanian toadfish, Halobatrachus didactylus

Abstract

Males of several fish species aggregate and vocalize together, increasing the detection range of the sounds and their chances of mating. In the Lusitanian toadfish (Halobatrachus didactylus), breeding males build nests under rocks in close proximity and produce hundreds of boatwhistles (BW) an hour to attract females to lay their demersal eggs on their nests. Chorusing behaviour includes fine-scale interactions between individuals, a behavioural dynamic worth investigating in this highly vocal fish. Here we present a study to further investigate this species' vocal temporal patterns on a fine (individual rhythms and male-male interactions) and large (chorus daily patterns) scales. Several datasets recorded in the Tagus estuary were labelled with the support of an automatic recognition system based on hidden Markov models. Fine-scale vocal temporal patterns exhibit high variability between and within individuals, varying from an almost isochronous to an apparent aperiodic pattern. When in a chorus, males exhibited alternation or synchrony calling patterns, possibly depending on motivation and social context (mating or male-male competition). When engaged in sustained calling, males usually alternated vocalizations with their close neighbours thus avoiding superposition of calls. Synchrony was observed mostly in fish with lower mean calling rate. Interaction patterns were less obvious in more distanced males. Daily choruses showed periods with several active calling males and periods of low activity with no significant diel patterns in shallower intertidal waters. Here, chorusing activity was mainly affected by tide level.In contrast, at a deeper location, although tidal currents causes a decrease in calling rate, tide level did not significantly influence calling, and there was a higher calling rate at night. These data show that photoperiod and tide levels can influence broad patterns of Lusitanian toadfish calling activity as in other shallow-water fishes, but fine temporal patterns in acoustic interactions among nesting males is more complex than previously known for fishes.

Boat noise interferes with Lusitanian toadfish acoustic communication

“…In previous studies, the active space of the Lusitanian toadfish (Alves et al, 2016), a vocal teleost fish that depends on acoustic communication for successful reproduction (Vasconcelos et al, 2012;Amorim et al, 2016), was evaluated and the vocal patterns of toadfish in choruses was described (Jordão et al, 2012;Vieira et al, 2021). The estimated range varied between 6 m and 13 m in a shallow breeding area (Alves et al, 2016).…”

“…The average range for BW1 was 6.4 m and for BW2 was 10.4 m (Table 1). While chorusing, males avoid overlapping their calls with their neighbours, leading to a pattern of antiphony alternation (Vieira et al, 2021). We assessed the impact that boat noise causes on this species' active space and vocal behaviour.…”

“…Because the inter-onset interval did not show much intra-individual variability, when one fish made more than one BW between consecutive BWs of the reference fish only the first BW was considered. We used the Rayleigh test of circular statistics (Fisher, 1995) to test if the noise playback altered the phase of the BWs between individuals (Vieira et al, 2021).…”

“…A reduction in BW active space will likely affect mate detection distance and vocal interactions amongst neighbouring territorial males, with implications for fitness. In previous studies, both active space (Alves et al, 2016) and vocal behaviour patterns (Amorim et al, 2011;Jordão et al, 2012;Vasconcelos et al, 2011;Vieira et al, 2021) have been studied. Alves et al (2016) showed that in shallow habitats BWs can be perceived up to about 13 m. Lusitanian toadfish adjust the rate of BW emission depending on the acoustic social environment, that is, whether they are calling alone or in a chorus (Amorim et al, 2011).…”

“…Alves et al (2016) showed that in shallow habitats BWs can be perceived up to about 13 m. Lusitanian toadfish adjust the rate of BW emission depending on the acoustic social environment, that is, whether they are calling alone or in a chorus (Amorim et al, 2011). In addition, this species exhibits finescale male-male interactions, such as matching a neighbour's calling rate (Jordão et al, 2012) and maintaining call alternation, thus avoiding the vocalizations of their neighbours (Vieira et al, 2021). Here, we aim to estimate to what extent boat noise reduces active space and how it affects vocal behaviour of Lusitanian toadfish breeding territorial males.…”